manning.spring.in.action.4th.edition.2014.11


MANNING Craig Walls FOURTH EDITION covers Spring 4 Praise for the Third Edition of Spring in Action Continues to be the de-facto reference guide to Spring. Offers clear explanations of concepts with very good examples in an easy-to-read format. —Dan Dobrin, CIBC An indispensable guide to the large landscape of Spring. —Mykel Alvis, Automaton Online The one book you need on your desk when working with Spring. —Josh Devins, Nokia Covers both the fundamentals and the breadth of Spring. —Chad Davis, Blackdog Software, Inc. Using Spring is not difficult—but with this book it becomes much easier. —Alberto Lagna, Biznology One of my favorite technology books. Great content delivered by a great teacher. —Robert Hanson, Author of Manning’s GWT in Action The right dose of humor with a load of technical wisdom is the perfect mix for learning Spring. —Valentin Crettaz, Goomzee Tremendous focus—and fun to read. —Doug Warren, Java Web Services Craig’s witty examples make complex concepts easy to understand. —Dan Alford Spring in Action FOURTH EDITION CRAIG WALLS MANNING SHELTER ISLAND For online information and ordering of this and other Manning books, please visit www.manning.com. The publisher offers discounts on this book when ordered in quantity. For more information, please contact Special Sales Department Manning Publications Co. 20 Baldwin Road PO Box 761 Shelter Island, NY 11964 Email: orders@manning.com ©2015 by Manning Publications Co. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by means electronic, mechanical, photocopying, or otherwise, without prior written permission of the publisher. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in the book, and Manning Publications was aware of a trademark claim, the designations have been printed in initial caps or all caps. Recognizing the importance of preserving what has been written, it is Manning’s policy to have the books we publish printed on acid-free paper, and we exert our best efforts to that end. Recognizing also our responsibility to conserve the resources of our planet, Manning books are printed on paper that is at least 15 percent recycled and processed without the use of elemental chlorine. Manning Publications Co. Development editor: Cynthia Kane 20 Baldwin Road Copyeditor: Andy Carroll Shelter Island, NY 11964 Proofreader: Alyson Brener Typesetter: Dottie Marsico Cover designer: Marija Tudor ISBN 9781617291203 Printed in the United States of America 12345678910–EBM–191817161514 v brief contents PART 1CORE SPRING ...............................................................1 1 ■ Springing into action 3 2 ■ Wiring beans 32 3 ■ Advanced wiring 64 4 ■ Aspect-oriented Spring 97 PART 2SPRING ON THE WEB.................................................129 5 ■ Building Spring web applications 131 6 ■ Rendering web views 164 7 ■ Advanced Spring MVC 194 8 ■ Working with Spring Web Flow 219 9 ■ Securing web applications 244 PART 3SPRING IN THE BACK END ..........................................279 10 ■ Hitting the database with Spring and JDBC 281 11 ■ Persisting data with object-relational mapping 305 12 ■ Working with NoSQL databases 327 13 ■ Caching data 362 14 ■ Securing methods 379 BRIEF CONTENTSvi PART 4INTEGRATING SPRING................................................391 15 ■ Working with remote services 393 16 ■ Creating REST APIs with Spring MVC 416 17 ■ Messaging in Spring 452 18 ■ Messaging with WebSocket and STOMP 485 19 ■ Sending email with Spring 511 20 ■ Managing Spring beans with JMX 523 21 ■ Simplifying Spring development with Spring Boot 540 vii contents preface xvii acknowledgments xix about this book xxi PART 1CORE SPRING....................................................1 1 Springing into action 3 1.1 Simplifying Java development 4 Unleashing the power of POJOs 5 ■ Injecting dependencies 5 Applying aspects 11 ■ Eliminating boilerplate code with templates 16 1.2 Containing your beans 18 Working with an application context 19 ■ A bean’s life 20 1.3 Surveying the Spring landscape 21 Spring modules 22 ■ The Spring portfolio 24 1.4 What’s new in Spring 27 What was new in Spring 3.1? 27 ■ What was new in Spring 3.2? 28 ■ What’s new in Spring 4.0? 29 1.5 Summary 30 CONTENTSviii 2 Wiring beans 32 2.1 Exploring Spring’s configuration options 33 2.2 Automatically wiring beans 34 Creating discoverable beans 34 ■ Naming a component-scanned bean 38 ■ Setting a base package for component scanning 38 Annotating beans to be automatically wired 39 ■ Verifying automatic configuration 41 2.3 Wiring beans with Java 43 Creating a configuration class 43 ■ Declaring a simple bean 44 ■ Injecting with JavaConfig 45 2.4 Wiring beans with XML 46 Creating an XML configuration specification 47 ■ Declaring a simple 48 ■ Initializing a bean with constructor injection 49 ■ Setting properties 54 2.5 Importing and mixing configurations 59 Referencing XML configuration in JavaConfig 59 Referencing JavaConfig in XML configuration 61 2.6 Summary 63 3 Advanced wiring 64 3.1 Environments and profiles 64 Configuring profile beans 66 ■ Activating profiles 70 3.2 Conditional beans 72 3.3 Addressing ambiguity in autowiring 75 Designating a primary bean 76 ■ Qualifying autowired beans 77 3.4 Scoping beans 81 Working with request and session scope 82 ■ Declaring scoped proxies in XML 84 3.5 Runtime value injection 84 Injecting external values 85 ■ Wiring with the Spring Expression Language 89 3.6 Summary 95 4 Aspect-oriented Spring 97 4.1 What is aspect-oriented programming? 98 Defining AOP terminology 99 ■ Spring’s AOP support 101 CONTENTS ix 4.2 Selecting join points with pointcuts 103 Writing pointcuts 104 ■ Selecting beans in pointcuts 106 4.3 Creating annotated aspects 106 Defining an aspect 106 ■ Creating around advice 110 Handling parameters in advice 112 ■ Annotating introductions 115 4.4 Declaring aspects in XML 117 Declaring before and after advice 118 ■ Declaring around advice 121 ■ Passing parameters to advice 122 Introducing new functionality with aspects 124 4.5 Injecting AspectJ aspects 125 4.6 Summary 127 PART 2SPRING ON THE WEB .....................................129 5 Building Spring web applications 131 5.1 Getting started with Spring MVC 132 Following the life of a request 132 ■ Setting up Spring MVC 134 ■ Introducing the Spittr application 138 5.2 Writing a simple controller 139 Testing the controller 140 ■ Defining class-level request handling 142 ■ Passing model data to the view 143 5.3 Accepting request input 148 Taking query parameters 149 ■ Taking input via path parameters 151 5.4 Processing forms 154 Writing a form-handling controller 156 ■ Validating forms 159 5.5 Summary 162 6 Rendering web views 164 6.1 Understanding view resolution 164 6.2 Creating JSP views 167 Configuring a JSP-ready view resolver 167 ■ Using Spring’s JSP libraries 169 6.3 Defining a layout with Apache Tiles views 182 Configuring a Tiles view resolver 182 CONTENTSx 6.4 Working with Thymeleaf 187 Configuring a Thymeleaf view resolver 187 ■ Defining Thymeleaf templates 189 6.5 Summary 193 7 Advanced Spring MVC 194 7.1 Alternate Spring MVC configuration 195 Customizing DispatcherServlet configuration 195 Adding additional servlets and filters 196 ■ Declaring DispatcherServlet in web.xml 197 7.2 Processing multipart form data 200 Configuring a multipart resolver 201 ■ Handling multipart requests 205 7.3 Handling exceptions 208 Mapping exceptions to HTTP status codes 209 ■ Writing exception-handling methods 211 7.4 Advising controllers 212 7.5 Carrying data across redirect requests 213 Redirecting with URL templates 214 ■ Working with flash attributes 215 7.6 Summary 217 8 Working with Spring Web Flow 219 8.1 Configuring Web Flow in Spring 220 Wiring a flow executor 220 ■ Configuring a flow registry 221 ■ Handling flow requests 222 8.2 The components of a flow 222 States 223 ■ Transitions 226 ■ Flow data 227 8.3 Putting it all together: the pizza flow 229 Defining the base flow 229 ■ Collecting customer information 232 ■ Building an order 238 Taking payment 240 8.4 Securing web flows 242 8.5 Summary 242 9 Securing web applications 244 9.1 Getting started with Spring Security 245 Understanding Spring Security modules 246 ■ Filtering web requests 246 ■ Writing a simple security configuration 248 CONTENTS xi 9.2 Selecting user details services 250 Working with an in-memory user store 251 ■ Authenticating against database tables 252 ■ Applying LDAP-backed authentication 255 ■ Configuring a custom user service 259 9.3 Intercepting requests 260 Securing with Spring Expressions 263 ■ Enforcing channel security 264 ■ Preventing cross-site request forgery 265 9.4 Authenticating users 267 Adding a custom login page 268 ■ Enabling HTTP Basic authentication 269 ■ Enabling remember-me functionality 270 Logging out 270 9.5 Securing the view 271 Using Spring Security’s JSP tag library 272 ■ Working with Thymeleaf’s Spring Security dialect 275 9.6 Summary 277 PART 3SPRING IN THE BACK END...............................279 10 Hitting the database with Spring and JDBC 281 10.1 Learning Spring’s data-access philosophy 282 Getting to know Spring’s data-access exception hierarchy 283 Templating data access 286 10.2 Configuring a data source 288 Using JNDI data sources 288 ■ Using a pooled data source 289 Using JDBC driver-based data sources 291 ■ Using an embedded data source 292 ■ Using profiles to select a data source 293 10.3 Using JDBC with Spring 295 Tackling runaway JDBC code 296 ■ Working with JDBC templates 299 10.4 Summary 304 11 Persisting data with object-relational mapping 305 11.1 Integrating Hibernate with Spring 307 Declaring a Hibernate session factory 307 ■ Building Spring-free Hibernate 309 11.2 Spring and the Java Persistence API 311 Configuring an entity manager factory 311 ■ Writing a JPA-based repository 316 CONTENTSxii 11.3 Automatic JPA repositories with Spring Data 318 Defining query methods 320 ■ Declaring custom queries 323 Mixing in custom functionality 324 11.4 Summary 326 12 Working with NoSQL databases 327 12.1 Persisting documents with MongoDB 328 Enabling MongoDB 329 ■ Annotating model types for MongoDB persistence 332 ■ Accessing MongoDB with MongoTemplate 335 ■ Writing a MongoDB repository 337 12.2 Working with graph data in Neo4j 341 Configuring Spring Data Neo4j 342 ■ Annotating graph entities 344 ■ Working with Neo4jTemplate 348 Creating automatic Neo4j repositories 349 12.3 Working with key-value data in Redis 354 Connecting to Redis 354 ■ Working with RedisTemplate 355 Setting key and value serializers 359 12.4 Summary 360 13 Caching data 362 13.1 Enabling cache support 363 Configuring a cache manager 364 13.2 Annotating methods for caching 368 Populating the cache 369 ■ Removing cache entries 373 13.3 Declaring caching in XML 374 13.4 Summary 378 14 Securing methods 379 14.1 Securing methods with annotations 380 Restricting method access with @Secured 380 ■ Using JSR-250’s @RolesAllowed with Spring Security 382 14.2 Using expressions for method-level security 383 Expressing method access rules 383 ■ Filtering method inputs and outputs 385 14.3 Summary 390 CONTENTS xiii PART 4INTEGRATING SPRING....................................391 15 Working with remote services 393 15.1 An overview of Spring remoting 394 15.2 Working with RMI 396 Exporting an RMI service 397 ■ Wiring an RMI service 399 15.3 Exposing remote services with Hessian and Burlap 402 Exposing bean functionality with Hessian/Burlap 402 Accessing Hessian/Burlap services 405 15.4 Using Spring’s HttpInvoker 407 Exposing beans as HTTP services 407 ■ Accessing services via HTTP 408 15.5 Publishing and consuming web services 410 Creating Spring-enabled JAX-WS endpoints 410 Proxying JAX-WS services on the client side 413 15.6 Summary 415 16 Creating REST APIs with Spring MVC 416 16.1 Getting REST 417 The fundamentals of REST 417 ■ How Spring supports REST 418 16.2 Creating your first REST endpoint 419 Negotiating resource representation 421 ■ Working with HTTP message converters 426 16.3 Serving more than resources 432 Communicating errors to the client 432 ■ Setting headers in the response 436 16.4 Consuming REST resources 439 Exploring RestTemplate’s operations 440 ■ GETting resources 441 ■ Retrieving resources 442 ■ Extracting response metadata 443 ■ PUTting resources 444 DELETEing resources 445 ■ POSTing resource data 446 Receiving object responses from POST requests 446 Receiving a resource location after a POST request 448 Exchanging resources 448 16.5 Summary 450 CONTENTSxiv 17 Messaging in Spring 452 17.1 A brief introduction to asynchronous messaging 453 Sending messages 454 ■ Assessing the benefits of asynchronous messaging 456 17.2 Sending messages with JMS 458 Setting up a message broker in Spring 458 ■ Using Spring’s JMS template 460 ■ Creating message-driven POJOs 469 ■ Using message-based RPC 472 17.3 Messaging with AMQP 474 A brief introduction to AMQP 475 ■ Configuring Spring for AMQP messaging 477 ■ Sending messages with RabbitTemplate 479 ■ Receiving AMQP messages 482 17.4 Summary 484 18 Messaging with WebSocket and STOMP 485 18.1 Working with Spring’s low-level WebSocket API 486 18.2 Coping with a lack of WebSocket support 491 18.3 Working with STOMP messaging 493 Enabling STOMP messaging 495 ■ Handling STOMP messages from the client 498 ■ Sending messages to the client 501 18.4 Working with user-targeted messages 505 Working with user messages in a controller 505 Sending messages to a specific user 507 18.5 Handling message exceptions 508 18.6 Summary 509 19 Sending email with Spring 511 19.1 Configuring Spring to send email 512 Configuring a mail sender 512 ■ Wiring and using the mail sender 514 19.2 Constructing rich email messages 515 Adding attachments 515 ■ Sending email with rich content 516 19.3 Generating email with templates 517 Constructing email messages with Velocity 518 ■ Using Thymeleaf to create email messages 520 19.4 Summary 522 CONTENTS xv 20 Managing Spring beans with JMX 523 20.1 Exporting Spring beans as MBeans 524 Exposing methods by name 527 ■ Using interfaces to define MBean operations and attributes 529 ■ Working with annotation-driven MBeans 530 ■ Handling MBean collisions 532 20.2 Remoting MBeans 533 Exposing remote MBeans 533 ■ Accessing remote MBeans 534 Proxying MBeans 536 20.3 Handling notifications 537 Listening for notifications 538 20.4 Summary 539 21 Simplifying Spring development with Spring Boot 540 21.1 Introducing Spring Boot 541 Adding starter dependencies 541 ■ Autoconfiguration 546 The Spring Boot CLI 546 ■ The Actuator 547 21.2 Building an application with Spring Boot 547 Handling requests 550 ■ Creating the view 552 Adding static artifacts 554 ■ Persisting the data 555 Try it out 557 21.3 Going Groovy with the Spring Boot CLI 560 Writing a Groovy controller 560 ■ Persisting with a Groovy repository 563 ■ Running the Spring Boot CLI 564 21.4 Gaining application insight with the Actuator 565 21.5 Summary 568 index 570 xvii preface The best keeps getting better. More than a dozen years ago, Spring entered the Java development scene with the ambitious goal of simplifying enterprise Java develop- ment. It challenged the heavyweight programming models of the time with a simpler and lighter programming model based on plain old Java objects. Now, several years and many releases later, we see that Spring has had a tremen- dous impact on enterprise application development. It has become a de facto stan- dard framework for countless Java projects and has had an impact on the evolution of some of the specifications and frameworks that it originally set out to replace. It’d be hard to deny that the current Enterprise JavaBeans (EJB) specification may have turned out very differently had Spring not challenged earlier versions of the EJB spec. But Spring itself continues to evolve and improve upon itself, always seeking to make the difficult development tasks simpler and empower Java developers with inno- vative features. Where Spring had first set out to challenge the status quo, Spring now has leapt ahead and is paving trails in Java application development. Therefore, it’s time for an updated edition of this book to expose the current state of Spring. There’s so much that has happened in the past few years since the previous edition of this book; it’d be impossible to cover everything in a single edition. Never- theless, I still tried to pack this fourth edition of Spring in Action with as much as I could. Here are just a few of the exciting new things that have been added in this edition: ■ An emphasis on Java-based Spring configuration with Java configuration options available for almost every area of Spring development ■ Conditional configuration and profiles that make runtime decisions regarding what Spring configuration should be used or ignored PREFACExviii ■ Several enhancements and improvements to Spring MVC, especially with regard to creating REST services ■ Using Thymeleaf with Spring web applications as an alternative to JSP ■ Enabling Spring Security with Java-based configuration ■ Using Spring Data to automatically generate repository implementations at runtime for JPA, MongoDB, and Neo4j ■ Spring’s new declarative caching support ■ Asynchronous web messaging with WebSocket and STOMP ■ Spring Boot, a game-changing new approach to working with Spring If you’re a seasoned Spring veteran, you’ll find that these new elements will become valuable additions to your Spring toolkit. On the other hand, if you’re new to Spring, you’ve picked a good time to learn Spring, and this book will help you get started. This is, indeed, an exciting time to be working with Spring. It’s been a blast to develop with Spring and write about it during the past 12 years. I can’t wait to see what Spring does next! xix acknowledgments Before this book goes to press, before it is bound, before it is boxed, before it is shipped, and before you get your hands on it, there are many other hands that have touched it along the way. Even if you have an eBook copy that didn’t go through that process, there were numerous hands on the bits and bytes that you downloaded— hands that edited it, reviewed it, typeset it, and proofread it. If it weren’t for all of those hands, this book wouldn’t exist. First, a big thank you to everyone at Manning for working hard, for their patience when the writing wasn’t moving as fast as it should have, and for prodding me along to get it done: Marjan Bace, Michael Stephens, Cynthia Kane, Andy Carroll, Benjamin Berg, Alyson Brener, Dottie Marisco, Mary Piergies, Janet Vail, and many others behind the scenes. Getting feedback early and often is just as critical when writing a book as it is when developing software. While the pages of this book were still in a very rough form, there were several great reviewers who took the time to read the drafts and provide feedback that helped shape the final product. Thanks to the following: Bob Casazza, Chaoho Hsieh, Christophe Martini, Gregor Zurowski, James Wright, Jeelani Basha, Jens Richter, Jonathan Thoms, Josh Hart, Karen Christenson, Mario Arias, Michael Roberts, Paul Balogh, and Ricardo da Silva Lima. And special thanks to John Ryan for his thor- ough technical review of the manuscript shortly before it went into production. Of course, I want to thank my beautiful wife for enduring yet another writing proj- ect and for her encouragement along the way. I love you more than you could possibly ever know. ACKNOWLEDGMENTSxx To Maisy and Madi, the most awesome little girls in the world, thank you again for your hugs, laughs, and unusual insights into what should go into the book. To my colleagues on the Spring team, what can I say? You guys ROCK! I’m humbled and grateful for being a part of the organization that drives Spring forward. I never cease to be amazed at the never-ending awesomeness that you crank out. And many thanks to everyone I encounter as I travel the country speaking at user groups and No Fluff/Just Stuff conferences. Finally, thank you to the Phoenicians. You (and Epcot fans) know what you did. xxi about this book The Spring Framework was created with a very specific goal in mind—to make devel- oping Java EE applications easier. Along the same lines, Spring in Action, Fourth Edition was written to make learning how to use Spring easier. My goal is not to give you a blow-by-blow listing of Spring APIs. Instead, I hope to present the Spring Framework in a way that is most relevant to a Java EE developer by providing practical code exam- ples from real-world experiences. Since Spring is a modular framework, this book was written in the same way. I recognize that not all developers have the same needs. Some may want to learn the Spring Framework from the ground up, while others may want to pick and choose different topics and go at their own pace. That way, the book can act as a tool for learning Spring for the first time as well as a guide and reference for those wanting to dig deeper into specific features. Spring in Action, Fourth Edition is for all Java developers, but enterprise Java develop- ers will find it particularly useful. While I will guide you along gently through code examples that build in complexity throughout each chapter, the true power of Spring lies in its ability to make enterprise applications easier to develop. Therefore, enter- prise developers will most fully appreciate the examples presented in this book. Because a vast portion of Spring is devoted to providing enterprise services, many par- allels can be drawn between Spring and EJB. Roadmap Spring in Action, Fourth Edition is divided into four parts. The first part introduces you to the essentials of the Spring Framework. Part 2 expands on that by showing how to build web applications with Spring. Part 3 steps behind the front end and shows where ABOUT THIS BOOKxxii Spring fits in the back end of an application. The final part shows how Spring can be used to integrate with other applications and services. In part 1, you’ll explore the Spring container, dependency injection (DI), and aspect-oriented programming…the essentials of the Spring Framework. This will give you a foundation upon which the rest of the book will build. ■ In chapter 1, you’ll be given an overview of Spring, including some basic exam- ples of DI and AOP. You’ll also get an overview of the greater Spring ecosystem. ■ Chapter 2 goes into more detail with DI, showing you various ways that the com- ponents in your application (the “beans”) can be wired together. This includes wiring with XML, Java, and automatic wiring. ■ With the basics of bean wiring down, chapter 3 presents several advanced wiring techniques. You won’t need these techniques that often, but when you do need them this chapter will show you how to get the most power out of the Spring container. ■ Chapter 4 explores how to use Spring AOP to decouple cross-cutting concerns from the objects that they service. This chapter also sets the stage for later chap- ters where you’ll use AOP to provide declarative services such as transactions, security, and caching. In part 2 you’ll see how to use Spring to build web applications. ■ Chapter 5 covers the basics of working with Spring MVC, the foundational web framework in Spring. You’ll see how to write controllers to handle web requests and respond with model data. ■ Once a controller is finished with its work, the model data must be rendered using a view. Chapter 6 will explore various view technologies that can be used with Spring, including JSP, Apache Tiles, and Thymeleaf. ■ Chapter 7 goes beyond the basics of Spring MVC. In this chapter, you’ll learn how to customize Spring MVC configuration, handle multipart file uploads, deal with exceptions that may occur in a controller, and pass data between requests with flash attributes. ■ Chapter 8 explores Spring Web Flow, an extension to Spring MVC that enables development of conversational web applications. In this chapter, you’ll learn how to build web applications that lead the user through a specific, guided flow. ■ In chapter 9 you’ll learn how to apply security to the web layer of your applica- tion using Spring Security. Part 3 goes behind the front end of an application and looks at how data is processed and persisted. ■ Data persistence is first tackled in chapter 10 using Spring’s abstraction over JDBC to work with data stored in a relational database. ■ Chapter 11 takes on data persistence from another angle, using the Java Persis- tence API (JPA) to store data in a relational database. ABOUT THIS BOOK xxiii ■ Chapter 12 looks at how Spring works with non-relational databases, such as MongoDB and Neo4j. Regardless of where the data is stored, caching can help improve performance by not hitting the database any more than necessary. ■ Chapter 13 introduces you to Spring’s support for declarative caching. ■ Chapter 14 revisits Spring Security, showing how to use AOP to apply security at the method level. The final part looks at ways to integrate your Spring applications with other systems. ■ Chapter 15 looks at how to create and consume remote services, including RMI, Hessian, Burlap, and SOAP-based services. ■ In chapter 16, Spring MVC is revisited to see how to create RESTful services using the same programming model as described previously in chapter 5. ■ Chapter 17 explores Spring support for asynchronous messaging. This chapter includes working with Java Message Service (JMS) as well as the Advanced Mes- sage Queuing Protocol (AMQP). ■ Asynchronous messaging takes a different twist in chapter 18 where you’ll see how to use Spring with WebSocket and STOMP for asynchronous communica- tion between the server and a client. ■ Chapter 19 looks at how to send emails with Spring. ■ Chapter 20 highlights Spring’s management support for Java Management Extensions (JMX), enabling you to monitor and modify runtime settings for a Spring application. ■ Finally, in chapter 21 you’ll be introduced to a game-changing and very new way to work with Spring called Spring Boot. You’ll see how Spring Boot can take away much of the boilerplate configuration required in a Spring application, enabling you to focus on the business functionality. Code conventions and downloads There are many code examples throughout this book. These examples will always appear in a fixed-width code font like this. Any class name, method name, or XML fragment within the normal text of the book will appear in code font as well. Many of Spring’s classes and packages have exceptionally long (but expressive) names. Because of this, line-continuation markers (➥) may be included when necessary. Not all code examples in this book will be complete. Often I only show a method or two from a class to focus on a particular topic. Complete source code for the appli- cations found throughout the book can be downloaded from the publisher’s website at www.manning.com/SpringinActionFourthEdition. Author Online Purchase of Spring in Action, Fourth Edition includes free access to a private web forum run by Manning Publications where you can make comments about the book, ask technical questions, and receive help from the author and from other users. To ABOUT THIS BOOKxxiv access the forum and subscribe to it, point your web browser to www.manning.com/ SpringinActionFourthEdition. This page provides information on how to get on the forum once you are registered, what kind of help is available, and the rules of con- duct on the forum. Manning’s commitment to our readers is to provide a venue where a meaningful dialogue between individual readers and between readers and the author can take place. It is not a commitment to any specific amount of participation on the part of the author, whose contribution to the book’s forum remains voluntary (and unpaid). We suggest you try asking the author some challenging questions, lest his interest stray! The Author Online forum and the archives of previous discussions will be accessi- ble from the publisher’s website as long as the book is in print. About the author Craig Walls is a senior engineer with Pivotal as the project lead for Spring Social and Spring Sync, and is the author of Manning’s Spring in Action books, now updated in this Fourth Edition. He’s a zealous promoter of the Spring Framework, speaking fre- quently at local user groups and conferences and writing about Spring. When he’s not slinging code, Craig spends as much time as he can with his wife, two daughters, two birds, and two dogs. About the cover illustration The figure on the cover of Spring in Action, Fourth Edition, is “Le Caraco,” or an inhab- itant of the province of Karak in southwest Jordan. Its capital is the city of Al-Karak, which boasts an ancient hilltop castle with magnificent views of the Dead Sea and sur- rounding plains. The illustration is taken from a French travel book, Encyclopédie des Voyages by J. G. St. Sauveur, published in 1796. Travel for pleasure was a relatively new phenomenon at the time and travel guides such as this one were popular, introducing both the tourist as well as the armchair traveler to the inhabitants of other regions of France and abroad. The diversity of the drawings in the Encyclopédie des Voyages speaks vividly of the dis- tinctiveness and individuality of the world’s towns and provinces just two hundred years ago. This was a time when the dress codes of two regions separated by a few dozen miles identified people uniquely as belonging to one or the other. The travel guide brings to life a sense of isolation and distance of that period, and of every other historic period except our own hyperkinetic present. Dress codes have changed since then and the diversity by region, so rich at the time, has faded away. It is now often hard to tell the inhabitants of one continent from another. Perhaps, trying to view it optimistically, we have traded a cultural and visual diversity for a more varied personal life—or a more varied and interesting intellectual and technical life. We at Manning celebrate the inventiveness, the initiative, and the fun of the com- puter business with book covers based on the rich diversity of regional life two centu- ries ago brought back to life by the pictures from this travel guide. Part 1 Core Spring Spring does a lot of things. But underneath all of the fantastic functionality it adds to enterprise development, its primary features are dependency injection (DI) and aspect-oriented programming (AOP). Starting in chapter 1, “Springing into action,” I’ll give you a quick overview of the Spring Framework, including a quick overview of DI and AOP in Spring and show how they help with decoupling application components. In chapter 2, “Wiring beans,” we’ll dive deeper into how to piece together the components of an application. We’ll look at automatic configuration, Java- based configuration, and XML configuration options offered by Spring. Chapter 3, “Advanced wiring,” goes beyond the basics and shows you a few tricks and techniques that will help you get the most power out of Spring, includ- ing conditional configuration, dealing with ambiguity when autowiring, scoping, and the Spring Expression Language. Chapter 4, “Aspect-oriented Spring,” explores how to use Spring’s AOP fea- tures to decouple system-wide services (such as security and auditing) from the objects they service. This chapter sets the stage for later chapters such as chap- ters 9, 13, and 14 where you’ll see how to leverage Spring AOP for declarative security and caching. 3 Springing into action It’s a good time to be a Java developer. In its almost 20 year history, Java has seen some good times and some bad times. Despite a handful of rough spots, such as applets, Enterprise JavaBeans (EJB), Java Data Objects (JDO), and countless logging frameworks, Java has enjoyed a rich and diverse history as the platform on which much enterprise software has been built. And Spring has been a big part of that story. In its early days, Spring was created as an alternative to heavier enterprise Java technologies, especially EJB. Spring offered a lighter and leaner programming model as compared to EJB. It empowered plain old Java objects (POJOs) with pow- ers previously only available using EJB and other enterprise Java specifications. Over time, EJB and the Java 2 Enterprise Edition (J2EE) evolved. EJB started offering a simple POJO-oriented programming model of its own. Now EJB employs ideas such as dependency injection (DI) and aspect-oriented programming (AOP), arguably inspired by the success of Spring. This chapter covers  Spring’s bean container  Exploring Spring’s core modules  The greater Spring ecosystem  What’s new in Spring 4 CHAPTER 1 Springing into action Although J2EE (now known as JEE) was able to catch up with Spring, Spring never stopped moving forward. Spring has continued to progress in areas where, even now, JEE is just starting to explore or isn’t innovating at all. Mobile development, social API integration, NoSQL databases, cloud computing, and big data are just a few areas where Spring has been and is innovating. And the future continues to look bright for Spring. As I said, it’s a good time to be a Java developer. This book is an exploration of Spring. In this chapter, we’ll examine Spring at a high level, providing you with a taste of what Spring is about. This chapter will give you a good idea of the types of problems Spring solves, and it will set the stage for the rest of the book. 1.1 Simplifying Java development Spring is an open source framework, originally created by Rod Johnson and described in his book Expert One-on-One: J2EE Design and Development (Wrox, 2002, http:// amzn.com/0764543857). Spring was created to address the complexity of enterprise application development and makes it possible to use plain-vanilla JavaBeans to achieve things that were previously only possible with EJB. But Spring’s usefulness isn’t limited to server-side development. Any Java application can benefit from Spring in terms of simplicity, testability, and loose coupling. A bean by any other name… Although Spring uses the words bean and JavaBean lib- erally when referring to application components, this doesn’t mean a Spring compo- nent must follow the JavaBeans specification to the letter. A Spring component can be any type of POJO. In this book, I assume a loose definition of JavaBean, which is synon- ymous with POJO. As you’ll see throughout this book, Spring does many things. But at the root of almost everything Spring provides are a few foundational ideas, all focused on Spring’s fundamental mission: Spring simplifies Java development. That’s a bold statement! A lot of frameworks claim to simplify something or other. But Spring aims to simplify the broad subject of Java development. This begs for more explanation. How does Spring simplify Java development? To back up its attack on Java complexity, Spring employs four key strategies:  Lightweight and minimally invasive development with POJOs  Loose coupling through DI and interface orientation  Declarative programming through aspects and common conventions  Eliminating boilerplate code with aspects and templates Almost everything Spring does can be traced back to one or more of these four strate- gies. Throughout the rest of this chapter, I’ll expand on each of these ideas, showing concrete examples of how Spring makes good on its promise to simplify Java develop- ment. Let’s start with seeing how Spring remains minimally invasive by encouraging POJO-oriented development. 5Simplifying Java development 1.1.1 Unleashing the power of POJOs If you’ve been doing Java development for long, you’ve probably seen (and may have even worked with) frameworks that lock you in by forcing you to extend one of their classes or implement one of their interfaces. The easy-target example of such an inva- sive programming model was EJB 2-era stateless session beans. But even though early EJBs were such an easy target, invasive programming could easily be found in earlier versions of Struts, WebWork, Tapestry, and countless other Java specifications and frameworks. Spring avoids (as much as possible) littering your application code with its API. Spring almost never forces you to implement a Spring-specific interface or extend a Spring-specific class. Instead, the classes in a Spring-based application often have no indication that they’re being used by Spring. At worst, a class may be annotated with one of Spring’s annotations, but it’s otherwise a POJO. To illustrate, consider the HelloWorldBean class shown in the following listing. package com.habuma.spring; public class HelloWorldBean { public String sayHello() { return "Hello World"; } } As you can see, this is a simple, garden-variety Java class—a POJO. Nothing special about it indicates that it’s a Spring component. Spring’s non-invasive programming model means this class could function equally well in a Spring application as it could in a non-Spring application. Despite their simple form, POJOs can be powerful. One of the ways Spring empow- ers POJOs is by assembling them using DI. Let’s see how DI can help keep application objects decoupled from each other. 1.1.2 Injecting dependencies The phrase dependency injection may sound intimidating, conjuring up notions of a complex programming technique or design pattern. But as it turns out, DI isn’t nearly as complex as it sounds. By applying DI in your projects, you’ll find that your code will become significantly simpler, easier to understand, and easier to test. HOW DI WORKS Any nontrivial application (pretty much anything more complex than a Hello World example) is made up of two or more classes that collaborate with each other to per- form some business logic. Traditionally, each object is responsible for obtaining its own references to the objects it collaborates with (its dependencies). This can lead to highly coupled and hard-to-test code. For example, consider the Knight class shown next. Listing 1.1 Spring doesn’t make any unreasonable demands on HelloWorldBean. This is all you need. 6 CHAPTER 1 Springing into action package com.springinaction.knights; public class DamselRescuingKnight implements Knight { private RescueDamselQuest quest; public DamselRescuingKnight() { this.quest = new RescueDamselQuest(); } public void embarkOnQuest() { quest.embark(); } } As you can see, DamselRescuingKnight creates its own quest, a RescueDamselQuest, in the constructor. This makes a DamselRescuingKnight tightly coupled to a Rescue- DamselQuest and severely limits the knight’s quest-embarking repertoire. If a damsel needs rescuing, this knight’s there. But if a dragon needs slaying or a round table needs … well … rounding, then this knight’s going to have to sit it out. What’s more, it’d be terribly difficult to write a unit test for DamselRescuing- Knight. In such a test, you’d like to be able to assert that the quest’s embark() method is called when the knight’s embarkOnQuest() method is called. But there’s no clear way to accomplish that here. Unfortunately, DamselRescuingKnight will remain untested. Coupling is a two-headed beast. On the one hand, tightly coupled code is difficult to test, difficult to reuse, and difficult to understand, and it typically exhibits “whack-a- mole” bug behavior (fixing one bug results in the creation of one or more new bugs). On the other hand, a certain amount of coupling is necessary—completely uncoupled code doesn’t do anything. In order to do anything useful, classes need to know about each other some- how. Coupling is necessary but should be carefully managed. With DI, objects are given their dependencies at creation time by some third party that coordinates each object in the system. Objects aren’t expected to create or obtain their dependencies. As illustrated in figure 1.1, dependencies are injected into the objects that need them. To illustrate this point, let’s look at BraveKnight in the next listing: a knight who’s not only brave, but also capable of embarking on any kind of quest that comes along. Listing 1.2 A DamselRescuingKnight can only embark on RescueDamselQuests. Tightly coupled to RescueDamselQuest Foo Bar Baz Injected into Injected into Figure 1.1 Dependency injection involves giving an object its dependencies as opposed to an object having to acquire those dependencies on its own. 7Simplifying Java development package com.springinaction.knights; public class BraveKnight implements Knight { private Quest quest; public BraveKnight(Quest quest) { this.quest = quest; } public void embarkOnQuest() { quest.embark(); } } As you can see, BraveKnight, unlike DamselRescuingKnight, doesn’t create his own quest. Instead, he’s given a quest at construction time as a constructor argument. This is a type of DI known as constructor injection. What’s more, the quest he’s given is typed as Quest, an interface that all quests imple- ment. So BraveKnight could embark on a RescueDamselQuest, a SlayDragonQuest, a MakeRoundTableRounderQuest, or any other Quest implementation he’s given. The point is that BraveKnight isn’t coupled to any specific implementation of Quest. It doesn’t matter to him what kind of quest he’s asked to embark on, as long as it implements the Quest interface. That’s the key benefit of DI—loose coupling. If an object only knows about its dependencies by their interface (not by their implementa- tion or how they’re instantiated), then the dependency can be swapped out with a dif- ferent implementation without the depending object knowing the difference. One of the most common ways a dependency is swapped out is with a mock imple- mentation during testing. You were unable to adequately test DamselRescuingKnight due to tight coupling, but you can easily test BraveKnight by giving it a mock imple- mentation of Quest, as shown next. package com.springinaction.knights; import static org.mockito.Mockito.*; import org.junit.Test; public class BraveKnightTest { @Test public void knightShouldEmbarkOnQuest() { Quest mockQuest = mock(Quest.class); BraveKnight knight = new BraveKnight(mockQuest); knight.embarkOnQuest(); verify(mockQuest, times(1)).embark(); } } Listing 1.3 A BraveKnight is flexible enough to take on any Quest he’s given. Listing 1.4 To test BraveKnight, inject it with a mock Quest. Quest is injected Create mock Quest Inject mock Quest 8 CHAPTER 1 Springing into action Here you use a mock object framework known as Mockito to create a mock implemen- tation of the Quest interface. With the mock object in hand, you create a new instance of BraveKnight, injecting the mock Quest via the constructor. After calling the embarkOnQuest() method, you ask Mockito to verify that the mock Quest’s embark() method was called exactly once. INJECTING A QUEST INTO A KNIGHT Now that the BraveKnight class is written in such a way that you can give a knight any quest you want, how can you specify which Quest to give him? Suppose, for instance, that you’d like for the BraveKnight to embark on a quest to slay a dragon. Perhaps SlayDragonQuest, shown in the following listing, would be appropriate. package com.springinaction.knights; import java.io.PrintStream; public class SlayDragonQuest implements Quest { private PrintStream stream; public SlayDragonQuest(PrintStream stream) { this.stream = stream; } public void embark() { stream.println("Embarking on quest to slay the dragon!"); } } As you can see, SlayDragonQuest implements the Quest interface, making it a good fit for BraveKnight. You may also notice that rather than lean on System.out .println() like many small getting-started Java samples, SlayDragonQuest more generically asks for a PrintStream through its constructor. The big question here is, how can you give SlayDragonQuest to BraveKnight? And how can you give a Print- Stream to SlayDragonQuest? The act of creating associations between application components is commonly referred to as wiring. In Spring, there are many ways to wire components together, but a common approach has always been via XML. The next listing shows a simple Spring configuration file, knights.xml, that wires a BraveKnight, a SlayDragonQuest, and a PrintStream together. Here, BraveKnight and SlayDragonQuest are declared as beans in Spring. In the case of the BraveKnight bean, it’s constructed, passing a reference to the SlayDragon- Quest bean as a constructor argument. Meanwhile, the SlayDragonQuest bean decla- ration uses the Spring Expression Language to pass System.out (which is a PrintStream) to SlayDragonQuest’s constructor. If XML configuration doesn’t suit your tastes, you might like to know that Spring also allows you to express configuration using Java. For example, here you see a Java- based equivalent to listing 1.6. package com.springinaction.knights.config; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import com.springinaction.knights.BraveKnight; import com.springinaction.knights.Knight; import com.springinaction.knights.Quest; import com.springinaction.knights.SlayDragonQuest; @Configuration public class KnightConfig { @Bean public Knight knight() { return new BraveKnight(quest()); } @Bean public Quest quest() { return new SlayDragonQuest(System.out); } } Whether you use XML-based or Java-based configuration, the benefits of DI are the same. Although BraveKnight depends on a Quest, it doesn’t know what type of Quest it will be given or where that Quest will come from. Likewise, SlayDragonQuest Listing 1.7 Spring offers Java-based configuration as an alternative to XML. Inject quest bean Create SlayDragonQuest 10 CHAPTER 1 Springing into action depends on a PrintStream, but it isn’t coded with knowledge of how that Print- Stream comes to be. Only Spring, through its configuration, knows how all the pieces come together. This makes it possible to change those dependencies with no changes to the depending classes. This example has shown a simple approach to wiring beans in Spring. Don’t con- cern yourself too much with the details right now. We’ll dig more into Spring configu- ration when we get to chapter 2. We’ll also look at other ways that beans can be wired in Spring, including a way to let Spring automatically discover beans and create the relationships between them. Now that you’ve declared the relationship between BraveKnight and a Quest, you need to load the XML configuration file and kick off the application. SEEING IT WORK In a Spring application, an application context loads bean definitions and wires them together. The Spring application context is fully responsible for the creation of and wiring of the objects that make up the application. Spring comes with several imple- mentations of its application context, each primarily differing only in how it loads its configuration. When the beans in knights.xml are declared in an XML file, an appropriate choice for application context might be ClassPathXmlApplicationContext.1 This Spring context implementation loads the Spring context from one or more XML files located in the application’s classpath. The main() method in the following listing uses Class- PathXmlApplicationContext to load knights.xml and to get a reference to the Knight object. package com.springinaction.knights; import org.springframework.context.support. ClassPathXmlApplicationContext; public class KnightMain { public static void main(String[] args) throws Exception { ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext( "META-INF/spring/knight.xml"); Knight knight = context.getBean(Knight.class); knight.embarkOnQuest(); context.close(); } } 1 For Java-based configurations, Spring offers AnnotationConfigApplicationContext. Listing 1.8 KnightMain.java loads the Spring context containing a Knight. Load Spring context Get knight bean Use knight 11Simplifying Java development Here the main() method creates the Spring application context based on the knights.xml file. Then it uses the application context as a factory to retrieve the bean whose ID is knight. With a reference to the Knight object, it calls the embarkOnQuest() method to have the knight embark on the quest he was given. Note that this class knows nothing about which type of Quest your hero has. For that matter, it’s blissfully unaware of the fact that it’s dealing with BraveKnight. Only the knights.xml file knows for sure what the implementations are. And with that you have a quick introduction to dependency injection. You’ll see a lot more DI throughout this book. But if you want even more DI, I encourage you to look at Dhanji R. Prasanna’s Dependency Injection (Manning, 2009, www.manning.com/ prasanna/), which covers DI in fine detail. Now let’s look at another of Spring’s Java-simplifying strategies: declarative pro- gramming through aspects. 1.1.3 Applying aspects Although DI makes it possible to tie software components together loosely, aspect- oriented programming (AOP) enables you to capture functionality that’s used throughout your application in reusable components. AOP is often defined as a technique that promotes separation of concerns in a soft- ware system. Systems are composed of several components, each responsible for a spe- cific piece of functionality. But often these components also carry additional respon- sibilities beyond their core functionality. System services such as logging, transaction management, and security often find their way into components whose core responsi- bilities is something else. These system services are commonly referred to as cross-cut- ting concerns because they tend to cut across multiple components in a system. By spreading these concerns across multiple components, you introduce two levels of complexity to your code:  The code that implements the system-wide concerns is duplicated across multi- ple components. This means that if you need to change how those concerns work, you’ll need to visit multiple components. Even if you’ve abstracted the concern to a separate module so that the impact to your components is a single method call, that method call is duplicated in multiple places.  Your components are littered with code that isn’t aligned with their core func- tionality. A method that adds an entry to an address book should only be con- cerned with how to add the address and not with whether it’s secure or transactional. Figure 1.2 illustrates this complexity. The business objects on the left are too inti- mately involved with the system services on the right. Not only does each object know that it’s being logged, secured, and involved in a transactional context, but each object also is responsible for performing those services for itself. 12 CHAPTER 1 Springing into action AOP makes it possible to modularize these services and then apply them declaratively to the components they should affect. This results in components that are more cohe- sive and that focus on their own specific concerns, completely ignorant of any system services that may be involved. In short, aspects ensure that POJOs remain plain. It may help to think of aspects as blankets that cover many components of an appli- cation, as illustrated in figure 1.3. At its core, an application consists of modules that implement business functionality. With AOP, you can then cover your core application with layers of functionality. These layers can be applied declaratively throughout your application in a flexible manner without your core application even knowing they exist. This is a powerful concept, because it keeps the security, transaction, and log- ging concerns from littering the application’s core business logic. To demonstrate how aspects can be applied in Spring, let’s revisit the knight exam- ple, adding a basic Spring aspect to the mix. Figure 1.2 Calls to system-wide concerns such as logging and security are often scattered about in modules where those tasks are not their primary concern. Student service Billing service Course service Instructor service Content service Logging module Security module Transaction manager Figure 1.3 Using AOP, system-wide concerns blanket the components they impact. This leaves the application components to focus on their specific business functionality. Secur ity modul e Logging m odule Transaction manager Student service Instructor service Course service Billing service Content service 13Simplifying Java development AOP IN ACTION Anyone who knows anything about knights only knows about them because their deeds were chronicled in song by the musically inclined storytellers known as min- strels. Let’s suppose that you want to record the comings and goings of your BraveK- night using the services of a minstrel. The following listing shows the Minstrel class you might use. package com.springinaction.knights; import java.io.PrintStream; public class Minstrel { private PrintStream stream; public Minstrel(PrintStream stream) { this.stream = stream; } public void singBeforeQuest() { stream.println("Fa la la, the knight is so brave!"); } public void singAfterQuest() { stream.println("Tee hee hee, the brave knight " + "did embark on a quest!"); } } As you can see, Minstrel is a simple class with two methods. The singBeforeQuest() method is intended to be invoked before a knight embarks on a quest, and the sing- AfterQuest() method should be invoked after the knight has completed a quest. In both cases, the Minstrel sings of the knight’s deeds via a PrintStream injected through its constructor. It should be simple to work this into your code—you can just inject it into Brave- Knight, right? Let’s make the appropriate tweaks to BraveKnight to use Minstrel. The next listing shows a first attempt at bringing BraveKnight and Minstrel together. package com.springinaction.knights; public class BraveKnight implements Knight { private Quest quest; private Minstrel minstrel; public BraveKnight(Quest quest, Minstrel minstrel) { Listing 1.9 A Minstrel is a musically inclined logging system from medieval times. Listing 1.10 A BraveKnight that must call Minstrel methods Called before quest Called after quest 14 CHAPTER 1 Springing into action this.quest = quest; this.minstrel = minstrel; } public void embarkOnQuest() throws QuestException { minstrel.singBeforeQuest(); quest.embark(); minstrel.singAfterQuest(); } } That should do the trick. Now all you need to do is go back to your Spring configura- tion to declare a Minstrel bean and inject it into the BraveKnight bean’s constructor. But hold on… Something doesn’t seem right. Is it really within the knight’s range of concern to manage his minstrel? It seems to me that minstrels should just do their job without having to be asked to do so. After all, that’s a minstrel’s job—to sing about the knight’s endeavors. Why should the knight have to keep reminding the minstrel? Furthermore, because the knight needs to know about the minstrel, you’re forced to inject Minstrel into BraveKnight. This not only complicates the BraveKnight code but also makes me wonder if you’d ever want a knight who didn’t have a minstrel. What if Minstrel is null? Should you introduce some null-checking logic to cover that case? Your simple BraveKnight class is starting to get more complicated and would become more so if you were to handle the nullMinstrel scenario. But using AOP, you can declare that the minstrel should sing about a knight’s quests and free the knight from having to deal with the Minstrel methods directly. To turn Minstrel into an aspect, all you need to do is declare it as one in the Spring configuration file. Here’s the updated knights.xml file, revised to declare Minstrel as an aspect. > Listing 1.11 Declaring the Minstrel as an aspect Should a knight manage his own minstrel? 15Simplifying Java development Here you’re using Spring’s aop configuration namespace to declare that the Minstrel bean is an aspect. First you declare Minstrel as a bean. Then you refer to that bean in the element. Defining the aspect further, you declare (using ) that before the embarkOnQuest() method is executed, the Minstrel’s singBefore- Quest() should be called. This is called before advice. And you (using ) declare that the singAfterQuest() method should be called after embarkOnQuest() has executed. This is known as after advice. In both cases, the pointcut-ref attribute refers to a pointcut named embark. This pointcut is defined in the preceding element with an expression attri- bute set to select where the advice should be applied. The expression syntax is AspectJ’s pointcut expression language. Don’t worry if you don’t know AspectJ or the details of how AspectJ pointcut expressions are written. We’ll talk more about Spring AOP later, in chapter 4. For now it’s enough to know that you’ve asked Spring to call Minstrel’s singBeforeQuest() and singAfterQuest() methods before and after BraveKnight embarks on a quest. That’s all there is to it! With a tiny bit of XML, you’ve turned Minstrel into a Spring aspect. Don’t worry if this doesn’t make complete sense yet—you’ll see plenty more examples of Spring AOP in chapter 4 that should help clear this up. For now, there are two important points to take away from this example. First, Minstrel is still a POJO—nothing about it indicates that it’s to be used as an aspect. Instead, Minstrel became an aspect when you declared it as such in the Spring context. Second, and most important, Minstrel can be applied to BraveKnight without BraveKnight needing to explicitly call on it. In fact, BraveKnight remains completely unaware of Minstrel’s existence. I should also point out that although you used some Spring magic to turn Minstrel into an aspect, it was declared as a Spring first. The point is that you Declare Minstrel bean Define pointcut Declare before advice Declare after advice 16 CHAPTER 1 Springing into action can do anything with Spring aspects that you can do with other Spring beans, such as inject them with dependencies. Using aspects to sing about knights can be fun. But Spring’s AOP can be used for even more practical things. As you’ll see later, Spring AOP can be employed to pro- vide services such as declarative transactions and security (chapters 9 and 14). But for now, let’s look at one more way that Spring simplifies Java development. 1.1.4 Eliminating boilerplate code with templates Have you ever written some code and then felt like you’d already written the same code before? That’s not déjà vu, my friend. That’s boilerplate code—the code that you often have to write over and over again to accomplish common and otherwise simple tasks. Unfortunately, there are a lot of places where Java APIs involve a bunch of boiler- plate code. A common example of boilerplate code can be seen when working with JDBC to query data from a database. If you’ve ever worked with JDBC, you’ve probably written something similar to the following. public Employee getEmployeeById(long id) { Connection conn = null; PreparedStatement stmt = null; ResultSet rs = null; try { conn = dataSource.getConnection(); stmt = conn.prepareStatement( "select id, firstname, lastname, salary from " + "employee where id=?"); stmt.setLong(1, id); rs = stmt.executeQuery(); Employee employee = null; if (rs.next()) { employee = new Employee(); employee.setId(rs.getLong("id")); employee.setFirstName(rs.getString("firstname")); employee.setLastName(rs.getString("lastname")); employee.setSalary(rs.getBigDecimal("salary")); } return employee; } catch (SQLException e) { } finally { if(rs != null) { try { rs.close(); } catch(SQLException e) {} } if(stmt != null) { try { stmt.close(); } catch(SQLException e) {} } Listing 1.12 Many Java APIs, such as JDBC, involve writing boilerplate code. Select employee Create object from data What should be done here? Clean up mess 17Simplifying Java development if(conn != null) { try { conn.close(); } catch(SQLException e) {} } } return null; } As you can see, this JDBC code queries the database for an employee’s name and sal- ary. But I’ll bet you had to look hard to see that. That’s because the small bit of code that’s specific to querying for an employee is buried in a heap of JDBC ceremony. You first have to create a connection, then create a statement, and finally query for the results. And, to appease JDBC’s anger, you must catch SQLException, a checked excep- tion, even though there’s not a lot you can do if it’s thrown. Finally, after all is said and done, you have to clean up the mess, closing down the connection, statement, and result set. This could also stir JDBC’s anger, so you must catch SQLException here as well. What’s most notable about listing 1.12 is that much of it is the exact same code you’d write for pretty much any JDBC operation. Little of it has anything to do with querying for an employee, and much of it is JDBC boilerplate. JDBC is not alone in the boilerplate code business. Many activities require similar boilerplate code. JMS, JNDI, and the consumption of REST services often involve a lot of commonly repeated code. Spring seeks to eliminate boilerplate code by encapsulating it in templates. Spring’s JdbcTemplate makes it possible to perform database operations without all the ceremony required by traditional JDBC. For example, using Spring’s SimpleJdbcTemplate (a specialization of Jdbc- Template that takes advantage of Java 5 features), the getEmployeeById() method can be rewritten so that its focus is on the task of retrieving employee data and not catering to the demands of the JDBC API. The following shows what such an updated getEmployeeById() method might look like. public Employee getEmployeeById(long id) { return jdbcTemplate.queryForObject( "select id, firstname, lastname, salary " + "from employee where id=?", new RowMapper() { public Employee mapRow(ResultSet rs, int rowNum) throws SQLException { Employee employee = new Employee(); employee.setId(rs.getLong("id")); employee.setFirstName(rs.getString("firstname")); employee.setLastName(rs.getString("lastname")); employee.setSalary(rs.getBigDecimal("salary")); return employee; Listing 1.13 Templates let your code focus on the task at hand. SQL query Map results to object 18 CHAPTER 1 Springing into action } }, id); } As you can see, this new version of getEmployeeById() is much simpler and acutely focused on selecting an employee from the database. The template’s queryFor- Object() method is given the SQL query, a RowMapper (for mapping result set data to a domain object), and zero or more query parameters. What you don’t see in get- EmployeeById() is any of the JDBC boilerplate from before. Everything is handled inside the template. I’ve shown you how Spring attacks complexity in Java development using POJO- oriented development, DI, aspects, and templates. Along the way, I showed you how to configure beans and aspects in XML-based configuration files. But how do those files get loaded? And what are they loaded into? Let’s look at the Spring container, the place where your application’s beans reside. 1.2 Containing your beans In a Spring-based application, your appli- cation objects live in the Spring container. As illustrated in figure 1.4, the container creates the objects, wires them together, configures them, and manages their com- plete lifecycle from cradle to grave (or new to finalize(), as the case may be). In the next chapter, you’ll see how to configure Spring so it knows what objects it should create, configure, and wire together. First, though, it’s important to get to know the container where your objects will be hanging out. Understand- ing the container will help you grasp how your objects will be managed. The container is at the core of the Spring Framework. Spring’s container uses DI to manage the components that make up an application. This includes creating associa- tions between collaborating components. As such, these objects are cleaner and easier to understand, they support reuse, and they’re easy to unit test. There’s no single Spring container. Spring comes with several container imple- mentations that can be categorized into two distinct types. Bean factories (defined by the org.springframework.beans.factory.BeanFactory interface) are the simplest of containers, providing basic support for DI. Application contexts (defined by the org.springframework.context.ApplicationContext interface) build on the notion of a bean factory by providing application-framework services, such as the ability to Specify query parameter Spring container Figure 1.4 In a Spring application, objects are created, are wired together, and live in the Spring container. 19Containing your beans resolve textual messages from a properties file and the ability to publish application events to interested event listeners. Although it’s possible to work with Spring using either bean factories or applica- tion contexts, bean factories are often too low-level for most applications. Therefore, application contexts are preferred over bean factories. We’ll focus on working with application contexts and not spend any more time talking about bean factories. 1.2.1 Working with an application context Spring comes with several flavors of application context. Here are a few that you’ll most likely encounter:  AnnotationConfigApplicationContext—Loads a Spring application context from one or more Java-based configuration classes  AnnotationConfigWebApplicationContext—Loads a Spring web application context from one or more Java-based configuration classes  ClassPathXmlApplicationContext—Loads a context definition from one or more XML files located in the classpath, treating context-definition files as class- path resources  FileSystemXmlApplicationContext—Loads a context definition from one or more XML files in the filesystem  XmlWebApplicationContext—Loads context definitions from one or more XML files contained in a web application We’ll talk more about AnnotationConfigWebApplicationContext and XmlWeb- ApplicationContext in chapter 8 when we discuss web-based Spring applications. For now, let’s load the application context from the filesystem using FileSystemXml- ApplicationContext or from the classpath using ClassPathXmlApplicationContext. Loading an application context from the filesystem or from the classpath is similar to how you load beans into a bean factory. For example, here’s how you’d load a File- SystemXmlApplicationContext: ApplicationContext context = new FileSystemXmlApplicationContext("c:/knight.xml"); Similarly, you can load an application context from the application’s classpath using ClassPathXmlApplicationContext: ApplicationContext context = new ClassPathXmlApplicationContext("knight.xml"); The difference between using FileSystemXmlApplicationContext and ClassPathXml- ApplicationContext is that FileSystemXmlApplicationContext looks for knight.xml in a specific location within the filesystem, whereas ClassPathXmlApplicationContext looks for knight.xml anywhere in the classpath (including JAR files). 20 CHAPTER 1 Springing into action Alternatively, if you’d rather load your application context from a Java configura- tion, you can use AnnotationConfigApplicationContext: ApplicationContext context = new AnnotationConfigApplicationContext( com.springinaction.knights.config.KnightConfig.class); Instead of specifying an XML file from which to load the Spring application context, AnnotationConfigApplicationContext has been given a configuration class from which to load beans. With an application context in hand, you can retrieve beans from the Spring con- tainer by calling the context’s getBean() method. Now that you know the basics of how to create a Spring container, let’s take a closer look at the lifecycle of a bean in the bean container. 1.2.2 A bean’s life In a traditional Java application, the lifecycle of a bean is simple. Java’s new keyword is used to instantiate the bean, and it’s ready to use. Once the bean is no longer in use, it’s eligible for garbage collection and eventually goes to the big bit bucket in the sky. In contrast, the lifecycle of a bean in a Spring container is more elaborate. It’s important to understand the lifecycle of a Spring bean, because you may want to take advantage of some of the opportunities that Spring offers to customize how a bean is created. Figure 1.5 shows the startup lifecycle of a typical bean as it’s loaded into a Spring application context. Instantiate Bean is ready to use Populate properties BeanNameAware’s setBeanName() BeanFactoryAware’s setBeanFactory() Pre-initialization BeanPostProcessors Post-initialization BeanPostProcessors InitializingBean ’s afterPropertiesSet() Call custom init-method Container is shutdown DisposableBean’s destroy() Call custom destroy-method ApplicationContextAware’s setApplicationContext() Figure 1.5 A bean goes through several steps between creation and destruction in the Spring container. Each step is an opportunity to customize how the bean is managed in Spring. 21Surveying the Spring landscape As you can see, a bean factory performs several setup steps before a bean is ready to use. Let’s break down figure 1.5 in more detail: 1 Spring instantiates the bean. 2 Spring injects values and bean references into the bean’s properties. 3 If the bean implements BeanNameAware, Spring passes the bean’s ID to the set- BeanName() method. 4 If the bean implements BeanFactoryAware, Spring calls the setBeanFactory() method, passing in the bean factory itself. 5 If the bean implements ApplicationContextAware, Spring calls the set- ApplicationContext() method, passing in a reference to the enclosing appli- cation context. 6 If the bean implements the BeanPostProcessor interface, Spring calls its post- ProcessBeforeInitialization() method. 7 If the bean implements the InitializingBean interface, Spring calls its after- PropertiesSet() method. Similarly, if the bean was declared with an init- method, then the specified initialization method is called. 8 If the bean implements BeanPostProcessor, Spring calls its postProcess- AfterInitialization() method. 9 At this point, the bean is ready to be used by the application and remains in the application context until the application context is destroyed. 10 If the bean implements the DisposableBean interface, Spring calls its destroy() method. Likewise, if the bean was declared with a destroy-method, the specified method is called. Now you know how to create and load a Spring container. But an empty container isn’t much good by itself; it doesn’t contain anything unless you put something in it. To achieve the benefits of Spring DI, you must wire your application objects into the Spring container. We’ll go into bean wiring in more detail in chapter 2. First, let’s survey the modern Spring landscape to see what the Spring Framework is made up of and what the latest versions of Spring have to offer. 1.3 Surveying the Spring landscape As you’ve seen, the Spring Framework is focused on simplifying enterprise Java devel- opment through DI, AOP, and boilerplate reduction. Even if that were all Spring did, it’d be worth using. But there’s more to Spring than meets the eye. Within the Spring Framework proper, you’ll find several ways that Spring can ease Java development. But beyond the Spring Framework is a greater ecosystem of proj- ects that build on the core framework, extending Spring into areas such as web ser- vices, REST, mobile, and NoSQL. Let’s first break down the core Spring Framework to see what it brings to the table. Then we’ll expand our sights to review the other members of the greater Spring portfolio. 22 CHAPTER 1 Springing into action 1.3.1 Spring modules When you download the Spring distribu- tion and dig into its libs folder, you’ll find several JAR files. As of Spring 4.0, there are 20 distinct modules in the Spring Framework distribution, with three JAR files for each module (the binary class library, the source JAR file, and a JavaDoc JAR file). The complete list of library JAR files is shown in figure 1.6. These modules can be arranged into six categories of functionality, as illus- trated in figure 1.7. Taken as a whole, these modules give you everything you need to develop enterprise-ready applications. But you don’t have to base your application fully on the Spring Framework. You’re free to choose the modules that suit your appli- cation and look to other options when Data access & integration JDBC OXM Transaction ORM JMS Web and remoting Web Web servlet Web portlet WebSocket Aspect-oriented programming AOP Aspects Instrumentation Instrument TomcatInstrument Core Spring container Beans Core Context Expression Context support Testing Test Messaging Figure 1.7 The Spring Framework is made up of six well-defined module categories. Figure 1.6 Spring 4.0 is made up of 20 distinct modules. 23Surveying the Spring landscape Spring doesn’t fit the bill. Spring even offers integration points with several other frameworks and libraries so that you don’t have to write them yourself. Let’s look at each of Spring’s modules, one at a time, to see how each fits in the overall Spring picture. CORE SPRING CONTAINER The centerpiece of the Spring Framework is a container that manages how the beans in a Spring-enabled application are created, configured, and managed. In this mod- ule is the Spring bean factory, which is the portion of Spring that provides DI. Build- ing on the bean factory, you’ll find several implementations of Spring’s application context, each of which provides a different way to configure Spring. In addition to the bean factory and application context, this module also supplies many enterprise services such as email, JNDI access, EJB integration, and scheduling. All of Spring’s modules are built on top of the core container. You’ll implicitly use these classes when you configure your application. We’ll discuss the core module throughout this book, starting in chapter 2 where we’ll dig deep into Spring DI. SPRING’S AOP MODULE Spring provides rich support for aspect-oriented programming in its AOP module. This module serves as the basis for developing your own aspects for your Spring- enabled application. Like DI, AOP supports loose coupling of application objects. But with AOP, application-wide concerns (such as transactions and security) are decoupled from the objects to which they’re applied. We’ll dig into Spring’s AOP support in chapter 4. DATA ACCESS AND INTEGRATION Working with JDBC often results in a lot of boilerplate code that gets a connection, creates a statement, processes a result set, and then closes the connection. Spring’s JDBC and data-access objects (DAO) module abstracts away the boilerplate code so that you can keep your database code clean and simple, and prevents problems that result from a failure to close database resources. This module also builds a layer of meaning- ful exceptions on top of the error messages given by several database servers. No more trying to decipher cryptic and proprietary SQL error messages! For those who prefer using an object-relational mapping (ORM) tool over straight JDBC, Spring provides the ORM module. Spring’s ORM support builds on the DAO sup- port, providing a convenient way to build DAOs for several ORM solutions. Spring doesn’t attempt to implement its own ORM solution but does provide hooks into sev- eral popular ORM frameworks, including Hibernate, Java Persistence API, Java Data Objects, and iBATIS SQL Maps. Spring’s transaction management supports each of these ORM frameworks as well as JDBC. You’ll see how Spring’s template-based JDBC abstraction can greatly simplify JDBC code when we look at Spring data access in chapter 10. This module also includes a Spring abstraction over the Java Message Service (JMS) for asynchronous integration with other applications through messaging. And, as of 24 CHAPTER 1 Springing into action Spring 3.0, this module includes the object-to-XML mapping features that were origi- nally part of the Spring Web Services project. In addition, this module uses Spring’s AOP module to provide transaction- management services for objects in a Spring application. WEB AND REMOTING The Model-View-Controller (MVC) paradigm is a commonly accepted approach to build- ing web applications such that the user interface is separate from the application logic. Java has no shortage of MVC frameworks, with Apache Struts, JSF, WebWork, and Tapestry being among the most popular MVC choices. Even though Spring integrates with several popular MVC frameworks, its web and remoting module comes with a capable MVC framework that promotes Spring’s loosely coupled techniques in the web layer of an application. We’ll look at Spring’s MVC framework in chapters 5–7. In addition to user-facing web applications, this module also provides several remoting options for building applications that interact with other applications. Spring’s remoting capabilities include Remote Method Invocation (RMI), Hessian, Bur- lap, JAX-WS, and Spring’s own HTTP invoker. Spring also offers first-class support for exposing and consuming REST APIs. In chapter 15, we’ll check out Spring remoting. And you’ll learn how to create and consume REST APIs in chapter 16. INSTRUMENTATION Spring’s instrumentation module includes support for adding agents to the JVM. Spe- cifically, it provides a weaving agent for Tomcat that transforms class files as they’re loaded by the classloader. If that sounds like a lot to understand, don’t worry too much about it. The instru- mentation provided by this module has a narrow set of use cases and we won’t be deal- ing with this module at all in this book. TESTING Recognizing the importance of developer-written tests, Spring provides a module ded- icated to testing Spring applications. In this module you’ll find a collection of mock object implementations for writing unit tests against code that works with JNDI, servlets, and portlets. For integration-level testing, this module provides support for loading a collection of beans in a Spring application context and working with the beans in that context. Throughout this book, many of the examples will be driven by tests, utilizing the testing facilities offered by Spring. 1.3.2 The Spring portfolio When it comes to Spring, there’s more than meets the eye. In fact, there’s more than what comes in the Spring Framework download. If you stop at just the core Spring Framework, you’ll miss out on a wealth of potential afforded by the larger Spring 25Surveying the Spring landscape portfolio. The whole Spring portfolio includes several frameworks and libraries that build on the core Spring Framework and on each other. All together, the entire Spring portfolio brings the Spring programming model to almost every facet of Java development. It would take several volumes to cover everything the Spring portfolio has to offer, and much of it is outside the scope of this book. But we’ll look at some of the elements of the Spring portfolio; here’s a taste of what lies beyond the core Spring Framework. SPRING WEB FLOW Spring Web Flow builds on Spring’s core MVC framework to provide support for build- ing conversational, flow-based web applications that guide users toward a goal (think wizards or shopping carts). We’ll talk more about Spring Web Flow in chapter 8, and you can learn more about it at http://projects.spring.io/spring-webflow/. SPRING WEB SERVICES Although the core Spring Framework provides for declaratively publishing Spring beans as web services, those services are based on an arguably architecturally inferior contract-last model. The contract for the service is determined from the bean’s inter- face. Spring Web Services offers a contract-first web services model where service implementations are written to satisfy the service contract. I won’t be talking about Spring-WS in this book, but you can read more about it at http://docs.spring.io/spring-ws/site/. SPRING SECURITY Security is a critical aspect of many applications. Implemented using Spring AOP, Spring Security offers a declarative security mechanism for Spring-based applications. You’ll see how to add Spring Security to an application’s web layer in chapter 9. We’ll return to Spring Security again in chapter 14 to examine how to secure method invocations. For further exploration, Spring Security’s home page is at http:// projects.spring.io/spring-security/. SPRING INTEGRATION Many enterprise applications must interact with other enterprise applications. Spring Integration offers implementations of several common integration patterns in Spring’s declarative style. We won’t cover Spring Integration in this book, but if you want more information, look at Spring Integration in Action by Mark Fisher, Jonas Partner, Marius Bogoevici, and Iwein Fuld (Manning, 2012, www.manning.com/fisher/). Or you can visit the Spring Integration home page at http://projects.spring.io/spring-integration/. SPRING BATCH When it’s necessary to perform bulk operations on data, nothing beats batch process- ing. If you’re going to be developing a batch application, you can use Spring’s robust, POJO-oriented development model to do it using Spring Batch. 26 CHAPTER 1 Springing into action Spring Batch is beyond the scope of this book, but Arnaud Cogoluegnes, Thierry Templier, Gary Gregory, and Olivier Bazoud will enlighten you in their book, Spring Batch in Action (Manning, 2011, www.manning.com/templier/). You can also learn about Spring Batch from its home page at http://projects.spring.io/spring-batch/. SPRING DATA Spring Data makes it easy to work with all kinds of databases in Spring. Although the relational database has been ubiquitous in enterprise applications for many years, mod- ern applications are recognizing that not all data is best served by columns and rows in a table. A new breed of databases, commonly referred to as NoSQL databases,2 offer new ways of working with data that are more fitting than the traditional relational database. Whether you’re using a document database like MongoDB, a graph database such as Neo4j, or even a traditional relational database, Spring Data offers a simplified pro- gramming model for persistence. This includes, for many database types, an auto- matic repository mechanism that creates repository implementations for you. We’ll look at using Spring Data to simplify Java Persistence API (JPA) development in chapter 11 and then expand the discussion to include a few NoSQL databases in chapter 12. SPRING SOCIAL Social networking is a rising trend on the internet, and more and more applications are being outfitted with integration into social networking sites such as Facebook and Twitter. If this is the kind of thing that interests you, you’ll want to look at Spring Social, a social networking extension to Spring. But Spring Social is about more than just tweets and friends. Despite its name, Spring Social is less about the word social and more about the word connect. It helps you connect your Spring application with REST APIs, including many that may not have any social purpose to them. Due to space constraints, we won’t cover Spring Social in this book. But if you’re interested in how Spring can help you connect with Facebook or Twitter, have a look at the Getting Started guides at https://spring.io/guides/gs/accessing-facebook/ and https://spring.io/guides/gs/accessing-twitter/. SPRING MOBILE Mobile applications are another significant area of software development. Smart- phones and tablet devices are taking over as the preferred client for many users. Spring Mobile is a new extension to Spring MVC to support development of mobile web applications. SPRING FOR ANDROID Related to Spring Mobile is the Spring Android project. This project aims to bring some of the simplicity afforded by the Spring Framework to development of native 2 I prefer the term non-relational or schema-less over NoSQL. Calling these databases NoSQL places the blame on the query language and not the database model. 27What’s new in Spring applications for Android-based devices. Initially, this project is offering a version of Spring’s RestTemplate that can be used in an Android application. It also works with Spring Social to enable native Android apps to connect with REST APIs. I won’t discuss Spring for Android in this book, but you can learn more about it at http://projects.spring.io/spring-android/. SPRING BOOT Spring greatly simplifies many programming tasks, reducing or even eliminating much of the boilerplate code you might normally be required to write without it. Spring Boot is an exciting new project that takes an opinionated view of developing with Spring to simplify Spring itself. Spring Boot heavily employs automatic configuration techniques that can elimi- nate most (and in many cases, all) Spring configuration. It also provides several starter projects to help reduce the size of your Spring project build files, whether you’re using Maven or Gradle. We’ll look at Spring Boot near the end of the book in chapter 21. 1.4 What’s new in Spring When the third edition of this book went to press, the latest version of Spring was ver- sion 3.0.5. That was around three years ago, and a lot has changed since then. The Spring Framework has seen three significant releases—3.1, 3.2, and now 4.0—each bringing new features and improvements to ease application development. And sev- eral of the other members of the Spring portfolio have undergone major changes. This edition of Spring in Action has been updated to cover many of the most excit- ing and useful features in these releases. But for now, let’s briefly size up what’s new in Spring. 1.4.1 What was new in Spring 3.1? Spring 3.1 had several useful new features and improvements, many of which were focused on simplifying and improving configuration. In addition, Spring 3.1 provided declarative caching support as well as many improvements to Spring MVC. Here’s a brief list of some of the highlights of Spring 3.1:  To address the common issue of selecting distinct configurations for various environments (such as development, test, and production), Spring 3.1 intro- duced environment profiles. Profiles make it possible, for instance, to select a different data source bean depending on which environment the application is deployed in.  Building on Spring 3.0’s Java-based configuration, Spring 3.1 added several enable annotations to switch on certain features of Spring with a single annotation.  Declarative caching support made its way into Spring, making it possible to declare caching boundaries and rules with simple annotations, similar to how you could already declare transaction boundaries. 28 CHAPTER 1 Springing into action  A new c namespace brought constructor injection the same succinct attribute- oriented style as Spring 2.0’s p namespace brought to property injection.  Spring began to support Servlet 3.0, including the ability to declare servlets and filters in Java-based configuration instead of web.xml.  Improvements to Spring’s JPA support made it possible to completely configure JPA in Spring without needing a persistence.xml file. Spring 3.1 also included several enhancements to Spring MVC:  Automatic binding of path variables to model attributes  @RequestMappingproduces and consumes attributes, for matching against a request’s Accept and Content-Type headers  A @RequestPart annotation that enables binding parts of a multipart request to handler method parameters  Support for flash attributes (attributes that survive a redirect) and a Redirect- Attributes type to carry the flash attributes between requests Just as important as what was new in Spring 3.1 is what was no longer available in Spring as of Spring 3.1. Specifically, Spring’s JpaTemplate and JpaDaoSupport classes were deprecated in favor of native EntityManager usage. Even though they were dep- recated, they were still around in Spring 3.2. But you shouldn’t use them, because they weren’t upgraded to support JPA 2.0 and have been removed in Spring 4. Now let’s look at what was new in Spring 3.2. 1.4.2 What was new in Spring 3.2? Whereas Spring 3.1 was largely focused on configuration improvements with a small set of other enhancements, including Spring MVC enhancements, Spring 3.2 was primarily a Spring MVC-focused release. Spring MVC 3.2 boasted the following improvements:  Spring 3.2 controllers can take advantage of Servlet 3’s asynchronous requests to spin off request processing in separate threads, freeing up the servlet thread to process more requests.  Although Spring MVC controllers have been easily testable as POJOs since Spring 2.5, Spring 3.2 included a Spring MVC test framework for writing richer tests against controllers, asserting their behavior as controllers, but without a servlet container.  In addition to improved controller testing, Spring 3.2 included support for testing RestTemplate-based clients without sending requests to the real REST endpoint.  An @ControllerAdvice annotation enables common @ExceptionHandler, @InitBinder, and @ModelAttributes methods to be collected in a single class and applied to all controllers.  Prior to Spring 3.2, full content negotiation support was only available via ContentNegotiatingViewResolver. But in Spring 3.2, full content negotiation 29What’s new in Spring became available throughout Spring MVC, even on controller methods relying on message converters for content consumption and production.  Spring MVC 3.2 included a new @MatrixVariable annotation for binding a request’s matrix variables to handler method parameters.  The abstract base class AbstractDispatcherServletInitializer can be used for conveniently configuring DispatcherServlet without web.xml. Likewise, a subclass named AbstractAnnotationConfigDispatcherServletInitializer can be used when you wish to configure Spring with Java-based configuration.  The ResponseEntityExceptionHandler class was added to be used as an alter- native to DefaultHandlerExceptionResolver. ResponseEntityException- Handler methods return ResponseEntity instead of ModelAndView.  RestTemplate and @RequestBody arguments support generic types.  RestTemplate and @RequestMapping methods support the HTTP PATCH method.  Mapped interceptors support URL patterns to be excluded from interceptor processing. Although Spring MVC was the main story of Spring 3.2, a few other non-MVC improve- ments were added as well. Here are a few of the most interesting new features in Spring 3.2:  @Autowired, @Value, and @Bean annotations can be used as meta-annotations to create custom injection and bean-declaration annotations.  The @DateTimeFormat annotation no longer has a hard dependency on Joda- Time. If JodaTime is present, it is used. Otherwise, SimpleDateFormat is used.  Spring’s declarative caching support has initial support for JCache 0.5.  You can define global formats for parsing and rendering dates and times.  Integration tests can configure and load a WebApplicationContext.  Integration tests can test against request- and session-scoped beans. You’ll see a lot of Spring 3.2’s features across several chapters in this book, especially in the web and REST chapters. 1.4.3 What’s new in Spring 4.0? Spring 4.0 is the freshest release of Spring available. There are a lot of exciting new features in Spring 4.0, including the following:  Spring now includes support for WebSocket programming, including support for JSR-356: Java API for WebSocket.  Recognizing that WebSocket offers a low-level API, screaming for a higher-level abstraction, Spring 4.0 includes a higher level message-oriented programming model on top of WebSocket that’s based on SockJS and includes STOMP sub- protocol support. 30 CHAPTER 1 Springing into action  A new messaging module with many types carried over from the Spring Integra- tion project. This messaging module supports Spring’s SockJS/STOMP support. It also includes template-based support for publishing messages.  Spring 4.0 is one of the first (if not the first) Java frameworks to support Java 8 features, including lambdas. Among other things, this makes working with cer- tain callback interfaces (such as RowMapper with JdbcTemplate) much cleaner and easier to read.  Along with Java 8 support comes support for JSR-310: Data and Time API, offer- ing the opportunity for developers to work with dates and times in a richer API than that offered with java.util.Date or java.util.Calendar.  A smooth programming experience for applications developed in Groovy has also been added, essentially enabling a Spring application to be developed eas- ily entirely in Groovy. With this comes the BeanBuilder from Grails, enabling Spring applications to be configured with Groovy.  Generalized support for conditional bean creation has been added, wherein beans can be declared to be created only if a developer-defined condition is met.  Spring 4.0 also includes a new asynchronous implementation of Spring’s Rest- Template that returns immediately but allows for callbacks once the operation completes.  Support for many JEE specs has been added, including JMS 2.0, JTA 1.2, JPA 2.1, and Bean Validation 1.1. As you can see, a lot of exciting new stuff has found its way into the latest versions of the Spring Framework. Throughout this book, we’ll look at many of these new fea- tures as well as many of the long-standing features of Spring. 1.5 Summary You should now have a good idea of what Spring brings to the table. Spring aims to make enterprise Java development easier and to promote loosely coupled code. Vital to this are dependency injection and aspect-oriented programming. In this chapter, you got a taste of DI in Spring. DI is a way of associating application objects such that the objects don’t need to know where their dependencies come from or how they’re implemented. Rather than acquiring dependencies on their own, dependent objects are given the objects that they depend on. Because dependent objects often only know about their injected objects through interfaces, coupling is kept low. In addition to DI, you also saw a glimpse of Spring’s AOP support. AOP enables you to centralize in one place—an aspect—logic that would normally be scattered throughout an application. When Spring wires your beans together, these aspects can be woven in at runtime, effectively giving the beans new behavior. 31Summary DI and AOP are central to everything in Spring. Thus you must understand how to use these principal functions of Spring to be able to use the rest of the framework. In this chapter, we’ve just scratched the surface of Spring’s DI and AOP features. Over the next few chapters, we’ll dig deeper into DI and AOP. Without further ado, let’s move on to chapter 2 to learn how to wire objects together in Spring using DI. 32 Wiring beans Have you ever stuck around long enough after a movie to watch the credits? It’s incredible how many different people it takes to pull together a major motion pic- ture. In addition to the obvious participants—the actors, scriptwriters, directors, and producers—there are the not-so-obvious—the musicians, special effects crew, and art directors. And that’s not to mention the key grip, sound mixer, costumers, makeup artists, stunt coordinators, publicists, first assistant to the cameraperson, second assistant to the cameraperson, set designers, gaffer, and (perhaps most important) caterers. Now imagine what your favorite movie would’ve been like had none of these people talked to one another. Let’s say that they all showed up at the studio and started doing their own thing without any coordination of any kind. If the director keeps to himself and doesn’t say “Roll ’em,” then the cameraperson wouldn’t start shooting. It probably wouldn’t matter anyway, because the lead actress would still be in her trailer and the lighting wouldn’t work because the gaffer wouldn’t have been hired. Maybe you’ve seen a movie where it looks like this is what happened. This chapter covers  Declaring beans  Injecting constructors and setters  Wiring beans  Controlling bean creation and destruction 33Exploring Spring’s configuration options But most movies (the good ones, anyway) are the product of thousands of people working together toward the common goal of making a blockbuster film. In this respect, a great piece of software isn’t much different. Any nontrivial appli- cation is made up of several objects that must work together to meet some business goal. These objects must be aware of one another and communicate with one another to get their jobs done. In an online shopping application, for instance, an order- manager component may need to work with a product-manager component and a credit-card authorization component. All of these will likely need to work with a data- access component to read from and write to a database. But as you saw in chapter 1, the traditional approach to creating associations between application objects (via construction or lookup) leads to complicated code that’s difficult to reuse and unit-test. At best, these objects do more work than they should. At worst, they’re highly coupled to one another, making them hard to reuse and hard to test. In Spring, objects aren’t responsible for finding or creating the other objects that they need to do their jobs. Instead, the container gives them references to the objects that they collaborate with. An order-manager component, for example, may need a credit-card authorizer—but it doesn’t have to create the credit-card authorizer. It just needs to show up empty-handed, and it’s given a credit-card authorizer to work with. The act of creating these associations between application objects is the essence of dependency injection (DI) and is commonly referred to as wiring. In this chapter, we’ll explore the basics of bean wiring using Spring. DI is the most elemental thing Spring does, so these are techniques you’ll use almost every time you develop Spring-based applications. There are many ways to wire beans in Spring. To begin, let’s take a moment to get a feel for the three most common approaches for configuring the Spring container. 2.1 Exploring Spring’s configuration options As mentioned in chapter 1, the Spring container is responsible for creating the beans in your application and coordinating the relationships between those objects via DI. But it’s your responsibility as a developer to tell Spring which beans to create and how to wire them together. When it comes to expressing a bean wiring specification, Spring is incredibly flexible, offering three primary wiring mechanisms:  Explicit configuration in XML  Explicit configuration in Java  Implicit bean discovery and automatic wiring At first glance, it may seem that offering these three configuration options compli- cates Spring. There is some overlap in what each configuration technique offers, and it can be overwhelming to decide which technique is most applicable for a given situa- tion. But don’t be distressed—in many cases, the choice is largely a matter of personal taste, and you’re welcome to choose the approach that feels best for you. 34 CHAPTER 2 Wiring beans It’s great that you have many choices about how to wire beans in Spring, but at some point you must select one. There’s no single right answer here. Any choice you make must be suitable for you and your project. And who says that you must make one choice? Spring’s configura- tion styles are mix-and-match, so you could choose XML to wire up some beans, use Spring’s Java-based configuration (JavaConfig) for other beans, and let other beans be automatically discovered by Spring. Even so, my recommendation is to lean on automatic configuration as much as you can. The less configuration you have to do explicitly, the better. When you must explicitly configure beans (such as when you’re configuring beans for which you don’t maintain the source code), I’d favor the type-safe and more powerful JavaConfig over XML. Finally, fall back on XML only in situations where there’s a convenient XML namespace you want to use that has no equivalent in JavaConfig. We’ll explore all three of these techniques in detail in this chapter and apply them throughout the book. At this point, let’s test-taste each one to get an idea of what they’re like. For your first sampling of Spring configuration, let’s look at Spring’s auto- matic configuration. 2.2 Automatically wiring beans A little bit later in this chapter, you’ll see how to express Spring wiring in both Java and XML. Even though you’ll find a lot of use for those explicit wiring techniques, nothing beats Spring’s automatic configuration for ease of use. Why bother explicitly wiring beans together if Spring can be configured to automatically do it for you? Spring attacks automatic wiring from two angles:  Component scanning—Spring automatically discovers beans to be created in the application context.  Autowiring—Spring automatically satisfies bean dependencies. Working together, component scanning and autowiring are a powerful force and can help keep explicit configuration to a minimum. To demonstrate component scanning and autowiring, you’re going to create a few beans that represent some of the components in a stereo system. You’ll start by creating a CompactDisc class that Spring will discover and create as a bean. Then you’ll create a CDPlayer class and have Spring discover it and inject it with the CompactDisc bean. 2.2.1 Creating discoverable beans In this age of MP3 files and streaming music, the compact disc may seem a bit quaint and archaic. Not as much as cassette tapes, eight-tracks, or vinyl records, of course, but CDs are becoming more and more scarce as the last remnant of physical music delivery. In spite of that, the CD provides a nice illustration of how DI works. CD players are of little value unless you insert (or inject) a CD into them. You could say that a CD player depends on a CD to do its job. 35Automatically wiring beans To bring this illustration to life in Spring, let’s establish the concept of a CD in Java. The following listing shows CompactDisc, an interface that defines a CD. package soundsystem; public interface CompactDisc { void play(); } The specifics of the CompactDisc interface aren’t important. What is important is that you’ve defined it as an interface. As an interface, it defines the contract through which a CD player can operate on the CD. And it keeps the coupling between any CD player implementation and the CD itself to a minimum. You still need an implementation of CompactDisc, though. In fact, you could have several CompactDisc implementations. In this case, you’ll start with one: the SgtPeppers class, as shown in the next listing. package soundsystem; import org.springframework.stereotype.Component; @Component public class SgtPeppers implements CompactDisc { private String title = "Sgt. Pepper's Lonely Hearts Club Band"; private String artist = "The Beatles"; public void play() { System.out.println("Playing " + title+"by"+artist); } } As with the CompactDisc interface, the specifics of SgtPeppers aren’t important to this discussion. What you should take note of is that SgtPeppers is annotated with @Component. This simple annotation identifies this class as a component class and serves as a clue to Spring that a bean should be created for the class. There’s no need to explicitly configure a SgtPeppers bean; Spring will do it for you because this class is annotated with @Component. Component scanning isn’t turned on by default, however. You’ll still need to write an explicit configuration to tell Spring to seek out classes annotated with @Component and to create beans from them. The configuration class in the following listing shows the minimal configuration to make this possible. Listing 2.1 The CompactDisc interface defines the concept of a CD in Java. Listing 2.2 @CompactDisc-annotated SgtPeppers implements CompactDisc 36 CHAPTER 2 Wiring beans package soundsystem; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.Configuration; @Configuration @ComponentScan public class CDPlayerConfig { } The CDPlayerConfig class defines a Spring wiring specification, expressed in Java. We’ll look at Java-based Spring configuration more in section 2.3. But for now, observe that CDPlayerConfig doesn’t explicitly define any beans itself. Instead, it’s annotated with @ComponentScan to enable component scanning in Spring. With no further configuration, @ComponentScan will default to scanning the same package as the configuration class. Therefore, because CDPlayerConfig is in the soundsystem package, Spring will scan that package and any subpackages underneath it, looking for classes that are annotated with @Component. It should find the Compact- Disc class and automatically create a bean for it in Spring. If you’d rather turn on component scanning via XML configuration, then you can use the element from Spring’s context namespace. Here is a minimal XML configuration to enable component scanning. Even though XML is an option for enabling component scanning, I’m going to focus on using the preferred Java-based configuration for the remainder of this discussion. If XML is more your style, though, you’ll be happy to know that the element has attributes and sub-elements that mirror the attributes you’ll use when working with @ComponentScan. Believe it or not, with only two classes created, you already have something that you can try out. To test that component scanning works, let’s write a simple JUnit test that creates a Spring application context and asserts that the CompactDisc bean is, in fact, created. CDPlayerTest in the next listing does precisely that. Listing 2.3 @ComponentScan enables component scanning Listing 2.4 Enabling component scanning in XML 37Automatically wiring beans package soundsystem; import static org.junit.Assert.*; import org.junit.Test; import org.junit.runner.RunWith; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.test.context.ContextConfiguration; import org.springframework.test.context.junit4.SpringJUnit4ClassRunner; @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration(classes=CDPlayerConfig.class) public class CDPlayerTest { @Autowired private CompactDisc cd; @Test public void cdShouldNotBeNull() { assertNotNull(cd); } } CDPlayerTest takes advantage of Spring’s SpringJUnit4ClassRunner to have a Spring application context automatically created when the test starts. And the @Context- Configuration annotation tells it to load its configuration from the CDPlayerConfig class. Because that configuration class includes @ComponentScan, the resulting applica- tion context should include the CompactDisc bean. To prove that, the test has a property of type CompactDisc that is annotated with @Autowired to inject the CompactDisc bean into the test. (I’ll talk more about @Autowired in a moment.) Finally, a simple test method asserts that the cd property isn’t null. If it’s not null, that means Spring was able to discover the CompactDisc class, automatically create it as a bean in the Spring application context, and inject it into the test. The test should pass with flying colors (or, hopefully, the color green in your test runner). Your first simple component-scanning exercise was a success! Even though you’ve only used it to create a single bean, that same small amount of configuration is good for discovering and creating any number of beans. Any classes in or under the soundsystem package that are annotated with @Component will also be created as beans. One line with @ComponentScan in exchange for countless automatically created beans is a good trade-off. Now let’s dig a bit deeper into @ComponentScan and @Component and see what else you can do with component scanning. Listing 2.5 Testing that a CompactDisc was found by component scanning 38 CHAPTER 2 Wiring beans 2.2.2 Naming a component-scanned bean All beans in a Spring application context are given an ID. What may not have been apparent from the previous example is that although you didn’t explicitly give the SgtPeppers bean an ID, it was given one derived from its class name. Specifically, the bean was given an ID of sgtPeppers by lowercasing the first letter of the class name. If you’d rather give the bean a different ID, all you have to do is pass the desired ID as a value to the @Component annotation. For example, if you wanted to identify the bean as lonelyHeartsClub, then you’d annotate the SgtPeppers class with @Component like this: @Component("lonelyHeartsClub") public class SgtPeppers implements CompactDisc { ... } Another way to name a bean is to not use the @Component annotation at all. Instead, you can use the @Named annotation from the Java Dependency Injection specification (JSR-330) to provide a bean ID: package soundsystem; import javax.inject.Named; @Named("lonelyHeartsClub") public class SgtPeppers implements CompactDisc { ... } Spring supports the @Named annotation as an alternative to @Component. There are a few subtle differences, but in most common cases they’re interchangeable. With that said, I have a strong preference for the @Component annotation, largely because @Named is … well … poorly named. It doesn’t describe what it does as well as @Component. Therefore, I won’t use @Named any further in this book or its examples. 2.2.3 Setting a base package for component scanning Thus far, you’ve used @ComponentScan with no attributes. That means it will default to the configuration class’s package as its base package to scan for components. But what if you want to scan a different package? Or what if you want to scan multiple base packages? One common reason for explicitly setting the base package is so that you can keep all of your configuration code in a package of its own, separate from the rest of your application’s code. In that case, the default base package won’t do. No problem. To specify a different base package, you only need to specify the pack- age in @ComponentScan’s value attribute: @Configuration @ComponentScan("soundsystem") public class CDPlayerConfig {} 39Automatically wiring beans Or, if you’d rather it be clear that you’re setting the base package, you can do so with the basePackages attribute: @Configuration @ComponentScan(basePackages="soundsystem") public class CDPlayerConfig {} You probably noticed that basePackages is plural. If you’re wondering whether that means you can specify multiple base packages, you can. All you need to do is set basePackages to an array of packages to be scanned: @Configuration @ComponentScan(basePackages={"soundsystem", "video"}) public class CDPlayerConfig {} The one thing about setting the base packages as shown here is that they’re expressed as String values. That’s fine, I suppose, but it’s not very type-safe. If you were to refac- tor the package names, the specified base packages would be wrong. Rather than specify the packages as simple String values, @ComponentScan also offers you the option of specifying them via classes or interfaces that are in the packages: @Configuration @ComponentScan(basePackageClasses={CDPlayer.class, DVDPlayer.class}) public class CDPlayerConfig {} As you can see, the basePackages attribute has been replaced with basePackage- Classes. And instead of identifying the packages with String names, the array given to basePackageClasses includes classes. Whatever packages those classes are in will be used as the base package for component scanning. Although I’ve specified component classes for basePackageClasses, you might consider creating an empty marker interface in the packages to be scanned. With a marker interface, you can still have a refactor-friendly reference to an interface, but without references to any actual application code (that could later be refactored out of the package you intended to component-scan). If all the objects in your applications were standalone and had no dependencies, like the SgtPeppers bean, then component scanning would be everything you need. But many objects lean on other objects for help to get their job done. You need a way to wire up your component-scanned beans with any dependencies they have. To do that, we’ll need to look at autowiring, the other side of automatic Spring configuration. 2.2.4 Annotating beans to be automatically wired Put succinctly, autowiring is a means of letting Spring automatically satisfy a bean’s dependencies by finding other beans in the application context that are a match to the bean’s needs. To indicate that autowiring should be performed, you can use Spring’s @Autowired annotation. For example, consider the CDPlayer class in the following listing. Its constructor is annotated with @Autowired, indicating that when Spring creates the CDPlayer bean, 40 CHAPTER 2 Wiring beans it should instantiate it via that constructor and pass in a bean that is assignable to CompactDisc. package soundsystem; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Component; @Component public class CDPlayer implements MediaPlayer { private CompactDisc cd; @Autowired public CDPlayer(CompactDisc cd) { this.cd = cd; } public void play() { cd.play(); } } The @Autowired annotation’s use isn’t limited to constructors. It can also be used on a property’s setter method. For example, if CDPlayer had a setCompactDisc() method, you might annotate it for autowiring like this: @Autowired public void setCompactDisc(CompactDisc cd) { this.cd = cd; } After Spring has instantiated the bean, it will try to satisfy the dependencies expressed through methods such as the setCompactDisc() method that are annotated with @Autowired. Actually, there’s nothing special about setter methods. @Autowired can also be applied on any method on the class. Pretending that CDPlayer has an insertDisc() method, @Autowired would work equally well there as on setCompactDisc(): @Autowired public void insertDisc(CompactDisc cd) { this.cd = cd; } Whether it’s a constructor, a setter method, or any other method, Spring will attempt to satisfy the dependency expressed in the method’s parameters. Assuming that one and only one bean matches, that bean will be wired in. If there are no matching beans, Spring will throw an exception as the application context is being created. To avoid that exception, you can set the required attribute on @Autowired to false: Listing 2.6 Injecting a CompactDisc into a CDPlayer bean using autowiring 41Automatically wiring beans @Autowired(required=false) public CDPlayer(CompactDisc cd) { this.cd = cd; } When required is false, Spring will attempt to perform autowiring; but if there are no matching beans, it will leave the bean unwired. You should be careful setting required to false, however. Leaving the property unwired could lead to NullPointer- Exceptions if you don’t check for null in your code. In the event that multiple beans can satisfy the dependency, Spring will throw an exception indicating ambiguity in selecting a bean for autowiring. We’ll talk more about managing ambiguity in autowiring later, in chapter 3. @Autowired is a Spring-specific annotation. If it troubles you to be scattering Spring-specific annotations throughout your code for autowiring, you might consider using the @Inject annotation instead: package soundsystem; import javax.inject.Inject; import javax.inject.Named; @Named public class CDPlayer { ... @Inject public CDPlayer(CompactDisc cd) { this.cd = cd; } ... } @Inject comes from the Java Dependency Injection specification, the same specifica- tion that gave us @Named. Spring supports the @Inject annotation for autowiring alongside its own @Autowired. Although there are some subtle differences between @Inject and @Autowired, they’re interchangeable in many cases. I have no strong preference between @Autowired and @Inject. In fact, I some- times find myself using both in a given project. For the purposes of the examples in this book, however, I’ll consistently use @Autowired. You’re welcome to use whichever one suits you best. 2.2.5 Verifying automatic configuration Now that you’ve annotated CDPlayer’s constructor with @Autowired, you can be assured that Spring will automatically inject it with a bean assignable to CompactDisc. To be certain, let’s change CDPlayerTest to play the compact disc through the CDPlayer bean: package soundsystem; import static org.junit.Assert.*; 42 CHAPTER 2 Wiring beans import org.junit.Rule; import org.junit.Test; import org.junit.contrib.java.lang.system.StandardOutputStreamLog; import org.junit.runner.RunWith; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.test.context.ContextConfiguration; import org.springframework.test.context.junit4.SpringJUnit4ClassRunner; @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration(classes=CDPlayerConfig.class) public class CDPlayerTest { @Rule public final StandardOutputStreamLog log = new StandardOutputStreamLog(); @Autowired private MediaPlayer player; @Autowired private CompactDisc cd; @Test public void cdShouldNotBeNull() { assertNotNull(cd); } @Test public void play() { player.play(); assertEquals( "Playing Sgt. Pepper's Lonely Hearts Club Band" + " by The Beatles\n", log.getLog()); } } Now, in addition to injecting CompactDisc, you’re injecting the CDPlayer bean into the test’s player member variable (as the more generic MediaPlayer type). In the play() test method, you call the play() method on the CDPlayer and assert that it does what you expect. Testing code that uses System.out.println() is a tricky business. Therefore, this example uses StandardOutputStreamLog, a JUnit rule from the System Rules library (http://stefanbirkner.github.io/system-rules/index.html) that lets you make asser- tions against whatever is written to the console. Here it’s asserting that the message from the SgtPeppers.play() method was sent to the console. Now you know the basics of component scanning and autowiring. We’ll revisit com- ponent scanning in chapter 3 when we look at ways to address autowiring ambiguity. But at this point, let’s set aside component scanning and autowiring and see how you can explicitly wire beans in Spring. We’ll start with Spring’s facility for expressing configuration in Java. 43Wiring beans with Java 2.3 Wiring beans with Java Although automatic Spring configuration with component scanning and automatic wiring is preferable in many cases, there are times when automatic configuration isn’t an option and you must configure Spring explicitly. For instance, let’s say that you want to wire components from some third-party library into your application. Because you don’t have the source code for that library, there’s no opportunity to annotate its classes with @Component and @Autowired. Therefore, automatic configu- ration isn’t an option. In that case, you must turn to explicit configuration. You have two choices for explicit configuration: Java and XML. In this section, we’ll look at how to use Java- Config. We’ll then follow up in the next section on Spring’s XML configuration. As I mentioned earlier, JavaConfig is the preferred option for explicit configura- tion because it’s more powerful, type-safe, and refactor-friendly. That’s because it’s just Java code, like any other Java code in your application. At the same time, it’s important to recognize that JavaConfig code isn’t just any other Java code. It’s conceptually set apart from the business logic and domain code in your application. Even though it’s expressed in the same language as those compo- nents, JavaConfig is configuration code. This means it shouldn’t contain any business logic, nor should JavaConfig invade any code where business logic resides. In fact, although it’s not required, JavaConfig is often set apart in a separate package from the rest of an application’s logic so there’s no confusion as to its purpose. Let’s see how to explicitly configure Spring with JavaConfig. 2.3.1 Creating a configuration class Earlier in this chapter, in listing 2.3, you got your first taste of JavaConfig. Let’s revisit CDPlayerConfig from that example: package soundsystem; import org.springframework.context.annotation.Configuration; @Configuration public class CDPlayerConfig { } The key to creating a JavaConfig class is to annotate it with @Configuration. The @Configuration annotation identifies this as a configuration class, and it’s expected to contain details on beans that are to be created in the Spring application context. So far, you’ve relied on component scanning to discover the beans that Spring should create. Although there’s no reason you can’t use component scanning and explicit configuration together, we’re focusing on explicit configuration in this sec- tion, so I’ve removed the @ComponentScan annotation from CDPlayerConfig. With @ComponentScan gone, the CDPlayerConfig class is ineffective. If you were to run CDPlayerTest now, the test would fail with a BeanCreationException. The test expects to be injected with CDPlayer and CompactDisc, but those beans are never cre- ated because they’re never discovered by component scanning. 44 CHAPTER 2 Wiring beans To make the test happy again, you could put @ComponentScan back in. Keeping the focus on explicit configuration, however, let’s see how you can wire the CDPlayer and CompactDisc beans in JavaConfig. 2.3.2 Declaring a simple bean To declare a bean in JavaConfig, you write a method that creates an instance of the desired type and annotate it with @Bean. For example, the following method declares the CompactDisc bean: @Bean public CompactDisc sgtPeppers() { return new SgtPeppers(); } The @Bean annotation tells Spring that this method will return an object that should be registered as a bean in the Spring application context. The body of the method contains logic that ultimately results in the creation of the bean instance. By default, the bean will be given an ID that is the same as the @Bean-annotated method’s name. In this case, the bean will be named compactDisc. If you’d rather it have a different name, you can either rename the method or prescribe a different name with the name attribute: @Bean(name="lonelyHeartsClubBand") public CompactDisc sgtPeppers() { return new SgtPeppers(); } No matter how you name the bean, this bean declaration is about as simple as they come. The body of the method returns a new instance of SgtPeppers. But because it’s expressed in Java, it has every capability afforded it by the Java language to do almost anything to arrive at the CompactDisc that is returned. Unleashing your imagination a bit, you might do something crazy like randomly selecting a CompactDisc from a selection of choices: @Bean public CompactDisc randomBeatlesCD() { int choice = (int) Math.floor(Math.random() * 4); if (choice == 0) { return new SgtPeppers(); } else if (choice == 1) { return new WhiteAlbum(); } else if (choice == 2) { return new HardDaysNight(); } else { return new Revolver(); } } I’ll let you daydream a bit about all the ways you can exploit the power of Java to pro- duce a bean from an @Bean-annotated method. When you’re done, we’ll pick it back up and look at how you can inject the CompactDisc bean into the CDPlayer in JavaConfig. 45Wiring beans with Java 2.3.3 Injecting with JavaConfig The CompactDisc bean you declared was simple and had no dependencies of its own. But now you must declare the CDPlayer bean, which depends on a CompactDisc. How can you wire that up in JavaConfig? The simplest way to wire up beans in JavaConfig is to refer to the referenced bean’s method. For example, here’s how you might declare the CDPlayer bean: @Bean public CDPlayer cdPlayer() { return new CDPlayer(sgtPeppers()); } The cdPlayer() method, like the sgtPeppers() method, is annotated with @Bean to indicate that it will produce an instance of a bean to be registered in the Spring appli- cation context. The ID of the bean will be cdPlayer, the same as the method’s name. The body of the cdPlayer() method differs subtly from that of the sgtPeppers() method. Rather than construct an instance via its default method, the CDPlayer instance is created by calling its constructor that takes a CompactDisc. It appears that the CompactDisc is provided by calling sgtPeppers, but that’s not exactly true. Because the sgtPeppers() method is annotated with @Bean, Spring will intercept any calls to it and ensure that the bean produced by that method is returned rather than allowing it to be invoked again. For example, suppose you were to introduce another CDPlayer bean that is just like the first: @Bean public CDPlayer cdPlayer() { return new CDPlayer(sgtPeppers()); } @Bean public CDPlayer anotherCDPlayer() { return new CDPlayer(sgtPeppers()); } If the call to sgtPeppers() was treated like any other call to a Java method, then each CDPlayer would be given its own instance of SgtPeppers. That would make sense if we were talking about real CD players and compact discs. If you have two CD players, there’s no physical way for a single compact disc to simultaneously be inserted into two CD players. In software, however, there’s no reason you couldn’t inject the same instance of SgtPeppers into as many other beans as you want. By default, all beans in Spring are singletons, and there’s no reason you need to create a duplicate instance for the sec- ond CDPlayer bean. So Spring intercepts the call to sgtPeppers() and makes sure that what is returned is the Spring bean that was created when Spring itself called sgtPeppers() to create the CompactDisc bean. Therefore, both CDPlayer beans will be given the same instance of SgtPeppers. 46 CHAPTER 2 Wiring beans I can see how referring to a bean by calling its method can be confusing. There’s another way that might be easier to digest: @Bean public CDPlayer cdPlayer(CompactDisc compactDisc) { return new CDPlayer(compactDisc); } Here, the cdPlayer() method asks for a CompactDisc as a parameter. When Spring calls cdPlayer() to create the CDPlayer bean, it autowires a CompactDisc into the configuration method. Then the body of the method can use it however it sees fit. With this technique, the cdPlayer() method can still inject the CompactDisc into the CDPlayer’s constructor without explicitly referring to the CompactDisc’s @Bean method. This approach to referring to other beans is usually the best choice because it doesn’t depend on the CompactDisc bean being declared in the same configuration class. In fact, there’s nothing that says the CompactDisc bean even needs to be declared in JavaConfig; it could have been discovered by component scanning or declared in XML. You could break up your configuration into a healthy mix of config- uration classes, XML files, and automatically scanned and wired beans. No matter how the CompactDisc was created, Spring will be happy to hand it to this configuration method to create the CDPlayer bean. In any event, it’s important to recognize that although you’re performing DI via the CDPlayer’s constructor, there’s no reason you couldn’t apply other styles of DI here. For example, if you wanted to inject a CompactDisc via a setter method, it might look like this: @Bean public CDPlayer cdPlayer(CompactDisc compactDisc) { CDPlayer cdPlayer = new CDPlayer(compactDisc); cdPlayer.setCompactDisc(compactDisc); return cdPlayer; } Once again, it bears repeating that the body of an @Bean method can utilize whatever Java is necessary to produce the bean instance. Constructor and setter injection just happen to be two simple examples of what you can do in an @Bean-annotated method. The possibilities are limited only by the capabilities of the Java language. 2.4 Wiring beans with XML So far, you’ve seen how to let Spring automatically discover and wire beans. And you’ve seen how to step in and explicitly wire beans using JavaConfig. But there’s another option for bean wiring that, although less desirable, has a long history with Spring. Since the beginning of Spring, XML has been the primary way of expressing con- figuration. Countless lines of XML have been created in the name of Spring. And for many, Spring has become synonymous with XML configuration. 47Wiring beans with XML Although it’s true that Spring has long been associated with XML, let’s be clear that XML isn’t the only option for configuring Spring. And now that Spring has strong sup- port for automatic configuration and Java-based configuration, XML should not be your first choice. Nevertheless, because so much XML-based Spring configuration has already been written, it’s important to understand how to use XML with Spring. I hope, however, that this section will only serve to help you work with existing XML configuration, and that you’ll lean on automatic configuration and JavaConfig for any new Spring work you do. 2.4.1 Creating an XML configuration specification Before you can start using XML to wire together beans in Spring, you’ll need to create the empty configuration specification. With JavaConfig, that meant creating a class annotated with @Configuration. For XML configuration, that means creating an XML file rooted with a element. The simplest possible Spring XML configuration looks like this: It doesn’t take much to see that this basic XML configuration is already much more complex than an equivalent JavaConfig class. Whereas JavaConfig’s @Configuration annotation was all you needed to get started, the XML elements for configuring Spring are defined in several XML schema (XSD) files that must be declared in the preamble of the XML configuration file. CREATING XML CONFIGURATIONS WITH SPRING TOOL SUITE An easy way to create and manage Spring XML configuration files is to use Spring Tool Suite (https://spring.io/tools/sts). Select File > New > Spring Bean Configuration File from Spring Tool Suite’s menu to create a Spring XML configuration file, and select from one of the available configuration namespaces. The most basic XML elements for wiring beans are contained in the spring-beans schema, which is declared as the root namespace of this XML file. The ele- ment, the root element of any Spring configuration file, is one of the elements in this schema. Several other schemas are available for configuring Spring in XML. Although I’m going to focus on automatic and Java configuration throughout this book, I’ll at least 48 CHAPTER 2 Wiring beans keep you informed along the way when some of these other schemas might come into play. As it is, you have a perfectly valid Spring XML configuration. It’s also a perfectly useless configuration, because it doesn’t (yet) declare any beans. To give it some life, let’s re-create the CD example, this time using XML configuration instead of Java- Config or automatic configuration. 2.4.2 Declaring a simple To declare a bean in Spring’s XML-based configuration, you’re going to use another element from the spring-beans schema: the element. The element is the XML analogue to JavaConfig’s @Bean annotation. You can use it to declare the CompactDisc bean like this: Here you declare a very simple bean. The class used to create this bean is specified in the class attribute and is expressed as the fully qualified class name. For lack of an explicitly given ID, the bean will be named according to the fully qualified class name. In this case, the bean’s ID will be soundsystem.SgtPeppers#0. The #0 is an enumeration used to differentiate this bean from any other bean of the same type. If you were to declare another SgtPeppers bean without explicitly identify- ing it, it would automatically be given an ID of soundsystem.SgtPeppers#1. Even though it’s convenient to have beans named automatically for you, the gener- ated names will be less useful if you need to refer to them later. Therefore, it’s usually a good idea to give each bean a name of your own choosing via the id attribute: You’ll use this explicit name in a moment when you wire this bean into the CDPlayer bean. REDUCING VERBOSITY To cut down on XML verbosity, only explicitly name a bean if you’ll need to refer to it by name (such as if you were to inject a refer- ence to it into another bean). But before we go any further, let’s take a moment to examine some of the characteris- tics of this simple bean declaration. The first thing to notice is that you aren’t directly responsible for creating an instance of SgtPeppers as you were when using JavaConfig. When Spring sees this element, it will create a SgtPeppers bean for you by calling its default con- structor. Bean creation is much more passive with XML configuration. But it’s also less powerful than JavaConfig, where you can do almost anything imaginable to arrive at the bean instance. Another notable thing about this simple declaration is that you express the type of the bean as a string set to the class attribute. Who’s to say that the value given to class even refers to a real class? Spring’s XML configuration doesn’t benefit from 49Wiring beans with XML compile-time verification of the Java types being referred to. And even if it does refer to an actual type, what will happen if you rename the class? CHECK XML VALIDITY WITH AN IDE Using a Spring-aware IDE such as Spring Tool Suite can help a lot to ensure the validity of your Spring XML configuration. These are just a few of the reasons why JavaConfig is preferable over XML configura- tion. I encourage you to be mindful of these shortcomings of XML configuration when choosing the configuration style for your application. Nevertheless, let’s continue this study of Spring’s XML configuration to see how you can inject your SgtPeppers bean into the CDPlayer. 2.4.3 Initializing a bean with constructor injection There’s only one way to declare a bean in Spring XML configuration: use the element, and specify a class attribute. Spring takes it from there. But when it comes to declaring DI in XML, there are several options and styles. With specific regard to constructor injection, you have two basic options to choose from:  The element  Using the c-namespace introduced in Spring 3.0 The difference between these two choices is largely one of verbosity. As you’ll see, the element is generally more verbose than using the c-namespace and results in XML that is more difficult to read. On the other hand, can do a few things that the c-namespace can’t. As we look at constructor injection in Spring XML, we’ll stack these two options side by side. First, let’s see how each fares at injecting bean references. INJECTING CONSTRUCTORS WITH BEAN REFERENCES As currently defined, the CDPlayer bean has a constructor that accepts a Compact- Disc. This makes it a perfect candidate for injection with a bean reference. Because you’ve already declared a SgtPeppers bean, and because the SgtPeppers class implements the CompactDisc interface, you have a bean to inject into a CDPlayer bean. All you need to do is declare the CDPlayer bean in XML and reference the SgtPeppers bean by its ID: When Spring encounters this element, it will create an instance of CDPlayer. The element tells it to pass a reference to the bean whose ID is compactDisc to the CDPlayer’s constructor. Alternatively, you can use Spring’s c-namespace. The c-namespace was introduced in Spring 3.0 as a more succinct way of expressing constructor args in XML. To use it, you must declare its schema in the preamble of the XML, like this: 50 CHAPTER 2 Wiring beans ... With the c-namespace and schema declared, you can use it to declare a constructor argument like this: Here you’re using the c-namespace to declare the constructor argument as an attribute of the element. And it’s a rather odd-looking attribute name. Figure 2.1 illustrates how the pieces of the attribute name come together. The attribute name starts with c:, the namespace prefix. Following that is the name of the constructor argument being wired. After that is -ref, a naming convention that indicates to Spring that you’re wiring a reference to a bean named compactDisc and not the literal String value "compactDisc". It’s clear that using c-namespace attributes is much more terse than using the element. That’s one of the reasons that I like it a lot. Aside from being slightly easier to read, c-namespace attributes are especially helpful when I have to write code samples that fit neatly within the margins of a book. But one thing that bugs me about the c-namespace as I’ve used it in the previous example is that it directly refers to the name of the constructor argument. Referring to a parameter name seems a bit flaky to me. Referring to a parameter by name requires that you compile your code with debug symbols stored in the class code. If you optimize your builds to leave out debug symbols, then this probably won’t work. Instead, you could refer to the parameter’s position in the parameter list: This c-namespace attribute looks even more bizarre than the last one. I’ve replaced the name of the parameter with 0, the parameter index. But because XML doesn’t allow digits as the first character of an attribute, I had to add an underscore as a prefix. Using an index to identify the constructor argument feels better than referencing it by its name. Even if debug symbols are excluded from the build, the parameters will c:cd-ref="compactDisc" c-namespace prefix Constructor argument name Injecting a bean reference The ID of the bean to inject Figure 2.1 Injecting a bean reference into a constructor argument with Spring’s c-namespace 51Wiring beans with XML still be in the same order. And if there were multiple constructor arguments, it would certainly be useful. But because you have only one constructor argument, you have one more option—don’t identify the parameter at all: This is by far the most peculiar c-namespace attribute. There’s no parameter index or parameter name. There’s just an underscore placeholder followed by -ref to indicate that you’re wiring a reference. Now that you’ve tried wiring a reference to other beans, let’s see how to wire literal values into constructors. INJECTING CONSTRUCTORS WITH LITERAL VALUES Although DI often refers to the type of wiring you’ve done thus far—wiring references to objects into other objects that depend on them—sometimes all you need to do is configure an object with a literal value. To illustrate, suppose you were to create a new implementation of CompactDisc, as shown here: package soundsystem; public class BlankDisc implements CompactDisc { private String title; private String artist; public BlankDisc(String title, String artist) { this.title = title; this.artist = artist; } public void play() { System.out.println("Playing " + title+"by"+artist); } } Unlike SgtPeppers, which was hard-coded with a title and artist, this implementation of CompactDisc is considerably more flexible. Much like a real-world blank disc, it can be set to contain any artist and title you want. Now you can change the existing SgtPeppers bean to use this class instead: Once again, the element is used to inject into constructor argu- ments. But this time, instead of using the ref attribute to reference another bean, you use the value attribute to indicate that the given value is to be taken literally and injected into the constructor. How would this look if you were to use c-namespace attributes instead? One possi- ble rendition might reference the constructor arguments by name: 52 CHAPTER 2 Wiring beans As you can see, wiring literal values via the c-namespace differs from wiring references in that the -ref suffix is left off the attribute name. Similarly, you could wire the same literal values using parameter indexes, like this: XML doesn’t allow more than one attribute on a given element to share the same name. Therefore, you can’t use the simple underscore when you have two or more constructor arguments. But you can use it when there’s only one constructor argu- ment. For the sake of completeness, let’s pretend that BlankDisc has a single- argument constructor that takes the album’s title. In that case, you could declare it in Spring like this: When it comes to wiring bean reference and literal values, both and the c-namespace attributes are equally capable. But there’s one thing that can do that the c-namespace can’t do. Let’s look at how to wire collections to constructor arguments. WIRING COLLECTIONS Up until now, we’ve assumed that CompactDisc was defined by merely a title and an artist name. But if that’s all that came with a real-world CD, the technology would’ve never taken off. What makes CDs worth buying is that they carry music on them. Most CDs carry roughly a dozen tracks, each holding a song. If CompactDisc is to truly model a real-world CD, then it must also have the notion of a list of tracks. Consider the new BlankDisc shown here: package soundsystem.collections; import java.util.List; import soundsystem.CompactDisc; public class BlankDisc implements CompactDisc { private String title; private String artist; private List tracks; public BlankDisc(String title, String artist, List tracks) { this.title = title; this.artist = artist; this.tracks = tracks; } 53Wiring beans with XML public void play() { System.out.println("Playing " + title+"by"+artist); for (String track : tracks) { System.out.println("-Track: " + track); } } } This change has implications for how you configure the bean in Spring. You must pro- vide a list of tracks when declaring the bean. The simplest thing you could do is leave the list null. Because it’s a constructor argument, you must specify it, but you can still pass null like this: The element does as you’d expect: it passes null into the constructor. It’s a dirty fix, but it will work at injection time. You’ll get a NullPointerException when the play() method is called, so it’s far from ideal. A better fix would be to supply a list of track names. For that you have a couple of options. First, you could specify it as a list, using the element: Sgt. Pepper's Lonely Hearts Club Band With a Little Help from My Friends Lucy in the Sky with Diamonds Getting Better Fixing a Hole The element is a child of and indicates that a list of values is to be passed into the constructor. The element is used to specify each ele- ment of the list. Similarly, a list of bean references could be wired using the element instead of . For example, suppose you have a Discography class with the following constructor: public Discography(String artist, List cds) { ... } You can then configure a Discography bean like this: 54 CHAPTER 2 Wiring beans ... It makes sense to use when wiring a constructor argument of type java.util.List. Even so, you could also use the element in the same way: Sgt. Pepper's Lonely Hearts Club Band With a Little Help from My Friends Lucy in the Sky with Diamonds Getting Better Fixing a Hole There’s little difference between and . The main difference is that when Spring creates the collection to be wired, it will create it as either a java.util.Set or a java.util.List. If it’s a Set, then any duplicate values will be discarded and the ordering may not be honored. But in either case, either a or a can be wired into a List, a Set, or even an array. Wiring collections is one place where the has an advantage over the c-namespace attributes. There’s no obvious way to wire collections like this via c-namespace attributes. There are a handful of other nuances to using both and the c- namespace for constructor injection. But what we’ve covered here should carry you quite far, especially considering my earlier advice to favor Java configuration over XML configuration. Therefore, rather than belabor the topic of constructor injection in XML, let’s move on to see how to wire properties in XML. 2.4.4 Setting properties Up to this point, the CDPlayer and BlankDisc classes have been configured entirely through constructor injection and don’t have any property setter methods. In con- trast, let’s examine how property injection works in Spring XML. Suppose that your new property-injected CDPlayer looks like this: 55Wiring beans with XML package soundsystem; import org.springframework.beans.factory.annotation.Autowired; import soundsystem.CompactDisc; import soundsystem.MediaPlayer; public class CDPlayer implements MediaPlayer { private CompactDisc compactDisc; @Autowired public void setCompactDisc(CompactDisc compactDisc) { this.compactDisc = compactDisc; } public void play() { compactDisc.play(); } } CHOOSING BETWEEN CONSTRUCTOR INJECTION AND PROPERTY INJECTION As a general rule, I favor constructor injection for hard dependencies and prop- erty injection for any optional dependencies. In light of that rule, we could argue that the title, artist, and track list are hard dependencies for a Blank- Disc and that constructor injection was the right choice. It’s debatable, how- ever, whether a CompactDisc is a hard or optional dependency for a CDPlayer. I stand by that choice, but you could say that a CDPlayer might still have some limited functionality even without a CompactDisc being injected into it. Now that CDPlayer doesn’t have any constructors (aside from the implicit default con- structor), it also doesn’t have any hard dependencies. Therefore, you could declare it as a Spring bean like this: Spring will have absolutely no problem creating that bean. Your CDPlayerTest would fail with a NullPointerException, however, because you never injected CDPlayer’s compactDisc property. But you can fix that with the following change to the XML: The element does for property setter methods what the element does for constructors. In this case, it references (with the ref attribute) the bean whose ID is compactDisc to be injected into the compactDisc property (via the setCompactDisc() method). Now if you run your test, it should pass. You may also like to know that just as Spring offers the c-namespace as an alterna- tive to the element, Spring also offers a succinct p-namespace as an alternative to the element. To enable the p-namespace, you must declare it among the other namespaces in the XML file: 56 CHAPTER 2 Wiring beans ... Using the p-namespace, you can wire the compactDisc property like this: The p-namespace attributes follow a nam- ing convention similar to that of the c-namespace attributes. Figure 2.2 illus- trates how this p-namespace attribute name breaks down. First, the attribute name is prefixed with p: to indicate that you’re setting a property. Next up is the name of the prop- erty to be injected. Finally, the name ends with -ref as a clue to Spring that you’re wiring a reference to a bean and not a literal value. INJECTING PROPERTIES WITH LITERAL VALUES Properties can be injected with literal values in much the same way as constructor arguments. As an example, let’s revisit the BlankDisc bean. This time, however, BlankDiscs will be configured entirely by property injection, not constructor injec- tion. The new BlankDisc class looks like this: package soundsystem; import java.util.List; import soundsystem.CompactDisc; public class BlankDisc implements CompactDisc { private String title; private String artist; private List tracks; public void setTitle(String title) { this.title = title; } public void setArtist(String artist) { this.artist = artist; } c:compactDisc-ref="compactDisc" p-namespace prefix Property name Injecting a bean reference The ID of the bean to inject Figure 2.2 Injecting a bean reference into a property with Spring’s p-namespace 57Wiring beans with XML public void setTracks(List tracks) { this.tracks = tracks; } public void play() { System.out.println("Playing " + title+"by"+artist); for (String track : tracks) { System.out.println("-Track: " + track); } } } Now you’re no longer obligated to wire any of these properties. You could create a BlankDisc bean in its most blank form as follows: Of course, wiring the bean without setting those properties wouldn’t play out well at runtime. The play() method would claim that it’s playing null by null just before a NullPointerException is thrown because there are no tracks. Therefore, you proba- bly should wire up those properties. You can do that using the value attribute of the element: Sgt. Pepper's Lonely Hearts Club Band With a Little Help from My Friends Lucy in the Sky with Diamonds Getting Better Fixing a Hole Aside from using the element’s value attribute to set the title and artist properties, notice how you set the tracks property with a nested element, the same as before when wiring the tracks through . Optionally, you can accomplish the same thing using p-namespace attributes: Sgt. Pepper's Lonely Hearts Club Band 58 CHAPTER 2 Wiring beans With a Little Help from My Friends Lucy in the Sky with Diamonds Getting Better Fixing a Hole As with c-namespace attributes, the only difference between wiring a bean reference and wiring a literal value is the presence or absence of a -ref suffix. Without the -ref suffix, you’re wiring literal values. Notice, however, that you can’t use the p-namespace when wiring a collection. Unfortunately, there’s no convenient way to specify a list of values (or bean refer- ences) with the p-namespace. But you can take advantage of something from Spring’s util-namespace to simplify the BlankDisc bean. First, you need to declare the util-namespace and its schema in the XML: ... One of the things that the util-namespace offers is the element, which creates a list bean. Using , you can shift the track list out of the Blank- Disc bean and into a bean of its own, like this: Sgt. Pepper's Lonely Hearts Club Band With a Little Help from My Friends Lucy in the Sky with Diamonds Getting Better Fixing a Hole Now you can wire the track-list bean into the BlankDisc bean’s tracks property just like any other bean: 59Importing and mixing configurations The element is just one of several elements in the util-namespace. Table 2.1 lists everything the util-namespace has to offer. You’ll occasionally call on members of the util-namespace as you need them. For now, though, let’s wrap up this chapter by seeing how you can mix and match auto- matic configuration, JavaConfig, and XML configuration. 2.5 Importing and mixing configurations In a typical Spring application, you’re likely to need to use both automatic and explicit configuration. And even if you favor JavaConfig for explicit configuration, there may be times when XML configuration is the best choice. Fortunately, none of the configuration options available in Spring are mutually exclusive. You’re free to mix component scanning and autowiring with JavaConfig and/or XML configuration. In fact, as you saw in section 2.2.1, you’ll need at least a lit- tle explicit configuration to enable component scanning and autowiring. The first thing to know about mixing configuration styles is that when it comes to autowiring, it doesn’t matter where the bean to be wired comes from. Autowiring con- siders all beans in the Spring container, regardless of whether they were declared in JavaConfig or XML or picked up by component scanning. That leaves you with how to reference beans when doing explicit configuration, either with XML configuration or with Java configuration. Let’s start by seeing how to reference XML-configured beans from JavaConfig. 2.5.1 Referencing XML configuration in JavaConfig Pretend for a moment that CDPlayerConfig is getting unwieldy and you want to split it apart. Sure, it only declares two beans, which is a far cry from a complex Spring con- figuration. Nevertheless, let’s pretend that two beans is two beans too many. What you could do is break out the BlankDisc bean from CDPlayerConfig into its own CDConfig class, like this: package soundsystem; import org.springframework.context.annotation.Bean; Table 2.1 Elements in Spring’s util-namespace Element Description References a public static field on a type and exposes it as a bean Creates a bean that is a java.util.List of values or references Creates a bean that is a java.util.Map of values or references Creates a bean that is a java.util.Properties References a bean property (or nested property) and exposes it as a bean Creates a bean that is a java.util.Set of values or references 60 CHAPTER 2 Wiring beans import org.springframework.context.annotation.Configuration; @Configuration public class CDConfig { @Bean public CompactDisc compactDisc() { return new SgtPeppers(); } } Now that the compactDisc() method is gone from CDPlayerConfig, you need a way to bring the two configuration classes together. One way is to import CDConfig from CDPlayerConfig using the @Import annotation: package soundsystem; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.Import; @Configuration @Import(CDConfig.class) public class CDPlayerConfig { @Bean public CDPlayer cdPlayer(CompactDisc compactDisc) { return new CDPlayer(compactDisc); } } Or, better yet, you can leave @Import out of CDPlayerConfig and instead create a higher-level SoundSystemConfig that uses @Import to bring both configurations together: package soundsystem; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.Import; @Configuration @Import({CDPlayerConfig.class, CDConfig.class}) public class SoundSystemConfig { } Either way, you’ve separated the configuration of CDPlayer from the configuration of BlankDisc. Now let’s suppose that (for whatever reason) you want to configure the BlankDisc bean in XML like this: Sgt. Pepper's Lonely Hearts Club Band With a Little Help from My Friends 61Importing and mixing configurations Lucy in the Sky with Diamonds Getting Better Fixing a Hole With BlankDisc being declared in XML, how can you have Spring load it in along with the rest of your Java-based configuration? The answer lies with the @ImportResource annotation. Assuming that the Blank- Disc bean is declared in a file named cd-config.xml that can be found at the root of the classpath, you can change SoundSystemConfig to use @ImportResource like this: package soundsystem; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.Import; import org.springframework.context.annotation.ImportResource; @Configuration @Import(CDPlayerConfig.class) @ImportResource("classpath:cd-config.xml") public class SoundSystemConfig { } Both beans—CDPlayer configured in JavaConfig and BlankDisc configured in XML— will be loaded into the Spring container. And because CDPlayer’s @Bean method accepts a CompactDisc as a parameter, the BlankDisc bean will be wired into it, even though it’s configured in XML. Let’s run through this exercise again. But this time, you’ll reference a JavaConfig- declared bean from XML. 2.5.2 Referencing JavaConfig in XML configuration Suppose you’re working with Spring’s XML-based configuration and you’ve decided that the XML is getting out of hand. As before, you’re only dealing with two beans, and things could be worse. But before you’re inundated with a flood of angle brackets, you decide to break the XML configuration file apart. With JavaConfig, I showed you how to use @Import and @ImportResource to split up your JavaConfig classes. In XML, you can use the element to split up the XML configuration. For example, suppose you were to split out the BlankDisc bean into its own config- uration file called cd-config.xml, as you did when working with @ImportResource. You can reference that file from the XML configuration file using : 62 CHAPTER 2 Wiring beans Now, suppose that instead of configuring BlankDisc in XML, you want to configure it in XML while leaving the CDPlayer configuration in JavaConfig. How can your XML- based configuration reference a JavaConfig class? As it turns out, the answer isn’t intuitive. The element only works to import other XML configuration files, and there isn’t an XML element whose job it is to import JavaConfig classes. There is, however, an element you already know that can be used to bring a Java configuration into an XML configuration: the element. To import a JavaConfig class into an XML configuration, you declare it as a bean like this: And just like that, the two configurations—one expressed in XML and one expressed in Java—have been brought together. Similarly, you might consider creating a higher- level configuration file that doesn’t declare any beans but that brings two or more configurations together. For example, you could leave the CDConfig bean out of the previous XML configuration and instead have a third configuration file that joins them: 63Summary Whether I’m using JavaConfig or XML wiring, I often create a root configuration, as I’ve shown here, that brings together two or more wiring classes and/or XML files. It’s in this root configuration that I’ll also usually turn on component scanning (with either or @ComponentScan). You’ll see this technique employed for many of the examples in this book. 2.6 Summary At the core of the Spring Framework is the Spring container. This container manages the lifecycle of the components of an application, creating those components and ensuring that their dependencies are met so that they can do their job. In this chapter, we’ve looked at three primary ways of wiring beans together in Spring: automatic configuration, explicit Java-based configuration, and explicit XML- based configuration. No matter which you choose, these techniques describe the com- ponents in a Spring application and the relationships between those components. I’ve also strongly recommended that you favor automatic configuration as much as possible to avoid the maintenance costs involved with explicit configuration. But when you must explicitly configure Spring, you should favor Java-based configuration— which is more powerful, type-safe, and refactorable—over XML configuration. This preference will guide my choice of wiring techniques as I present the examples throughout this book. Because dependency injection is an essential part of working with Spring, the tech- niques shown in this chapter will play a role in almost everything else you do in this book. Building on this foundation, the next chapter will present some more advanced bean-wiring techniques that will help you make the most of the Spring container. 64 Advanced wiring In the previous chapter, we looked at some essential bean-wiring techniques. You’re likely to find a lot of use for what you learned in that chapter. But there’s more to bean wiring than what we explored in chapter 2. Spring has several other tricks up its sleeve for more advanced bean wiring. In this chapter, we’ll dig in to some of these advanced techniques. You won’t get as much day-to-day use out of the techniques in this chapter, but that doesn’t mean they’re any less valuable. 3.1 Environments and profiles One of the most challenging things about developing software is transitioning an application from one environment to another. Certain environment-specific choices made for development aren’t appropriate or won’t work when the application transitions from development to production. Database configuration, encryption This chapter covers  Spring profiles  Conditional bean declaration  Autowiring and ambiguity  Bean scoping  The Spring Expression Language 65Environments and profiles algorithms, and integration with external systems are just a few examples of things that are likely to vary across deployment environments. Consider database configuration, for instance. In a development environment, you’re likely to use an embedded database preloaded with test data. For example, in a Spring configuration class, you might use EmbeddedDatabaseBuilder in an @Bean method like this: @Bean(destroyMethod="shutdown") public DataSource dataSource() { return new EmbeddedDatabaseBuilder() .addScript("classpath:schema.sql") .addScript("classpath:test-data.sql") .build(); } This will create a bean of type javax.sql.DataSource. But it’s how that bean is created that’s most interesting. Using EmbeddedDatabaseBuilder sets up an embedded Hyper- sonic database whose schema is defined in schema.sql and loaded with test data from test-data.sql. This DataSource is useful in a development environment when you’re running integration tests or firing up an application for manual testing. You can count on your database being in a given state every time you start it. Although that makes an EmbeddedDatabaseBuilder-created DataSource perfect for development, it makes it a horrible choice for production. In a production setting, you may want to retrieve a DataSource from your container using JNDI. In that case, the following @Bean method is more appropriate: @Bean public DataSource dataSource() { JndiObjectFactoryBean jndiObjectFactoryBean = new JndiObjectFactoryBean(); jndiObjectFactoryBean.setJndiName("jdbc/myDS"); jndiObjectFactoryBean.setResourceRef(true); jndiObjectFactoryBean.setProxyInterface(javax.sql.DataSource.class); return (DataSource) jndiObjectFactoryBean.getObject(); } Retrieving a DataSource from JNDI allows your container to make decisions about how it’s created, including handing off a DataSource from a container-managed con- nection pool. Even so, using a JNDI-managed DataSource is more fitting for produc- tion and unnecessarily complicated for a simple integration test or developer test. Meanwhile, in a QA environment you could select a completely different Data- Source configuration. You might choose to configure a Commons DBCP connection pool like this: @Bean(destroyMethod="close") public DataSource dataSource() { BasicDataSource dataSource = new BasicDataSource(); dataSource.setUrl("jdbc:h2:tcp://dbserver/~/test"); dataSource.setDriverClassName("org.h2.Driver"); 66 CHAPTER 3 Advanced wiring dataSource.setUsername("sa"); dataSource.setPassword("password"); dataSource.setInitialSize(20); dataSource.setMaxActive(30); return dataSource; } Clearly, all three versions of the dataSource() method presented here are different from each other. They all produce a bean whose type is javax.sql.DataSource, but that’s where the similarities end. Each applies a completely different strategy for pro- ducing the DataSource bean. Again, this discussion isn’t about how to configure a DataSource (we’ll talk more about that in chapter 10). But certainly the seemingly simple DataSource bean isn’t so simple. It’s a good example of a bean that might vary across different environments. You must find a way to configure a DataSource bean so that the most appropriate con- figuration is chosen for each environment. One way of doing this is to configure each bean in a separate configuration class (or XML file) and then make a build-time decision (perhaps using Maven profiles) about which to compile into the deployable application. The problem with this solu- tion is that it requires that the application be rebuilt for each environment. A rebuild might not be that big a problem when going from development to QA. But requiring a rebuild between QA and production has the potential to introduce bugs and cause an epidemic of ulcers among the members of your QA team. Fortunately, Spring has a solution that doesn’t require a rebuild. 3.1.1 Configuring profile beans Spring’s solution for environment-specific beans isn’t much different from build-time solutions. Certainly, an environment-specific decision is made as to which beans will and won’t be created. But rather than make that decision at build time, Spring waits to make the decision at runtime. Consequently, the same deployment unit (perhaps a WAR file) will work in all environments without being rebuilt. In version 3.1, Spring introduced bean profiles. To use profiles, you must gather all the varying bean definitions into one or more profiles and then make sure the proper profile is active when your application is deployed in each environment. In Java configuration, you can use the @Profile annotation to specify which pro- file a bean belongs to. For example, the embedded database DataSource bean might be configured in a configuration class like this: package com.myapp; import javax.activation.DataSource; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.Profile; import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseBuilder; import 67Environments and profiles org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType; @Configuration @Profile("dev") public class DevelopmentProfileConfig { @Bean(destroyMethod="shutdown") public DataSource dataSource() { return new EmbeddedDatabaseBuilder() .setType(EmbeddedDatabaseType.H2) .addScript("classpath:schema.sql") .addScript("classpath:test-data.sql") .build(); } } The main thing I want to draw your attention to is the @Profile annotation applied at the class level. It tells Spring that the beans in this configuration class should be cre- ated only if the dev profile is active. If the dev profile isn’t active, then the @Bean meth- ods will be ignored. Meanwhile, you may have another configuration class for production that looks like this: package com.myapp; import javax.activation.DataSource; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.Profile; import org.springframework.jndi.JndiObjectFactoryBean; @Configuration @Profile("prod") public class ProductionProfileConfig { @Bean public DataSource dataSource() { JndiObjectFactoryBean jndiObjectFactoryBean = new JndiObjectFactoryBean(); jndiObjectFactoryBean.setJndiName("jdbc/myDS"); jndiObjectFactoryBean.setResourceRef(true); jndiObjectFactoryBean.setProxyInterface( javax.sql.DataSource.class); return (DataSource) jndiObjectFactoryBean.getObject(); } } In this case, the bean won’t be created unless the prod profile is active. In Spring 3.1, you could only use the @Profile annotation at the class level. Start- ing with Spring 3.2, however, you can use @Profile at the method level, alongside the @Bean annotation. This makes it possible to combine both bean declarations into a single configuration class, as shown in the following listing. 68 CHAPTER 3 Advanced wiring package com.myapp; import javax.activation.DataSource; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.Profile; import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseBuilder; import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType; import org.springframework.jndi.JndiObjectFactoryBean; @Configuration public class DataSourceConfig { @Bean(destroyMethod="shutdown") @Profile("dev") public DataSource embeddedDataSource() { return new EmbeddedDatabaseBuilder() .setType(EmbeddedDatabaseType.H2) .addScript("classpath:schema.sql") .addScript("classpath:test-data.sql") .build(); } @Bean @Profile("prod") public DataSource jndiDataSource() { JndiObjectFactoryBean jndiObjectFactoryBean = new JndiObjectFactoryBean(); jndiObjectFactoryBean.setJndiName("jdbc/myDS"); jndiObjectFactoryBean.setResourceRef(true); jndiObjectFactoryBean.setProxyInterface(javax.sql.DataSource.class); return (DataSource) jndiObjectFactoryBean.getObject(); } } What’s not apparent here is that although each of the DataSource beans is in a profile and will only be created if the prescribed profile is active, there are probably other beans that aren’t defined in the scope of a given profile. Any bean that isn’t given a profile will always be created, regardless of what profile is active. CONFIGURING PROFILES IN XML You can also configure profiled beans in XML by setting the profile attribute of the element. For example, to define the embedded database DataSource bean for development in XML, you can create a configuration XML file that looks like this: Likewise, you could create another configuration file, with profile set to prod for the production-ready JNDI-obtained DataSource bean. And you could create yet another XML file for the connection pool–defined DataSource bean specified by the qa profile. All the configuration XML files are collected into the deployment unit (likely a WAR file), but only those whose profile attribute matches the active profile will be used. Rather than creating a proliferation of XML files for each environment, you also have the option of defining elements embedded in the root ele- ment. This helps to collect all profiled bean definitions into a single XML file, as shown next. elements can be repeated to specify multiple profiles “dev” profile beans “qa” profile beans 70 CHAPTER 3 Advanced wiring p:initialSize="20" p:maxActive="30" /> Aside from the fact that all these beans are now defined in the same XML file, the effect is the same as if they were defined in separate XML files. There are three beans, all of type javax.sql.DataSource and all with an ID of dataSource. But at runtime, only one bean will be created, depending on which profile is active. That raises the question: how do you make a profile active? 3.1.2 Activating profiles Spring honors two separate properties when determining which profiles are active: spring.profiles.active and spring.profiles.default. If spring.profiles.active is set, then its value determines which profiles are active. But if spring .profiles.active isn’t set, then Spring looks to spring.profiles.default. If neither spring.profiles.active nor spring.profiles.default is set, then there are no active profiles, and only those beans that aren’t defined as being in a profile are created. There are several ways to set these properties:  As initialization parameters on DispatcherServlet  As context parameters of a web application  As JNDI entries  As environment variables  As JVM system properties  Using the @ActiveProfiles annotation on an integration test class I’ll leave it to you to choose the best combination of spring.profiles.active and spring.profiles.default to suit your needs. One approach that I like is to set spring.profiles.default to the development profile using parameters on DispatcherServlet and in the servlet context (for the sake of ContextLoaderListener). For example, a web application’s web.xml file might set spring.profiles.default as shown in the next listing. Listing 3.3 Setting default profiles in a web application’s web.xml file “prod” profile beans 71Environments and profiles contextConfigLocation /WEB-INF/spring/root-context.xml spring.profiles.default dev org.springframework.web.context.ContextLoaderListener appServlet org.springframework.web.servlet.DispatcherServlet spring.profiles.default dev 1 appServlet / With spring.profiles.default set this way, any developer can retrieve the applica- tion code from source control and run it using development settings (such as an embedded database) without any additional configuration. Then, when the application is deployed in a QA, production, or other environ- ment, the person responsible for deploying it can set spring.profiles.active using system properties, environment variables, or JNDI as appropriate. When spring .profiles.active is set, it doesn’t matter what spring.profiles.default is set to; the profiles set in spring.profiles.active take precedence. You’ve probably noticed that the word profiles is plural in spring.profiles.active and spring.profiles.default. This means you can activate multiple profiles at the same time by listing the profile names, separated by commas. Of course, it probably doesn’t make much sense to enable both dev and prod profiles at the same time, but you could enable multiple orthogonal profiles simultaneously. TESTING WITH PROFILES When running an integration test, you’ll often want to test using the same configura- tion (or some subset thereof) you’d use in production. But if your configuration Set default profile for context Set default profile for servlet 72 CHAPTER 3 Advanced wiring references beans that are in profiles, you need a way to enable the appropriate profile when running those tests. Spring offers the @ActiveProfiles annotation to let you specify which profile(s) should be active when a test is run. Often it’s the development profile that you’ll want to activate during an integration test. For example, here’s a snippet of a test class that uses @ActiveProfiles to activate the dev profile: @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration(classes={PersistenceTestConfig.class}) @ActiveProfiles("dev") public class PersistenceTest { ... } Spring profiles are a great way to conditionally define beans where the condition is based on which profile is active. But Spring 4 offers a more general-purpose mecha- nism for conditional bean definitions where the condition is up to you. Let’s see how to define conditional beans using Spring 4 and the @Conditional annotation. 3.2 Conditional beans Suppose you want one or more beans to be configured if and only if some library is available in the application’s classpath. Or let’s say you want a bean to be created only if a certain other bean is also declared. Maybe you want a bean to be created if and only if a specific environment variable is set. Until Spring 4, it was difficult to achieve this level of conditional configuration, but Spring 4 introduced a new @Conditional annotation that can be applied to @Bean methods. If the prescribed condition evaluates to true, then the bean is created. Oth- erwise the bean is ignored. For example, suppose you have a class named MagicBean that you only want Spring to instantiate if a magic environment property has been set. If the environment has no such property, then the MagicBean should be ignored. The following listing shows a configuration that conditionally configures the MagicBean using @Conditional. @Bean @Conditional(MagicExistsCondition.class) public MagicBean magicBean() { return new MagicBean(); } As you can see, @Conditional is given a Class that specifies the condition—in this case, MagicExistsCondition. @Conditional comes paired with a Condition interface: public interface Condition { boolean matches(ConditionContext ctxt, AnnotatedTypeMetadata metadata); } Listing 3.4 Conditionally configuring a bean Conditionally create bean 73Conditional beans The class given to @Conditional can be any type that implements the Condition interface. As you can see, it’s a straightforward interface to implement, requiring only that you provide an implementation for the matches() method. If the matches() method returns true, then the @Conditional-annotated beans are created. If matches() returns false, then those beans aren’t created. For this example, you need to create an implementation of Condition that hinges its decision on the presence of a magic property in the environment. The next listing shows MagicExistsCondition, an implementation of Condition that does the trick. package com.habuma.restfun; import org.springframework.context.annotation.Condition; import org.springframework.context.annotation.ConditionContext; import org.springframework.core.type.AnnotatedTypeMetadata; import org.springframework.util.ClassUtils; public class MagicExistsCondition implements Condition { public boolean matches( ConditionContext context, AnnotatedTypeMetadata metadata) { Environment env = context.getEnvironment(); return env.containsProperty("magic"); } } The matches() method in this listing is simple but powerful. It uses the Environment obtained from the given ConditionContext object to check for the presence of an environment property named magic. For this example, the value of the property is irrelevant; it only needs to exist. This results in true being returned from matches(). Consequently, the condition is met, and any beans whose @Conditional annotation refers to MagicExistsCondition will be created. On the other hand, if the property doesn’t exist, the condition will fail, false will be returned from matches(), and none of those beans will be created. MagicExistsCondition only uses the Environment from the ConditionContext, but there’s much more that a Condition implementation can consider. The matches() method is given a ConditionContext and an AnnotatedTypeMetadata to use in making its decision. ConditionContext is an interface that looks something like this: public interface ConditionContext { BeanDefinitionRegistry getRegistry(); ConfigurableListableBeanFactory getBeanFactory(); Environment getEnvironment(); ResourceLoader getResourceLoader(); ClassLoader getClassLoader(); } Listing 3.5 Checking for the presence of magic in a Condition Check for “magic” property 74 CHAPTER 3 Advanced wiring From the ConditionContext, you can do the following:  Check for bean definitions via the BeanDefinitionRegistry returned from getRegistry().  Check for the presence of beans, and even dig into bean properties via the ConfigurableListableBeanFactory returned from getBeanFactory().  Check for the presence and values of environment variables via the Environment retrieved from getEnvironment().  Read and inspect the contents of resources loaded via the ResourceLoader returned from getResourceLoader().  Load and check for the presence of classes via the ClassLoader returned from getClassLoader(). As for the AnnotatedTypeMetadata, it offers you a chance to inspect annotations that may also be placed on the @Bean method. Like ConditionContext, Annotated- TypeMetadata is an interface. It looks like this: public interface AnnotatedTypeMetadata { boolean isAnnotated(String annotationType); Map getAnnotationAttributes(String annotationType); Map getAnnotationAttributes( String annotationType, boolean classValuesAsString); MultiValueMap getAllAnnotationAttributes( String annotationType); MultiValueMap getAllAnnotationAttributes( String annotationType, boolean classValuesAsString); } Using the isAnnotated() method, you can check to see if the @Bean method is anno- tated with any particular annotation type. Using the other methods, you can check on the attributes of any annotation applied to the @Bean method. Interestingly, starting with Spring 4, the @Profile annotation has been refactored to be based on @Conditional and the Condition interface. As another example of how to work with @Conditional and Condition, let’s look at how @Profile is imple- mented in Spring 4. The @Profile annotation looks like this: @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE, ElementType.METHOD}) @Documented @Conditional(ProfileCondition.class) public @interface Profile { String[] value(); } Notice that @Profile is itself annotated with @Conditional and refers to Profile- Condition as the Condition implementation. As shown next, ProfileCondition implements Condition and considers several factors from both ConditionContext and AnnotatedTypeMetadata in making its decision. 75Addressing ambiguity in autowiring class ProfileCondition implements Condition { public boolean matches( ConditionContext context, AnnotatedTypeMetadata metadata) { if (context.getEnvironment() != null) { MultiValueMap attrs = metadata.getAllAnnotationAttributes(Profile.class.getName()); if (attrs != null) { for (Object value : attrs.get("value")) { if (context.getEnvironment() .acceptsProfiles(((String[]) value))) { return true; } } return false; } } return true; } } As you can see, ProfileCondition fetches all the annotation attributes for the @Profile annotation from AnnotatedTypeMetadata. With that, it checks explicitly for the value attribute, which contains the name of the bean’s profile. It then consults with the Environment retrieved from the ConditionContext to see whether the pro- file is active (by calling the acceptsProfiles() method). 3.3 Addressing ambiguity in autowiring In chapter 2, you saw how to use autowiring to let Spring do all the work when inject- ing bean references into constructor arguments or properties. Autowiring is a huge help because it reduces the amount of explicit configuration necessary to assemble application components. But autowiring only works when exactly one bean matches the desired result. When there’s more than one matching bean, the ambiguity prevents Spring from autowiring the property, constructor argument, or method parameter. To illustrate autowiring ambiguity, suppose you’ve annotated the following set- Dessert() method with @Autowired: @Autowired public void setDessert(Dessert dessert) { this.dessert = dessert; } In this example, Dessert is an interface and is implemented by three classes: Cake, Cookies, and IceCream: @Component public class Cake implements Dessert { ... } @Component public class Cookies implements Dessert { ... } Listing 3.6 ProfileCondition checking whether a bean profile is acceptable 76 CHAPTER 3 Advanced wiring @Component public class IceCream implements Dessert { ... } Because all three implementations are annotated by @Component, they’re all picked up during component-scanning and created as beans in the Spring application context. Then, when Spring tries to autowire the Dessert parameter in setDessert(), it doesn’t have a single, unambiguous choice. Although most people wouldn’t have any problem making choices when faced with multiple dessert options, Spring can’t choose. Spring has no option but to fail and throw an exception. To be precise, Spring throws a NoUniqueBeanDefinitionException: nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.desserteater.Dessert] is defined: expected single matching bean but found 3: cake,cookies,iceCream Of course, this dessert-eating example is contrived to illustrate how autowiring can run into trouble with ambiguity. In reality, autowiring ambiguity is more rare than you’d expect. Even though such ambiguity is a real problem, more often than not there’s only one implementation of a given type, and autowiring works perfectly. For those times when ambiguity does happen, however, Spring offers a couple of options. You can declare one of the candidate beans as the primary choice, or you can use qualifiers to help Spring narrow its choices to a single candidate. 3.3.1 Designating a primary bean If you’re like me, you enjoy all kinds of desserts. Cake … cookies … ice cream … it’s all good. But if you were forced to choose only a single dessert, which is your favorite? When declaring beans, you can avoid autowiring ambiguity by designating one of the candidate beans as a primary bean. In the event of any ambiguity, Spring will choose the primary bean over any other candidate beans. Essentially, you’re declaring your “favorite” bean. Let’s say that ice cream is your favorite dessert. You can express that favorite choice in Spring using the @Primary annotation. @Primary can be used either alongside @Component for beans that are component-scanned or alongside @Bean for beans declared in Java configuration. For example, here’s how you might declare the @Component-annotated IceCream bean as the primary choice: @Component @Primary public class IceCream implements Dessert { ... } Or, if you’re declaring the IceCream bean explicitly in Java configuration, the @Bean method might look like this: @Bean @Primary public Dessert iceCream() { return new IceCream(); } 77Addressing ambiguity in autowiring If you’re configuring your beans in XML, you’re not left out. The element has a primary attribute to specify a primary bean: No matter how you designate a primary bean, the effect is the same. You’re telling Spring that it should choose the primary bean in the case of ambiguity. This works well right up to the point where you designate two or more primary beans. For example, suppose the Cake class looks like this: @Component @Primary public class Cake implements Dessert { ... } Now there are two primary Dessert beans: Cake and IceCream. This poses a new ambi- guity issue. Just as Spring couldn’t choose among multiple candidate beans, it can’t choose among multiple primary beans. Clearly, when more than one bean is desig- nated as primary, there are no primary candidates. For a more powerful ambiguity-busting mechanism, let’s look at qualifiers. 3.3.2 Qualifying autowired beans The limitation of primary beans is that @Primary doesn’t limit the choices to a single unambiguous option. It only designates a preferred option. When there’s more than one primary, there’s not much else you can do to narrow the choices further. In contrast, Spring’s qualifiers apply a narrowing operation to all candidate beans, ultimately arriving at the single bean that meets the prescribed qualifications. If ambi- guity still exists after applying all qualifiers, you can always apply more qualifiers to narrow the choices further. The @Qualifier annotation is the main way to work with qualifiers. It can be applied alongside @Autowired or @Inject at the point of injection to specify which bean you want to be injected. For example, let’s say you want to ensure that the IceCream bean is injected into setDessert(): @Autowired @Qualifier("iceCream") public void setDessert(Dessert dessert) { this.dessert = dessert; } This is a prime example of qualifiers in their simplest form. The parameter given to @Qualifier is the ID of the bean that you want to inject. All @Component-annotated classes will be created as beans whose ID is the uncapitalized class name. Therefore, @Qualifier("iceCream") refers to the bean created when component-scanning cre- ated an instance of the IceCream class. Actually, there’s a bit more to the story than that. To be more precise, @Qualifier ("iceCream") refers to the bean that has the String “iceCream” as a qualifier. For 78 CHAPTER 3 Advanced wiring lack of having specified any other qualifiers, all beans are given a default qualifier that’s the same as their bean ID. Therefore, the setDessert() method will be injected with the bean that has “iceCream” as a qualifier. That just happens to be the bean whose ID is iceCream, created when the IceCream class was component-scanned. Basing qualification on the default bean ID qualifier is simple but can pose some problems. What do you suppose would happen if you refactored the IceCream class, renaming it Gelato? In that case, the bean’s ID and default qualifier would be gelato, which doesn’t match the qualifier on setDessert(). Autowiring would fail. The problem is that you specified a qualifier on setDessert() that is tightly cou- pled to the class name of the bean being injected. Any change to that class name will render the qualifier ineffective. CREATING CUSTOM QUALIFIERS Instead of relying on the bean ID as the qualifier, you can assign your own qualifier to a bean. All you need to do is place the @Qualifier annotation on the bean declara- tion. For example, it can be applied alongside @Component like this: @Component @Qualifier("cold") public class IceCream implements Dessert { ... } In this case, a qualifier of cold is assigned to the IceCream bean. Because it’s not cou- pled to the class name, you can refactor the name of the IceCream class all you want without worrying about breaking autowiring. It will work as long as you refer to the cold qualifier at the injection point: @Autowired @Qualifier("cold") public void setDessert(Dessert dessert) { this.dessert = dessert; } It’s worth noting that @Qualifier can also be used alongside the @Bean annotation when explicitly defining beans with Java configuration: @Bean @Qualifier("cold") public Dessert iceCream() { return new IceCream(); } When defining custom @Qualifier values, it’s a good practice to use a trait or descrip- tive term for the bean, rather than using an arbitrary name. In this case, I’ve described the IceCream bean as a “cold” bean. At the injection point, it reads as “give me the cold dessert,” which happens to describe IceCream. Similarly, I might describe Cake as “soft” and Cookies as “crispy.” 79Addressing ambiguity in autowiring DEFINING CUSTOM QUALIFIER ANNOTATIONS Trait-oriented qualifiers are better than those based on the bean ID. But they still run into trouble when you have multiple beans that share common traits. For example, imagine what would happen if you introduced this new Dessert bean: @Component @Qualifier("cold") public class Popsicle implements Dessert { ... } Oh no! Now you have two “cold” desserts. Once again you’re faced with ambiguity in autowiring dessert beans. You need more qualifiers to narrow the selection to a single bean. Perhaps the solution is to tack on another @Qualifier at both the injection point and at the bean definition. Maybe the IceCream class could look like this: @Component @Qualifier("cold") @Qualifier("creamy") public class IceCream implements Dessert { ... } Perhaps the Popsicle class could also use another @Qualifier: @Component @Qualifier("cold") @Qualifier("fruity") public class Popsicle implements Dessert { ... } And at the injection point, you could narrow it down to IceCream like this: @Autowired @Qualifier("cold") @Qualifier("creamy") public void setDessert(Dessert dessert) { this.dessert = dessert; } There’s only one small problem: Java doesn’t allow multiple annotations of the same type to be repeated on the same item.1 The compiler will complain with errors if you try this. There’s no way you can use @Qualifier (at least not directly) to narrow the list of autowiring candidates to a single choice. What you can do, however, is create custom qualifier annotations to represent the traits you want your beans to be qualified with. All you have to do is create an annota- tion that is itself annotated with @Qualifier. Rather than use @Qualifier("cold"), you can use a custom @Cold annotation that’s defined like this: @Target({ElementType.CONSTRUCTOR, ElementType.FIELD, ElementType.METHOD, ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface Cold { } 1 Java 8 allows repeated annotations, as long as the annotation is annotated with @Repeatable. Even so, Spring’s @Qualifier annotation isn’t annotated with @Repeatable. 80 CHAPTER 3 Advanced wiring Likewise, you can create a new @Creamy annotation as a replacement for @Qualifier ("creamy"): @Target({ElementType.CONSTRUCTOR, ElementType.FIELD, ElementType.METHOD, ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface Creamy { } And, similarly, you can create @Soft, @Crispy, and @Fruity annotations to use wher- ever you’d otherwise use the @Qualifier annotation. By annotating these annotations with @Qualifier, they take on the characteristics of @Qualifier. They are, in fact, qualifier annotations in their own right. Now you can revisit IceCream and annotate it with @Cold and @Creamy, like this: @Component @Cold @Creamy public class IceCream implements Dessert { ... } Similarly, the Popsicle class can be annotated with @Cold and @Fruity: @Component @Cold @Fruity public class Popsicle implements Dessert { ... } Finally, at the injection point, you can use any combination of qualifier annotations necessary to narrow the selection to the one bean that meets your specifications. To arrive at the IceCream bean, the setDessert() method can be annotated like this: @Autowired @Cold @Creamy public void setDessert(Dessert dessert) { this.dessert = dessert; } By defining custom qualifier annotations, you’re able to use multiple qualifiers together with no limitations or complaints from the Java compiler. Also, your custom annotations are more type-safe than using the raw @Qualifier annotation and specify- ing the qualifier as a String. Take a closer look at the setDessert() method and how it’s annotated. Nowhere do you explicitly say that you want that method to be autowired with the IceCream bean. Instead, you identify the desired bean by its traits, @Cold and @Creamy. Thus set- Dessert() remains decoupled from any specific Dessert implementation. Any bean that satisfies those traits will do fine. It just so happens that in your current selection of Dessert implementations, the IceCream bean is the single matching candidate. In this section and the previous section, we explored a couple of ways to extend Spring with custom annotations. To create a custom conditional annotation, you create a new annotation and annotate it with @Conditional. And to create a custom qualifier annotation, you can create a new annotation and annotate it with @Qualifier. This 81Scoping beans technique can be applied using many of Spring’s annotations, composing them into custom special-purpose annotations. Now let’s take a moment to see how you can declare beans to be created in differ- ent scopes. 3.4 Scoping beans By default, all beans created in the Spring application context are created as single- tons. That is to say, no matter how many times a given bean is injected into other beans, it’s always the same instance that is injected each time. Most of the time, singleton beans are ideal. The cost of instantiating and garbage- collecting instances of objects that are only used for small tasks can’t be justified when an object is stateless and can be reused over and over again in an application. But sometimes you may find yourself working with a mutable class that does main- tain some state and therefore isn’t safe for reuse. In that case, declaring the class as a singleton bean probably isn’t a good idea because that object can be tainted and cre- ate unexpected problems when reused later. Spring defines several scopes under which a bean can be created, including the following:  Singleton—One instance of the bean is created for the entire application.  Prototype—One instance of the bean is created every time the bean is injected into or retrieved from the Spring application context.  Session—In a web application, one instance of the bean is created for each session.  Request—In a web application, one instance of the bean is created for each request. Singleton scope is the default scope, but as we’ve discussed, it isn’t ideal for mutable types. To select an alternative type, you can use the @Scope annotation, either in con- junction with the @Component annotation or with the @Bean annotation. For example, if you’re relying on component-scanning to discover and declare a bean, then you can annotate the bean class with @Scope to make it a prototype bean: @Component @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE) public class Notepad { ... } Here, you specify prototype scope by using the SCOPE_PROTOTYPE constant from the ConfigurableBeanFactory class. You could also use @Scope("prototype"), but using the SCOPE_PROTOTYPE constant is safer and less prone to mistakes. Alternatively, if you’re configuring the Notepad bean as a prototype in Java config- uration, you can use @Scope along with @Bean to specify the desired scoping: @Bean @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE) public Notepad notepad() { return new Notepad(); } 82 CHAPTER 3 Advanced wiring And, in the event that you’re configuring the bean in XML, you can set the scope using the scope attribute of the element: Regardless of how you specify prototype scope, an instance of the bean will be created each and every time it’s injected into or retrieved from the Spring application context. Consequently, everyone gets their own instance of Notepad. 3.4.1 Working with request and session scope In a web application, it may be useful to instantiate a bean that’s shared within the scope of a given request or session. For instance, in a typical e-commerce application, you may have a bean that represents the user’s shopping cart. If the shopping cart bean is a singleton, then all users will be adding products to the same cart. On the other hand, if the shopping cart is prototype-scoped, then products added to the cart in one area of the application may not be available in another part of the application where a different prototype-scoped shopping cart was injected. In the case of a shopping cart bean, session scope makes the most sense, because it’s most directly attached to a given user. To apply session scope, you can use the @Scope annotation in a way similar to how you specified prototype scope: @Component @Scope( value=WebApplicationContext.SCOPE_SESSION, proxyMode=ScopedProxyMode.INTERFACES) public ShoppingCart cart() { ... } Here you’re setting the value attribute to the SCOPE_SESSION constant from Web- ApplicationContext (which has a value of session). This tells Spring to create an instance of the ShoppingCart bean for each session in a web application. There will be multiple instances of the ShoppingCart bean, but only one will be created for a given session and it will essentially be a singleton as far as that session is concerned. Notice that @Scope also has a proxyMode attribute set to ScopedProxyMode.INTERFACES. This attribute addresses a problem encountered when injecting a session- or request- scoped bean into a singleton-scoped bean. But before I explain proxyMode, let’s look at a scenario that presents the problem that proxyMode addresses. Suppose you want to inject the ShoppingCart bean into the following setter method on a singleton StoreService bean: @Component public class StoreService { @Autowired public void setShoppingCart(ShoppingCart shoppingCart) { this.shoppingCart = shoppingCart; } ... } 83Scoping beans Because StoreService is a singleton bean, it will be created as the Spring application context is loaded. As it’s created, Spring will attempt to inject ShoppingCart into the setShoppingCart() method. But the ShoppingCart bean, being session scoped, doesn’t exist yet. There won’t be an instance of ShoppingCart until a user comes along and a session is created. Moreover, there will be many instances of ShoppingCart: one per user. You don’t want Spring to inject just any single instance of ShoppingCart into StoreService. You want StoreService to work with the ShoppingCart instance for whichever session happens to be in play when StoreService needs to work with the shopping cart. Instead of injecting the actual ShoppingCart bean into StoreService, Spring should inject a proxy to the ShoppingCart bean, as illustrated in listing 3.2. This proxy will expose the same methods as ShoppingCart so that for all StoreService knows, it is the shopping cart. But when StoreService calls methods on ShoppingCart, the proxy will lazily resolve it and delegate the call to the actual session-scoped Shopping- Cart bean. See figure 3.1. Now let’s take this understanding of scoped proxies and discuss the proxyMode attribute. As configured, proxyMode is set to ScopedProxyMode.INTERFACES, indicat- ing that the proxy should implement the ShoppingCart interface and delegate to the implementation bean. This is fine (and the most ideal proxy mode) as long as ShoppingCart is an inter- face and not a class. But if ShoppingCart is a concrete class, there’s no way Spring can create an interface-based proxy. Instead, it must use CGLib to generate a class-based proxy. So, if the bean type is a concrete class, you must set proxyMode to ScopedProxy- Mode.TARGET_CLASS to indicate that the proxy should be generated as an extension of the target class. Although I’ve focused on session scope, know that request-scoped beans pose the same wiring challenges as session-scoped beans. Therefore, request-scoped beans should also be injected as scoped proxies. ... Singleton bean Injected into Delegates to Delegates to Delegates to Delegates to Session/request- scoped bean Interf ace Session/request- scoped bean Interf ace Session/request- scoped bean Interf ace Session/request- scoped bean Interf ace Scoped proxy Interf ace Figure 3.1 Scoped proxies enable deferred injection of request- and session-scoped beans. 84 CHAPTER 3 Advanced wiring 3.4.2 Declaring scoped proxies in XML If you’re declaring your session-scoped or request-scoped beans in XML, then you can’t use the @Scope annotation or its proxyMode attribute. The scope attribute of the element lets you set the bean scope, but how can you specify the proxy mode? To set the proxy mode, you must use a new element from Spring’s aop namespace: is the Spring XML configuration’s counterpart to the @Scope annotation’s proxyMode attribute. It tells Spring to create a scoped proxy for the bean. By default, it uses CGLib to create a target class proxy. But you can ask it to generate an interface-based proxy by setting the proxy-target-class attribute to false: In order to use the element, you must declare Spring’s aop namespace in your XML configuration: ... We’ll talk more about Spring’s aop namespace in chapter 4 when you work with Spring and aspect-oriented programming. For now, let’s wrap up this chapter by look- ing at one more of Spring’s advanced wiring options: Spring Expression Language. 3.5 Runtime value injection When we talk about dependency injection and wiring, we’re often talking about wir- ing a bean reference into a property or constructor argument of another bean. It’s often about associating one object with another object. But another side to bean wiring is when you wire a value into a bean property or into its constructor as an argument. You did a lot of value wiring in chapter 2, such as 85Runtime value injection wiring the name of an album into the constructor or title property of a BlankDisc bean. For example, you might have wired up a BlankDisc like this: @Bean public CompactDisc sgtPeppers() { return new BlankDisc( "Sgt. Pepper's Lonely Hearts Club Band", "The Beatles"); } Although this accomplished what you needed, setting the title and artist for the BlankDisc bean, it did so with values hard-coded in the configuration class. Likewise, if you had done this in XML, the values would have also been hard-coded: Sometimes hard-coded values are fine. Other times, however, you may want to avoid hard-coded values and let the values be determined at runtime. For those cases, Spring offers two ways of evaluating values at runtime:  Property placeholders  The Spring Expression Language (SpEL) You’ll soon see that the application of these two techniques is similar, although their purposes and behavior are different. Let’s start with a look at property placeholders, the simpler of the two, and then dig into the more powerful SpEL. 3.5.1 Injecting external values The simplest way to resolve external values in Spring is to declare a property source and retrieve the properties via the Spring Environment. For example, the following listing shows a basic Spring configuration class that uses external properties to wire up a BlankDisc bean. package com.soundsystem; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.PropertySource; import org.springframework.core.env.Environment; @Configuration @PropertySource("classpath:/com/soundsystem/app.properties") public class ExpressiveConfig { @Autowired Environment env; @Bean Listing 3.7 Using the @PropertySource annotation and Environment Declare a property source 86 CHAPTER 3 Advanced wiring public BlankDisc disc() { return new BlankDisc( env.getProperty("disc.title"), env.getProperty("disc.artist")); } } In this example, @PropertySource references a file named app.properties in the class- path. It might look something like this: disc.title=Sgt. Peppers Lonely Hearts Club Band disc.artist=The Beatles This properties file is loaded into Spring’s Environment, from which it can be retrieved later. Meanwhile, in the disc() method, a new BlankDisc is created; its con- structor arguments are resolved from the properties file by calling getProperty(). DIGGING INTO SPRING’S ENVIRONMENT While we’re on the subject of Environment, you might find it helpful to know that the getProperty() method shown in listing 3.7 isn’t the only method you can use to fetch a property value. getProperty() is overloaded into four variations:  String getProperty(String key)  String getProperty(String key, String defaultValue)  T getProperty(String key, Class type)  T getProperty(String key, Class type, T defaultValue) The first two forms of getProperty() always return a String value. You saw how to use the first form in listing 3.7. But you can tweak the @Bean method slightly to work with default values if the specified properties don’t exist: @Bean public BlankDisc disc() { return new BlankDisc( env.getProperty("disc.title", "Rattle and Hum"), env.getProperty("disc.artist", "U2")); } The second two forms of getProperty() work much like the first two, but they recog- nize that not all values may be Strings. For example, suppose you’re retrieving a value representing the number of connections to maintain in a connection pool. If you receive a String value from the properties file, then you’ll need to convert it to an Integer before you can use it. But using one of the overloaded getProperty() meth- ods handles that conversion for you: int connectionCount = env.getProperty("db.connection.count", Integer.class, 30); A few more property-related methods are offered by Environment. If you use either of the getProperty() methods without specifying a default value, you’ll receive null if the property isn’t defined. If you want to require that the property be defined, you can use getRequiredProperty() like this: Retrieve property values 87Runtime value injection @Bean public BlankDisc disc() { return new BlankDisc( env.getRequiredProperty("disc.title"), env.getRequiredProperty("disc.artist")); } Here, if either the disc.title property or the disc.artist property is undefined, an IllegalStateException will be thrown. If you want to check for the existence of a property, you can call contains- Property() on Environment: boolean titleExists = env.containsProperty("disc.title"); Finally, if you need to resolve a property into a Class, you can use the getProperty- AsClass() method: Class cdClass = env.getPropertyAsClass("disc.class", CompactDisc.class); Digressing a bit from the subject of properties, Environment also offers some methods for checking which profiles are active:  String[] getActiveProfiles()—Returns an array of active profile names  String[] getDefaultProfiles()—Returns an array of default profile names  boolean acceptsProfiles(String... profiles)—Returns true if the envi- ronment supports the given profile(s) You saw how to use the acceptsProfiles() method in listing 3.6. In that case, Environment was retrieved from ConditionContext, and the acceptsProfiles() method was used to ensure that a given bean’s profile was in play before allowing the bean to be created. You often won’t need the profile-focused methods from Environment, but it’s good to know that they’re available. Retrieving properties directly from Environment is handy, especially when you’re wiring beans in Java configuration. But Spring also offers the option of wiring proper- ties with placeholder values that are resolved from a property source. RESOLVING PROPERTY PLACEHOLDERS Spring has always supported the option of externalizing properties into a properties file and then plugging them into Spring beans using placeholder values. In Spring wiring, placeholder values are property names wrapped with ${ ... }. As an exam- ple, you can resolve the constructor arguments for a BlankDisc in XML like this: As shown here, the title constructor argument is given a value that’s resolved from the property whose name is disc.title. And the artist argument is wired with the value of the property whose name is disc.artist. In this way, the XML configuration 88 CHAPTER 3 Advanced wiring doesn’t use any hard-coded values. Instead, the values are resolved from a source external to the configuration file. (We’ll talk about how those properties are resolved in a moment.) When relying on component-scanning and autowiring to create and initialize your application components, there’s no configuration file or class where you can specify the placeholders. Instead, you can use the @Value annotation in much the same way as you might use the @Autowired annotation. In the BlankDisc class, for example, the constructor might be written like this: public BlankDisc( @Value("${disc.title}") String title, @Value("${disc.artist}") String artist) { this.title = title; this.artist = artist; } In order to use placeholder values, you must configure either a PropertyPlaceholder- Configurer bean or a PropertySourcesPlaceholderConfigurer bean. Starting with Spring 3.1, PropertySourcesPlaceholderConfigurer is preferred because it resolves placeholders against the Spring Environment and its set of property sources. The following @Bean method configures PropertySourcesPlaceholderConfigurer in Java configuration: @Bean public static PropertySourcesPlaceholderConfigurer placeholderConfigurer() { return new PropertySourcesPlaceholderConfigurer(); } If you’d rather use XML configuration, the ele- ment from Spring’s context namespace will give you a PropertySourcesPlaceholder- Configurer bean: Resolving external properties is one way to defer value resolution until runtime, but its focus is finely tuned on resolving properties, by name, from Spring’s Environment and property sources. Spring Expression Language, on the other hand, offers a more general way of calculating values for injection at runtime. 89Runtime value injection 3.5.2 Wiring with the Spring Expression Language Spring 3 introduced Spring Expression Language (SpEL), a powerful yet succinct way of wiring values into a bean’s properties or constructor arguments using expressions that are evaluated at runtime. Using SpEL, you can pull off amazing feats of bean wiring that would be much more difficult (or in some cases impossible) using other wiring techniques. SpEL has a lot of tricks up its sleeves, including the following:  The ability to reference beans by their IDs  Invoking methods and accessing properties on objects  Mathematical, relational, and logical operations on values  Regular expression matching  Collection manipulation As you’ll see later in this book, SpEL can also be used for purposes other than depen- dency injection. Spring Security, for example, supports defining security constraints using SpEL expressions. And if you’re using Thymeleaf templates as the views in your Spring MVC application, those templates can use SpEL expressions to reference model data. To get started, let’s consider a few examples of SpEL expressions and see how to wire them into beans. Then we’ll take a deeper dive into some of SpEL’s primitive expressions that can be pieced together into more powerful expressions. A FEW SPEL EXAMPLES SpEL is such a flexible expression language that it would be impossible to show you all the ways it can be used in the space allowed in this book. But there is enough room to show you a few basic examples from which you can draw inspiration for your own expressions. The first thing to know is that SpEL expressions are framed with #{ ... }, much as property placeholders are framed with ${ ... }. What follows is possibly one of the simplest SpEL expressions you can write: #{1} Stripping away the #{ ... } markers, what’s left is the body of a SpEL expression, which is a numeric constant. It probably won’t surprise you much to learn that this expression evaluates to the numeric value of 1. Of course, you’re not likely to use such a simple expression in a real application. You’re more likely to build up more interesting expressions, such as this one: #{T(System).currentTimeMillis()} Ultimately this expression evaluates to the current time in milliseconds at the moment when the expression is evaluated. The T() operator evaluates java.lang.System as a type so that the staticcurrentTimeMillis() method can be invoked. 90 CHAPTER 3 Advanced wiring SpEL expressions can also refer to other beans or properties on those beans. For example, the following expression evaluates to the value of the artist property on a bean whose ID is sgtPeppers: #{sgtPeppers.artist} You can also refer to system properties via the systemProperties object: #{systemProperties['disc.title']} These are just a few basic examples of SpEL. You’ll see more before this chapter ends. But first, let’s consider how you might use these expressions during bean wiring. When injecting properties and constructor arguments on beans that are created via component-scanning, you can use the @Value annotation, much as you saw earlier with property placeholders. Rather than use a placeholder expression, however, you use a SpEL expression. For example, here’s what the BlankDisc constructor might look like, drawing the album title and artist from system properties: public BlankDisc( @Value("#{systemProperties['disc.title']}") String title, @Value("#{systemProperties['disc.artist']}") String artist) { this.title = title; this.artist = artist; } In XML configuration, you can pass in the SpEL expression to the value attribute of or , or as the value given to a p-namespace or c- namespace entry. For example, here’s the XML declaration of the BlankDisc bean that has its constructor arguments set from a SpEL expression: Now that we’ve looked at a few simple examples and how to inject values resolved from SpEL expressions, let’s go over some of the primitive expressions supported in SpEL. EXPRESSING LITERAL VALUES You’ve already seen an example of using SpEL to express a literal integer value. But it can also be used for floating-point numbers, String values, and Boolean values. Here’s an example of a SpEL expression that is a floating-point value: #{3.14159} Numbers can also be expressed in scientific notation. For example, the following expression evaluates to 98,700: #{9.87E4} A SpEL expression can also evaluate literal String values, such as 91Runtime value injection #{'Hello'} Finally, Boolean literals true and false are evaluated to their Boolean value. For example, #{false} Working with literal values in SpEL is mundane. After all, you don’t need SpEL to set an integer property to 1 or a Boolean property to false. I admit there’s not much use in SpEL expressions that only contain literal values. But remember that more interest- ing SpEL expressions are composed of simpler expressions, so it’s good to know how to work with literal values in SpEL. You’ll eventually need them as you compose more complex expressions. REFERENCING BEANS, PROPERTIES, AND METHODS Another basic thing that a SpEL expression can do is reference another bean by its ID. For example, you could use SpEL to wire one bean into another bean’s property by using the bean ID as the SpEL expression (in this case, a bean whose ID is sgtPeppers): #{sgtPeppers} Now let’s say that you want to refer to the artist property of the sgtPeppers bean in an expression: #{sgtPeppers.artist} The first part of the expression body refers to the bean whose ID is sgtPeppers. What follows the period delimiter is a reference to the artist property. In addition to referencing a bean’s properties, you can also call methods on a bean. For example, suppose you have another bean whose ID is artistSelector. You can call that bean’s selectArtist() method in a SpEL expression like this: #{artistSelector.selectArtist()} You can also call methods on the value returned from the invoked method. For exam- ple, if selectArtist() returns a String, you can call toUpperCase() to make the entire artist name uppercase lettering: #{artistSelector.selectArtist().toUpperCase()} This will work fine, as long as selectArtist() doesn’t return null. To guard against a NullPointerException, you can use the type-safe operator: #{artistSelector.selectArtist()?.toUpperCase()} Instead of a lonely dot (.) to access the toUpperCase() method, now you’re using the ?. operator. This operator makes sure the item to its left isn’t null before access- ing the thing on its right. So, if selectArtist() returns null, then SpEL won’t even try to invoke toUpperCase(). The expression will evaluate to null. 92 CHAPTER 3 Advanced wiring WORKING WITH TYPES IN EXPRESSIONS The key to working with class-scoped methods and constants in SpEL is to use the T() operator. For example, to express Java’s Math class in SpEL, you need to use the T() operator like this: T(java.lang.Math) The result of the T() operator, as shown here, is a Class object that represents java.lang.Math. You can even wire it into a bean property of type Class, if you want. But the real value of the T() operator is that it gives you access to static methods and constants on the evaluated type. For example, suppose you need to wire the value of pi into a bean property. The following SpEL expression does the trick: T(java.lang.Math).PI Similarly, static methods can be invoked in the type resolved with the T() operator. You’ve seen an example of using T() to make a call to System.currentTimeMillis(). Here’s another example that evaluates to a random value between 0 and 1: T(java.lang.Math).random() SPEL OPERATORS SpEL offers several operators that you can apply on values in SpEL expressions. Table 3.1 summarizes these operators. As a simple example of using one of these operators, consider the following SpEL expression: #{2 * T(java.lang.Math).PI * circle.radius} Not only is this a great example of using SpEL’s multiplication operator (*), but it also shows how you can compose simpler expressions into a more complex expression. Here the value of pi is multiplied by 2, and that result is multiplied by the value of the radius property of a bean whose ID is circle. Essentially, it evaluates to the circum- ference of the circle defined in the circle bean. Table 3.1 SpEL operators for manipulating expression values Operator type Operators Arithmetic +, -, *, /, %, ^ Comparison <, lt, >, gt, ==, eq, <=, le, >=, ge Logical and, or, not, | Conditional ?: (ternary), ?: (Elvis) Regular expression matches 93Runtime value injection Similarly, you can use the carat symbol (^) in an expression to calculate a circle’s area: #{T(java.lang.Math).PI * circle.radius ^ 2} The carat symbol is the power-of operator. In this case, it’s used to calculate the square of the circle’s radius. When working with String values, the + operator performs concatenation, just as in Java: #{disc.title+'by'+disc.artist} SpEL also offers comparison operators for comparing values in an expression. Notice in table 3.1 that the comparison operators come in two forms: symbolic and textual. For the most part, the symbolic operators are equivalent to their textual counterparts, and you’re welcome to use whichever one suits you best. For example, to compare two numbers for equality, you can use the double-equal (==) operator: #{counter.total == 100} Or you can use the textual eq operator: #{counter.total eq 100} Either way, the result is the same. The expression evaluates to a Boolean: true if counter.total is equal to 100 or false if it’s not. SpEL also offers a ternary operator that works much like Java’s ternary operator. For example, the following expression evaluates to the String “Winner!” if scoreboard .score > 1000 or “Loser” if not: #{scoreboard.score > 1000 ? "Winner!" : "Loser"} A common use of the ternary operator is to check for a null value and offer a default value in place of the null. For example, the following expression evaluates to the value of disc.title if it isn’t null. If disc.title is null, then the expression evalu- ates to “Rattle and Hum”. #{disc.title ?: 'Rattle and Hum'} This expression is commonly referred to as the Elvis operator. This strange name comes from using the operator as an emoticon, where the question mark appears to form the shape of Elvis Presley’s hair style.2 EVALUATING REGULAR EXPRESSIONS When working with text, it’s sometimes useful to check whether that text matches a certain pattern. SpEL supports pattern matching in expressions with its matches oper- ator. The matches operator attempts to apply a regular expression (given as its right- side argument) against a String value (given as the left-side argument). The result of 2 Don’t blame me. I didn’t come up with that name. But you gotta admit—it does kinda look like Elvis’s hair. 94 CHAPTER 3 Advanced wiring a matches evaluation is a Boolean value: true if the value matches the regular expres- sion, and false otherwise. To demonstrate, suppose you want to check whether a String contains a valid email address. In that case, you can apply matches like this: #{admin.email matches '[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.com'} Exploring the mysteries of the enigmatic regular-expression syntax is outside the scope of this book. And I realize that the regular expression given here isn’t robust enough to cover all scenarios. But for the purposes of showing off the matches opera- tor, it’ll have to suffice. EVALUATING COLLECTIONS Some of SpEL’s most amazing tricks involve working with collections and arrays. The most basic thing you can do is reference a single element from a list: #{jukebox.songs[4].title} This evaluates to the title property of the fifth (zero-based) element from the songs collection property on the bean whose ID is jukebox. To spice things up a bit, I suppose you could randomly select a song from the jukebox: #{jukebox.songs[T(java.lang.Math).random() * jukebox.songs.size()].title} As it turns out, the [] operator used to fetch an indexed element from a collection or array can also be used to fetch a single character from a String. For example, #{'This is a test'[3]} This references the fourth (zero-based) character in the String, or s. SpEL also offers a selection operator (.?[]) to filter a collection into a subset of the collection. As a demonstration, suppose you want a list of all songs in the jukebox where the artist property is Aerosmith. The following expression uses the selection operator to arrive at the list of available Aerosmith songs: #{jukebox.songs.?[artist eq 'Aerosmith']} As you can see, the selection operator accepts another expression within its square brackets. As SpEL iterates over the list of songs, it evaluates that expression for each entry in the songs collection. If the expression evaluates to true, then the entry is car- ried over into the new collection. Otherwise it’s left out of the new collection. In this case, the inner expression checks to see if the song’s artist property equals Aerosmith. SpEL also offers two other selection operations: .^[] for selecting the first match- ing entry and .$[] for selecting the last matching entry. To demonstrate, consider this expression, which finds the first song in the list whose artist property is Aerosmith: #{jukebox.songs.^[artist eq 'Aerosmith']} 95Summary Finally, SpEL offers a projection operator (.![]) to project properties from the ele- ments in the collection onto a new collection. As an example, suppose you don’t want a collection of the song objects, but a collection of all the song titles. The following expression projects the title property into a new collection of Strings: #{jukebox.songs.![title]} Naturally, the projection operator can be combined with any of SpEL’s other opera- tors, including the selection operator. For example, you could use this expression to obtain a list of all of Aerosmith’s songs: #{jukebox.songs.?[artist eq 'Aerosmith'].![title]} We’ve only scratched the surface of what SpEL can do. There will be more opportunities to tinker with SpEL throughout this book, especially when defining security constraints. For now, however, let me wrap up this discussion of SpEL with a warning. SpEL expressions are handy and powerful ways to dynamically inject values into Spring beans. It can be tempting to get crafty and write very involved expressions. But take care not to get too clever with your expressions. The more clever your expressions become, the more important it will be to test them. Ultimately, SpEL expressions are given as String values and can be difficult to test. For that reason, I encourage you to keep your expressions simple so that testing isn’t as big a concern. 3.6 Summary We’ve covered a lot of ground in this chapter. In doing so, we’ve built on the founda- tional bean-wiring techniques explored in chapter 2 with some powerful advanced wiring tricks. We started by using Spring profiles to address a common problem where Spring beans must vary across deployment environments. By resolving environment-specific beans at runtime by matching them against one or more active profiles, Spring makes it possible to deploy the same deployment unit across multiple environments without rebuilding. Profiled beans are one way to conditionally create beans at runtime, but Spring 4 offers a more generic way to declare beans that are created (or not created) depend- ing on the outcome of a given condition. The @Conditional annotation, paired with an implementation of Spring’s Condition interface, offers developers a powerful and flexible mechanism for conditionally creating beans. We also looked at two techniques for resolving autowiring ambiguity: primary beans and qualifiers. Although designating a bean as a primary bean is simple, it’s also limited, so we discussed using qualifiers to narrow the list of autowire candidates to a single bean. In addition, you saw how to create custom qualifier annotations that describe a bean by its traits. Although most Spring beans are created as singletons, there are times when other creation strategies are more appropriate. Out of the box, Spring allows beans to be created as singletons, prototypes, request-scoped, or session-scoped. When declaring 96 CHAPTER 3 Advanced wiring request- or session-scoped beans, you also learned how to control the way scoped proxies are created, either as class-based proxies or interface-based proxies. Finally, we looked at the Spring Expression Language, which gives you a way to resolve values to be injected into bean properties at runtime. With a strong foundation in bean wiring established, we’ll now turn our attention to aspect-oriented programming (AOP). Much as dependency injection helps decouple components from the other components they collaborate with, AOP helps decouple your application components from tasks that span multiple components in an applica- tion. In the next chapter, we’ll dig into creating and working with aspects in Spring. 97 Aspect-oriented Spring As I’m writing this chapter, summertime is upon Texas (where I reside). And in Texas, it’s very common to go through several days of record-high temperatures. It’s hot. In weather like this, air conditioning is a must. But the downside of air con- ditioning is that it uses electricity, and electricity costs money. There’s little we can do to avoid paying for a cool and comfortable home. That’s because every home has a meter that measures every kilowatt, and once a month someone comes by to read that meter so that the electric company knows how much to bill us. Now imagine what would happen if the meter went away and nobody came by to measure our electricity usage. Suppose it was up to each homeowner to contact the electric company and report their electricity usage. Although it’s possible that some obsessive homeowners would keep careful records of how much they used their lights, televisions, and air conditioning, most wouldn’t bother. Electricity on the honor system might be great for consumers, but it would be less than ideal for the electric companies. This chapter covers  Basics of aspect-oriented programming  Creating aspects from POJOs  Using @AspectJ annotations  Injecting dependencies into AspectJ aspects 98 CHAPTER 4 Aspect-oriented Spring Monitoring electricity consumption is an important function, but it isn’t foremost in most homeowners’ minds. Mowing the lawn, vacuuming the carpet, and cleaning the bathroom are the kinds of things that homeowners are actively involved in. Moni- toring the amount of electricity used by their house is a passive event from the home- owner’s point of view. (Although it’d be great if mowing the lawn was also a passive event—especially on these hot days.) Some functions of software systems are like the electric meters on our homes. The functions need to be applied at multiple points within the application, but it’s unde- sirable to explicitly call them at every point. Logging, security, and transaction man- agement are important, but should they be activities that your application objects are actively participating in? Or would it be better for your application objects to focus on the business domain problems they’re designed for, and to leave certain aspects to be handled by someone else? In software development, functions that span multiple points of an application are called cross-cutting concerns. Typically, these cross-cutting concerns are conceptually separate from (but often embedded directly within) the application’s business logic. Separating these cross-cutting concerns from the business logic is where aspect- oriented programming (AOP) goes to work. In chapter 2, you learned how to use dependency injection to manage and config- ure application objects. Whereas DI helps you decouple application objects from each other, AOP helps you decouple cross-cutting concerns from the objects they affect. Logging is a common example of the application of aspects, but it’s not the only thing aspects are good for. Throughout this book, you’ll see several practical applica- tions of aspects, including declarative transactions, security, and caching. This chapter explores Spring’s support for aspects, including how to declare regu- lar classes to be aspects and how to use annotations to create aspects. In addition, you’ll see how AspectJ—another popular AOP implementation—can complement Spring’s AOP framework. But first, before we get carried away with transactions, secu- rity, and caching, let’s see how aspects are implemented in Spring, starting with a primer on a few of AOP’s fundamentals. 4.1 What is aspect-oriented programming? As stated earlier, aspects help to modularize cross-cutting concerns. In short, a cross- cutting concern can be described as any functionality that affects multiple points of an application. Security, for example, is a cross-cutting concern, in that many methods in an application can have security rules applied to them. Figure 4.1 gives a visual depic- tion of cross-cutting concerns. This figure represents a typical application that’s broken down into modules. Each module’s main concern is to provide services for its particular domain. But each module also requires similar ancillary functionality, such as security and transaction management. 99What is aspect-oriented programming? A common object-oriented technique for reusing common functionality is to apply inheritance or delegation. But inheri- tance can lead to a brittle object hierarchy if the same base class is used throughout an application, and delegation can be cumber- some because complicated calls to the dele- gate object may be required. Aspects offer an alternative to inheri- tance and delegation that can be cleaner in many circumstances. With AOP, you still define the common functionality in one place, but you can declaratively define how and where this functionality is applied without having to modify the class to which you’re applying the new feature. Cross- cutting concerns can now be modularized into special classes called aspects. This has two benefits. First, the logic for each concern is in one place, as opposed to being scat- tered all over the code base. Second, your service modules are cleaner because they only contain code for their primary concern (or core functionality), and secondary concerns have been moved to aspects. 4.1.1 Defining AOP terminology Like most technologies, AOP has its own jargon. Aspects are often described in terms of advice, pointcuts, and join points. Figure 4.2 illustrates how these concepts are tied together. Unfortunately, many of the terms used to describe AOP features aren’t intuitive. Nevertheless, they’re now part of the AOP idiom, and in order to understand AOP, you must know these terms. Before you walk the walk, you have to learn to talk the talk. ADVICE When a meter reader shows up at your house, his purpose is to report the number of kilowatt hours back to the electric company. Sure, he has a list of houses that he must visit, and the information he reports is important. But the actual act of recording elec- tricity usage is the meter reader’s main job. Likewise, aspects have a purpose—a job they’re meant to do. In AOP terms, the job of an aspect is called advice. Advice defines both the what and the when of an aspect. In addition to describing the job that an aspect will perform, advice addresses the question of when to perform CourseService StudentService MiscService Security Transactions Other Figure 4.1 Aspects modularize cross- cutting concerns, applying logic that spans multiple application objects. Figure 4.2 An aspect’s functionality (advice) is woven into a program’s execution at one or more join points. 100 CHAPTER 4 Aspect-oriented Spring the job. Should it be applied before a method is invoked? After the method is invoked? Both before and after method invocation? Or should it be applied only if a method throws an exception? Spring aspects can work with five kinds of advice:  Before—The advice functionality takes place before the advised method is invoked.  After—The advice functionality takes place after the advised method completes, regardless of the outcome.  After-returning—The advice functionality takes place after the advised method successfully completes.  After-throwing—The advice functionality takes place after the advised method throws an exception.  Around—The advice wraps the advised method, providing some functionality before and after the advised method is invoked. JOIN POINTS An electric company services several houses, perhaps even an entire city. Each house has an electric meter that needs to be read, so each house is a potential target for the meter reader. The meter reader could potentially read all kinds of devices, but to do her job, she needs to target electric meters that are attached to houses. In the same way, your application may have thousands of opportunities for advice to be applied. These opportunities are known as join points. A join point is a point in the execution of the application where an aspect can be plugged in. This point could be a method being called, an exception being thrown, or even a field being modified. These are the points where your aspect’s code can be inserted into the normal flow of your application to add new behavior. POINTCUTS It’s not possible for any one meter reader to visit all houses serviced by the electric company. Instead, each one is assigned a subset of all the houses to visit. Likewise, an aspect doesn’t necessarily advise all join points in an application. Pointcuts help narrow down the join points advised by an aspect. If advice defines the what and when of aspects, then pointcuts define the where. A pointcut definition matches one or more join points at which advice should be woven. Often you specify these pointcuts using explicit class and method names or through regular expressions that define matching class and method name patterns. Some AOP frameworks allow you to create dynamic pointcuts that determine whether to apply advice based on runtime decisions, such as the value of method parameters. ASPECTS When a meter reader starts his day, he knows both what he’s supposed to do (report electricity usage) and which houses to collect that information from. Thus he knows everything he needs to know to get his job done. 101What is aspect-oriented programming? An aspect is the merger of advice and pointcuts. Taken together, advice and point- cuts define everything there is to know about an aspect—what it does and where and when it does it. INTRODUCTIONS An introduction allows you to add new methods or attributes to existing classes. For example, you could create an Auditable advice class that keeps the state of when an object was last modified. This could be as simple as having one method, setLast- Modified(Date), and an instance variable to hold this state. The new method and instance variable can then be introduced to existing classes without having to change them, giving them new behavior and state. WEAVING Weaving is the process of applying aspects to a target object to create a new proxied object. The aspects are woven into the target object at the specified join points. The weaving can take place at several points in the target object’s lifetime:  Compile time—Aspects are woven in when the target class is compiled. This requires a special compiler. AspectJ’s weaving compiler weaves aspects this way.  Class load time—Aspects are woven in when the target class is loaded into the JVM. This requires a special ClassLoader that enhances the target class’s byte- code before the class is introduced into the application. AspectJ 5’s load-time weaving (LTW) support weaves aspects this way.  Runtime—Aspects are woven in sometime during the execution of the applica- tion. Typically, an AOP container dynamically generates a proxy object that del- egates to the target object while weaving in the aspects. This is how Spring AOP aspects are woven. That’s a lot of new terms to get to know. Revisiting figure 4.1, you can now see how advice contains the cross-cutting behavior that needs to be applied to an application’s objects. The join points are all the points within the execution flow of the application that are candidates to have advice applied. The pointcut defines where (at what join points) that advice is applied. The key concept you should take from this is that point- cuts define which join points get advised. Now that you’re familiar with some basic AOP terminology, let’s see how these core AOP concepts are implemented in Spring. 4.1.2 Spring’s AOP support Not all AOP frameworks are created equal. They may differ in how rich their join point models are. Some allow you to apply advice at the field-modification level, whereas others only expose the join points related to method invocations. They may also differ in how and when they weave the aspects. Whatever the case, the ability to create pointcuts that define the join points at which aspects should be woven is what makes it an AOP framework. 102 CHAPTER 4 Aspect-oriented Spring Because this is a Spring book, we’ll focus on Spring AOP. Even so, there’s a lot of synergy between the Spring and AspectJ projects, and the AOP support in Spring bor- rows a lot from the AspectJ project. Spring’s support for AOP comes in four styles:  Classic Spring proxy-based AOP  Pure-POJO aspects  @AspectJ annotation-driven aspects  Injected AspectJ aspects (available in all versions of Spring) The first three styles are all variations on Spring’s own AOP implementation. Spring AOP is built around dynamic proxies. Consequently, Spring’s AOP support is limited to method interception. The term classic usually carries a good connotation. Classic cars, classic golf tourna- ments, and classic Coca-Cola are all good things. But Spring’s classic AOP program- ming model isn’t so great. Oh, it was good in its day. But now Spring supports much cleaner and easier ways to work with aspects. When held up against simple declarative AOP and annotation-based AOP, Spring’s classic AOP seems bulky and overcompli- cated. Therefore, I won’t be covering classic Spring AOP. With Spring’s aop namespace, you can turn pure POJOs into aspects. In truth, those POJOs will only supply methods that are called in reaction to a pointcut. Unfor- tunately, this technique requires XML configuration, but it’s an easy way to declara- tively turn any object into an aspect. Spring borrows AspectJ’s aspects to enable annotation-driven AOP. Under the cov- ers, it’s still Spring’s proxy-based AOP, but the programming model is almost identical to writing full-blown AspectJ annotated aspects. The perk of this AOP style is that it can be done without any XML configuration. If your AOP needs exceed simple method interception (constructor or property interception, for example), you’ll want to consider implementing aspects in AspectJ. In that case, the fourth style listed will enable you to inject values into AspectJ-driven aspects. We’ll explore more of these Spring AOP techniques in this chapter. But before we get started, it’s important to understand a few key points of Spring’s AOP framework. SPRING ADVICE IS WRITTEN IN JAVA All the advice you create in Spring is written in a standard Java class. That way, you get the benefit of developing your aspects in the same integrated development environ- ment (IDE) you’d use for normal Java development. The pointcuts that define where advice should be applied may be specified with annotations or configured in a Spring XML configuration, but either will be familiar to Java developers. Contrast this with AspectJ. Although AspectJ now supports annotation-based aspects, it also comes as a language extension to Java. This approach has benefits and drawbacks. By having an AOP-specific language, you get more power and fine-grained control, as well as a richer AOP toolset. But you’re required to learn a new tool and syntax to accomplish this. 103Selecting join points with pointcuts SPRING ADVISES OBJECTS AT RUNTIME In Spring, aspects are woven into Spring-managed beans at runtime by wrapping them with a proxy class. As illustrated in figure 4.3, the proxy class poses as the target bean, intercepting advised method calls and forwarding those calls to the target bean. Between the time when the proxy intercepts the method call and the time when it invokes the target bean’s method, the proxy performs the aspect logic. Spring doesn’t create a proxied object until that proxied bean is needed by the application. If you’re using an ApplicationContext, the proxied objects will be cre- ated when it loads all the beans from the BeanFactory. Because Spring creates prox- ies at runtime, you don’t need a special compiler to weave aspects in Spring’s AOP. SPRING ONLY SUPPORTS METHOD JOIN POINTS As mentioned earlier, multiple join-point models are available through various AOP implementations. Because it’s based on dynamic proxies, Spring only supports method join points. This is in contrast to some other AOP frameworks, such as AspectJ and JBoss, which provide field and constructor join points in addition to method pointcuts. Spring’s lack of field pointcuts prevents you from creating very fine-grained advice, such as intercepting updates to an object’s field. And without constructor pointcuts, there’s no way to apply advice when a bean is instantiated. But method interception should suit most, if not all, of your needs. If you find yourself in need of more than method interception, you’ll want to complement Spring AOP with AspectJ. Now you have a general idea of what AOP does and how it’s supported by Spring. It’s time to get your hands dirty creating aspects in Spring. Let’s start with Spring’s declarative AOP model. 4.2 Selecting join points with pointcuts As mentioned before, pointcuts are used to pinpoint where an aspect’s advice should be applied. Along with an aspect’s advice, pointcuts are among the most fundamental elements of an aspect. Therefore, it’s important to know how to write pointcuts. In Spring AOP, pointcuts are defined using AspectJ’s pointcut expression language. If you’re already familiar with AspectJ, then defining pointcuts in Spring should feel Proxy TargetCaller Figure 4.3 Spring aspects are implemented as proxies that wrap the target object. The proxy handles method calls, performs additional aspect logic, and then invokes the target method. 104 CHAPTER 4 Aspect-oriented Spring natural. But in case you’re new to AspectJ, this section will serve as a quick lesson on writing AspectJ-style pointcuts. For a more detailed discussion of AspectJ and AspectJ’s pointcut expression language, I strongly recommend Ramnivas Laddad’s AspectJ in Action, Second Edition (Manning, 2009, www.manning.com/laddad2/). The most important thing to know about AspectJ pointcuts as they pertain to Spring AOP is that Spring only supports a subset of the pointcut designators available in AspectJ. Recall that Spring AOP is proxy-based, and certain pointcut expressions aren’t relevant to proxy-based AOP. Table 4.1 lists the AspectJ pointcut designators that are supported in Spring AOP. Attempting to use any of AspectJ’s other designators will result in an IllegalArgumentException being thrown. As you browse through the supported designators, note that the execution desig- nator is the only one that actually performs matches. The other designators are used to limit those matches. This means execution is the primary designator you’ll use in every pointcut definition you write. You’ll use the other designators to constrain the pointcut’s reach. 4.2.1 Writing pointcuts To demonstrate aspects in Spring, you need something to be the subject of the aspect’s pointcuts. For that purpose, let’s define a Performance interface: Table 4.1 Spring uses AspectJ’s pointcut expression language to define Spring aspects. AspectJ designator Description args() Limits join-point matches to the execution of methods whose arguments are instances of the given types @args() Limits join-point matches to the execution of methods whose arguments are annotated with the given annotation types execution() Matches join points that are method executions this() Limits join-point matches to those where the bean reference of the AOP proxy is of a given type target() Limits join-point matches to those where the target object is of a given type @target() Limits matching to join points where the class of the executing object has an annotation of the given type within() Limits matching to join points within certain types @within() Limits matching to join points within types that have the given annotation (the execution of methods declared in types with the given annotation when using Spring AOP) @annotation Limits join-point matches to those where the subject of the join point has the given annotation 105Selecting join points with pointcuts package concert; public interface Performance { public void perform(); } Performance represents any kind of live performance, such as a stage play, a movie, or a concert. Let’s say that you want to write an aspect that triggers off Performance’s perform() method. Figure 4.4 shows a pointcut expression that can be used to apply advice whenever the perform() method is executed. You use the execution() designator to select Performance’s perform() method. The method specification starts with an asterisk, which indicates that you don’t care what type the method returns. Then you specify the fully qualified class name and the name of the method you want to select. For the method’s parameter list, you use the double dot (..), indicating that the pointcut should select any perform() method, no matter what the argument list is. Now let’s suppose that you want to confine the reach of that pointcut to only the concert package. In that case, you can limit the match by tacking on a within() des- ignator, as shown in figure 4.5. Note that you use the && operator to combine the execution() and within() desig- nators in an “and” relationship (where both designators must match for the pointcut to match). Similarly, you could use the || operator to indicate an “or” relationship. And the ! operator can be used to negate the effect of a designator. Because ampersands have special meaning in XML, you’re free to use and in place of && when specifying pointcuts in a Spring XML-based configuration. Likewise, or and not can be used in place of || and !, respectively. execution(* concert.Performance.perform(..)) Trigger on a method’s execution Returning any type The type that the method belongs to Method specifiction The method Taking any arguments Figure 4.4 Selecting Performance’s perform() method with an AspectJ pointcut expression execution(* concert.Performance.perform(..)) && within(concert.*)) Combination (and) operator The execution of the Instrument.play() method When the method is called from within any class in the concert package Figure 4.5 Limiting a pointcut’s reach by using the within() designator 106 CHAPTER 4 Aspect-oriented Spring 4.2.2 Selecting beans in pointcuts In addition to the designators listed in table 4.1, Spring adds a bean() designator that lets you identify beans by their ID in a pointcut expression. bean() takes a bean ID or name as an argument and limits the pointcut’s effect to that specific bean. For example, consider the following pointcut: execution(* concert.Performance.perform()) and bean('woodstock') Here you’re saying that you want to apply aspect advice to the execution of Performance’s perform() method, but limited to the bean whose ID is woodstock. Narrowing a pointcut to a specific bean may be valuable in some cases, but you can also use negation to apply an aspect to all beans that don’t have a specific ID: execution(* concert.Performance.perform()) and !bean('woodstock') In this case, the aspect’s advice will be woven into all beans whose ID isn’t woodstock. Now that we’ve covered the basics of writing pointcuts, let’s see how to write the advice and declare the aspects that use those pointcuts. 4.3 Creating annotated aspects A key feature introduced in AspectJ 5 is the ability to use annotations to create aspects. Prior to AspectJ 5, writing AspectJ aspects involved learning a Java language extension. But AspectJ’s annotation-oriented model makes it simple to turn any class into an aspect by sprinkling a few annotations around. You’ve already defined the Performance interface as the subject of your aspect’s pointcuts. Now let’s use AspectJ annotations to create an aspect. 4.3.1 Defining an aspect A performance isn’t a performance without an audience. Or is it? When you think about it from the perspective of a performance, an audience is important but isn’t central to the function of the performance itself; it’s a separate concern. Therefore, it makes sense to define the audience as an aspect that’s applied to a performance. The following listing shows the Audience class that defines the aspect you’ll need. package concert; import org.aspectj.lang.annotation.AfterReturning; import org.aspectj.lang.annotation.AfterThrowing; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Before; @Aspect public class Audience { @Before("execution(** concert.Performance.perform(..))") public void silenceCellPhones() { Listing 4.1 Audience class: an aspect that watches a performance Before performance 107Creating annotated aspects System.out.println("Silencing cell phones"); } @Before("execution(** concert.Performance.perform(..))") public void takeSeats() { System.out.println("Taking seats"); } @AfterReturning("execution(** concert.Performance.perform(..))") public void applause() { System.out.println("CLAP CLAP CLAP!!!"); } @AfterThrowing("execution(** concert.Performance.perform(..))") public void demandRefund() { System.out.println("Demanding a refund"); } } Notice how the Audience class is annotated with @Aspect. This annotation indicates that Audience isn’t just any POJO—it’s an aspect. And throughout the Audience class are methods that are annotated to define the specifics of the aspect. Audience has four methods that define things an audience might do as it observes a performance. Before the performance, the audience should take their seats (takeSeats()) and silence their cell phones (silenceCellPhones()). If the perfor- mance goes well, the audience should applaud (applause()). But if the performance fails to meet the audience’s expectations, then the audience should demand a refund (demandRefund()). As you can see, those methods are annotated with advice annotations to indicate when those methods should be called. AspectJ provides five annotations for defining advice, as listed in table 4.2. The Audience class makes use of three out of the five advice annotations. The takeSeats() and silenceCellPhones() methods are both annotated with @Before, indicating that they should be called before a performance is performed. The applause() method is annotated with @AfterReturning so that it will be called Table 4.2 Spring uses AspectJ annotations to declare advice methods. Annotation Advice @After The advice method is called after the advised method returns or throws an exception. @AfterReturning The advice method is called after the advised method returns. @AfterThrowing The advice method is called after the advised method throws an exception. @Around The advice method wraps the advised method. @Before The advice method is called before the advised method is called. Before performance After performance After bad performance 108 CHAPTER 4 Aspect-oriented Spring after a performance returns successfully. And the @AfterThrowing annotation is placed on demandRefund() so that it will be called if any exceptions are thrown dur- ing a performance. You’ve probably noticed that all of these annotations are given a pointcut expres- sion as a value. And you may have noticed that it’s the same pointcut expression on all four methods. They could each be given a different pointcut expression, but this par- ticular pointcut suits your needs for all the advice methods. Taking a closer look at the pointcut expression given to the advice annotations, you’ll see that it triggers on the execution of the perform() method on a Performance. It’s a shame that you had to repeat that same pointcut expression four times. Duplication like this doesn’t feel right. It’d be nice if you could define the pointcut once and then reference it every time you need it. Fortunately, there’s a way: the @Pointcut annotation defines a reusable pointcut within an @AspectJ aspect. The next listing shows the Audience aspect, updated to use @Pointcut. package concert; import org.aspectj.lang.annotation.AfterReturning; import org.aspectj.lang.annotation.AfterThrowing; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Before; import org.aspectj.lang.annotation.Pointcut; @Aspect public class Audience { @Pointcut("execution(** concert.Performance.perform(..))") public void performance() {} @Before("performance()") public void silenceCellPhones() { System.out.println("Silencing cell phones"); } @Before("performance()") public void takeSeats() { System.out.println("Taking seats"); } @AfterReturning("performance()") public void applause() { System.out.println("CLAP CLAP CLAP!!!"); } @AfterThrowing("performance()") public void demandRefund() { System.out.println("Demanding a refund"); } } Listing 4.2 Declaring a frequently used pointcut expression with @Pointcut Define named pointcut Before performance After performance After bad performance 109Creating annotated aspects In Audience, the performance() method is annotated with @Pointcut. The value given to the @Pointcut annotation is a pointcut expression, just like the ones you used previously with the advice annotations. By annotating performance() with @Pointcut in this way, you essentially extend the pointcut expression language so that you can use performance() in your pointcut expressions anywhere you’d otherwise use the longer expression. As you can see, you replace the longer expression in all the advice annotations with performance(). The body of the performance() method is irrelevant and, in fact, should be empty. The method itself is just a marker, giving the @Pointcut annotation something to attach itself to. Note that aside from the annotations and the no-op performance() method, the Audience class is essentially a POJO. Its methods can be called just like methods on any other Java class. Its methods can be individually unit-tested just as in any other Java class. Audience is just another Java class that happens to be annotated to be used as an aspect. And, just like any other Java class, it can be wired as a bean in Spring: @Bean public Audience audience() { return new Audience(); } If you were to stop here, Audience would only be a bean in the Spring container. Even though it’s annotated with AspectJ annotations, it wouldn’t be treated as an aspect without something that interpreted those annotations and created the proxies that turn it into an aspect. If you’re using JavaConfig, you can turn on auto-proxying by applying the @EnableAspectJAutoProxy annotation at the class level of the configuration class. The following configuration class shows how to enable auto-proxying in JavaConfig. package concert; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.EnableAspectJAutoProxy; @Configuration @EnableAspectJAutoProxy @ComponentScan public class ConcertConfig { @Bean public Audience audience() { return new Audience(); } } Listing 4.3 Enabling auto-proxying of AspectJ annotations in JavaConfig Enable AspectJ auto-proxying Declare Audience bean 110 CHAPTER 4 Aspect-oriented Spring If, however, you’re using XML to wire your beans in Spring, then you need to use the element from Spring’s aop namespace. The XML config- uration in the following listing shows how this is done. Whether you use JavaConfig or XML, AspectJ auto-proxying uses the @Aspect- annotated bean to create a proxy around any other beans for which the aspect’s point- cuts are a match. In this case, a proxy will be created for the Concert bean, with the advice methods in Audience being applied before and after the perform() method. It’s important to understand that Spring’s AspectJ auto-proxying only uses @AspectJ annotations as a guide for creating proxy-based aspects. Under the covers, it’s still Spring’s proxy-based aspects. This is significant because it means that although you’re using @AspectJ annotations, you’re still limited to proxying method invoca- tions. If you want to be able to exploit the full power of AspectJ, you’ll have to use the AspectJ runtime and not rely on Spring to create proxy-based aspects. At this point, your aspect is defined using distinct advice methods for before and after advice. But table 4.2 mentions another kind of advice: around advice. Around advice is just different enough from the other advice types that it’s worth spending a moment seeing how to write it. 4.3.2 Creating around advice Around advice is the most powerful advice type. It allows you to write logic that com- pletely wraps the advised method. It’s essentially like writing both before advice and after advice in a single advice method. To illustrate around advice, let’s rewrite the Audience aspect. This time you’ll use a single around advice method instead of distinct before and after advice methods. Listing 4.4 Enabling AspectJ auto-proxying in XML using Spring’s aop namespace Declare Spring’s aop namespace Enable AspectJ auto- proxying Declare the Audience bean 111Creating annotated aspects package concert; import org.aspectj.lang.ProceedingJoinPoint; import org.aspectj.lang.annotation.Around; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Pointcut; @Aspect public class Audience { @Pointcut("execution(** concert.Performance.perform(..))") public void performance() {} @Around("performance()") public void watchPerformance(ProceedingJoinPoint jp) { try { System.out.println("Silencing cell phones"); System.out.println("Taking seats"); jp.proceed(); System.out.println("CLAP CLAP CLAP!!!"); } catch (Throwable e) { System.out.println("Demanding a refund"); } } } Here the @Around annotation indicates that the watchPerformance() method is to be applied as around advice to the performance() pointcut. In this advice, the audi- ence will silence their cell phones and take their seats before the performance and will applaud after the performance. And just like before, if an exception is thrown during the performance, the audience will ask for their money back. As you can see, the effect of this advice is identical to what you did earlier with before and after advice. But here it’s all in a single advice method, whereas before it was spread across four distinct advice methods. The first thing you’ll notice about this new advice method is that it’s given a ProceedingJoinPoint as a parameter. This object is necessary because it’s how you can invoke the advised method from within your advice. The advice method will do everything it needs to do; and when it’s ready to pass control to the advised method, it will call ProceedingJoinPoint’s proceed() method. Note that it’s crucial that you remember to include a call to the proceed() method. If you don’t, then your advice will effectively block access to the advised method. Maybe that’s what you want, but chances are good that you do want the advised method to be executed at some point. What’s also interesting is that just as you can omit a call to the proceed() method to block access to the advised method, you can also invoke it multiple times from within the advice. One reason for doing this may be to implement retry logic to per- form repeated attempts on the advised method should it fail. Listing 4.5 Reimplementing the Audience aspect using around advice Declare named pointcut Around advice method 112 CHAPTER 4 Aspect-oriented Spring 4.3.3 Handling parameters in advice So far, your aspects have been simple, taking no parameters. The only exception is that the watchPerformance() method you wrote for the around advice example took a ProceedingJoinPoint as a parameter. Other than that, the advice you’ve written hasn’t bothered to look at any parameters passed to the advised methods. That’s been okay, though, because the perform() method you were advising didn’t take any parameters. But what if your aspect was to advise a method that does take parameters? Could the aspect access the parameters that are passed into the method and use them? To illustrate, let’s revisit the BlankDisc class from section 2.4.4. As it is, the play() method cycles through all the tracks and calls playTrack() for each track. But you could call the playTrack() method directly to play an individual track. Suppose you want to keep a count of how many times each track is played. One way to do this is to change the playTrack() method to directly keep track of that count each time it’s called. But track-counting logic is a separate concern from playing a track and therefore doesn’t belong in the playTrack() method. This looks like a job for an aspect. To keep a running count of how many times a track is played, let’s create Track- Counter, an aspect that advises playTrack(). The following listing shows just such an aspect. package soundsystem; import java.util.HashMap; import java.util.Map; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Before; import org.aspectj.lang.annotation.Pointcut; @Aspect public class TrackCounter { private Map trackCounts = new HashMap(); @Pointcut( "execution(* soundsystem.CompactDisc.playTrack(int)) " + "&& args(trackNumber)") public void trackPlayed(int trackNumber) {} @Before("trackPlayed(trackNumber)") public void countTrack(int trackNumber) { int currentCount = getPlayCount(trackNumber); trackCounts.put(trackNumber, currentCount + 1); } public int getPlayCount(int trackNumber) { return trackCounts.containsKey(trackNumber) ? trackCounts.get(trackNumber) : 0; } } Listing 4.6 Using parameterized advice to count how many times a track is played Advise the playTrack() method Count a track before it’s played 113Creating annotated aspects As with the other aspects you’ve created so far, this aspect uses @Pointcut to define a named pointcut and @Before to declare a method as supplying before advice. What’s different here, however, is that the pointcut also declares parameters to be supplied to the advice method. Figure 4.6 breaks down the pointcut expression to show where the parameter is specified. The thing to focus on in the figure is the args(trackNumber) qualifier in the pointcut expression. This indicates that any int argument that is passed into the exe- cution of playTrack() should also be passed into the advice. The parameter name, trackNumber, also matches the parameter in the pointcut method signature. That carries over into the advice method where the @Before annotation is defined with the named pointcut, trackPlayed(trackNumber). The parameter in the point- cut aligns with the parameter of the same name in the pointcut method, completing the path of the parameter from the named pointcut to the advice method. Now you can configure BlankDisc and TrackCounter as beans in the Spring con- figuration and enable AspectJ auto-proxying, as shown next. package soundsystem; import java.util.ArrayList; import java.util.List; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.EnableAspectJAutoProxy; @Configuration @EnableAspectJAutoProxy public class TrackCounterConfig { @Bean public CompactDisc sgtPeppers() { BlankDisc cd = new BlankDisc(); cd.setTitle("Sgt. Pepper's Lonely Hearts Club Band"); cd.setArtist("The Beatles"); List tracks = new ArrayList(); tracks.add("Sgt. Pepper's Lonely Hearts Club Band"); tracks.add("With a Little Help from My Friends"); tracks.add("Lucy in the Sky with Diamonds"); tracks.add("Getting Better"); tracks.add("Fixing a Hole"); Listing 4.7 Configuring TrackCounter to count the number of times a track is played execution(* soundsystem.CompactDisc.playTrack(int)) && args(trackNumber) Returning any type The type that the method belongs to Arguments specifiction The method Taking an int argument Figure 4.6 Declaring a parameter in a pointcut expression that’s to be passed into an advice method Enable AspectJ auto-proxying CompactDisc bean 114 CHAPTER 4 Aspect-oriented Spring // ...other tracks omitted for brevity... cd.setTracks(tracks); return cd; } @Bean public TrackCounter trackCounter() { return new TrackCounter(); } } Finally, to prove that this all works, you can write the following simple test. It plays a few tracks and then asserts the play count through the TrackCounter bean. package soundsystem; import static org.junit.Assert.*; import org.junit.Assert; import org.junit.Rule; import org.junit.Test; import org.junit.contrib.java.lang.system.StandardOutputStreamLog; import org.junit.runner.RunWith; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.test.context.ContextConfiguration; import org.springframework.test.context.junit4.SpringJUnit4ClassRunner; @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration(classes=TrackCounterConfig.class) public class TrackCounterTest { @Rule public final StandardOutputStreamLog log = new StandardOutputStreamLog(); @Autowired private CompactDisc cd; @Autowired private TrackCounter counter; @Test public void testTrackCounter() { cd.playTrack(1); cd.playTrack(2); cd.playTrack(3); cd.playTrack(3); cd.playTrack(3); cd.playTrack(3); cd.playTrack(7); cd.playTrack(7); assertEquals(1, counter.getPlayCount(1)); assertEquals(1, counter.getPlayCount(2)); assertEquals(4, counter.getPlayCount(3)); assertEquals(0, counter.getPlayCount(4)); Listing 4.8 Testing the TrackCounter aspect TrackCounter bean Play some tracks Assert the expected counts 115Creating annotated aspects assertEquals(0, counter.getPlayCount(5)); assertEquals(0, counter.getPlayCount(6)); assertEquals(2, counter.getPlayCount(7)); } } The aspects you’ve worked with thus far wrap existing methods on the advised object. But method wrapping is just one of the tricks that aspects can perform. Let’s see how to write aspects that introduce completely new functionality into an advised object. 4.3.4 Annotating introductions Some languages, such as Ruby and Groovy, have the notion of open classes. They make it possible to add new methods to an object or class without directly changing the definition of those objects or classes. Unfortunately, Java isn’t that dynamic. Once a class has been compiled, there’s little you can do to append new functionality to it. But if you think about it, isn’t that what you’ve been doing in this chapter with aspects? Sure, you haven’t added any new methods to objects, but you’re adding new functionality around the methods that the objects already have. If an aspect can wrap existing methods with additional functionality, why not add new methods to the object? In fact, using an AOP concept known as introduction, aspects can attach new methods to Spring beans. Recall that in Spring, aspects are proxies that implement the same interfaces as the beans they wrap. What if, in addition to implementing those interfaces, the proxy is also exposed through some new interface? Then any bean that’s advised by the aspect will appear to implement the new interface, even if its underlying implementation class doesn’t. Figure 4.7 illustrates how this works. Advised bean Caller Introduction delegateIntroduced method Proxy Existing method Figure 4.7 With Spring AOP, you can introduce new methods to a bean. A proxy intercepts the calls and delegates to a different object that provides the implementation. 116 CHAPTER 4 Aspect-oriented Spring Notice that when a method on the introduced interface is called, the proxy dele- gates the call to some other object that provides the implementation of the new interface. Effectively, this gives you one bean whose implementation is split across multiple classes. Putting this idea to work, let’s say you want to introduce the following Encoreable interface to any implementation of Performance: package concert; public interface Encoreable { void performEncore(); } Setting aside any debates as to whether Encoreable is a real word, you need a way to apply this interface to your Performance implementations. I suppose you could visit all implementations of Performance and change them so that they also implement Encoreable. But from a design standpoint, that may not be the best move. Not all Performances will necessarily be Encoreable. Moreover, it may not be possible to change all implementations of Performance, especially if you’re working with third- party implementations and don’t have the source code. Fortunately, AOP introductions can help you without compromising design choices or requiring invasive changes to the existing implementations. To pull it off, you cre- ate a new aspect: package concert; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.DeclareParents; @Aspect public class EncoreableIntroducer { @DeclareParents(value="concert.Performance+", defaultImpl=DefaultEncoreable.class) public static Encoreable encoreable; } As you can see, EncoreableIntroducer is an aspect. But unlike the aspects you’ve created so far, it doesn’t provide before, after, or around advice. Instead, it introduces the Encoreable interface to Performance beans using the @DeclareParents anno- tation. The @DeclareParents annotation is made up of three parts:  The value attribute identifies the kinds of beans that should be introduced with the interface. In this case, that’s anything that implements the Performance interface. (The plus sign at the end specifies any subtype of Performance, as opposed to Performance itself.)  The defaultImpl attribute identifies the class that will provide the implemen- tation for the introduction. Here you’re saying that DefaultEncoreable will provide that implementation. 117Declaring aspects in XML  The static property that is annotated by @DeclareParents specifies the inter- face that’s to be introduced. In this case, you’re introducing the Encoreable interface. As with any aspect, you need to declare EncoreableIntroducer as a bean in the Spring application context: Spring auto-proxying will take it from there. When Spring discovers a bean annotated with @Aspect, it will automatically create a proxy that delegates calls to either the proxied bean or to the introduction implementation, depending on whether the method called belongs to the proxied bean or to the introduced interface. Annotations and auto-proxying provide a convenient programming model for cre- ating aspects in Spring. It’s simple and involves only minimal Spring configuration. But annotation-oriented aspect declaration has one clear disadvantage: you must be able to annotate the advice class. And to do that, you must have the source code. When you don’t have the source code, or if you don’t want to place AspectJ anno- tations in your code, Spring offers another option for aspects. Let’s see how you can declare aspects in a Spring XML configuration file. 4.4 Declaring aspects in XML Early in this book, I established a preference for annotation-based configuration over Java-based configuration, and Java-based configuration over XML configuration. But if you need to declare aspects without annotating the advice class, then you must turn to XML configuration. Spring’s aop namespace offers several elements that are useful for declaring aspects in XML, as described in table 4.3. Table 4.3 Spring’s AOP configuration elements enable non-invasive declaration of aspects. AOP configuration element Purpose Defines an AOP advisor. Defines an AOP after advice (regardless of whether the advised method returns successfully). Defines an AOP after-returning advice. Defines an AOP after-throwing advice. Defines an AOP around advice. Defines an aspect. Enables annotation-driven aspects using @AspectJ. Defines an AOP before advice. 118 CHAPTER 4 Aspect-oriented Spring You’ve already seen the element and how it can enable auto-proxying of AspectJ-annotated advice classes. But the other elements in the aop namespace let you declare aspects directly in your Spring configuration without using annotations. For example, let’s have another look at the Audience class. This time, let’s remove all of those AspectJ annotations: package concert; public class Audience { public void silenceCellPhones() { System.out.println("Silencing cell phones"); } public void takeSeats() { System.out.println("Taking seats"); } public void applause() { System.out.println("CLAP CLAP CLAP!!!"); } public void demandRefund() { System.out.println("Demanding a refund"); } } As you can see, without the AspectJ annotations, there’s nothing remarkable about the Audience class. It’s a basic Java class with a handful of methods. And you can reg- ister it as a bean in the Spring application context like any other class. Despite its unassuming appearance, what’s remarkable about Audience is that it has all the makings of AOP advice. It just needs a little help to become the advice it’s meant to be. 4.4.1 Declaring before and after advice You could put back all the AspectJ annotations, but that isn’t the point of this section. Instead, you’ll use some of the elements from Spring’s aop namespace to turn the annotation-free Audience into an aspect. The next listing shows the XML you need. The top-level AOP element. Most \ elements must be contained within \. Introduces additional interfaces to advised objects that are trans- parently implemented. Defines a pointcut. Table 4.3 Spring’s AOP configuration elements enable non-invasive declaration of aspects. (continued) AOP configuration element Purpose 119Declaring aspects in XML The first thing to notice about the Spring AOP configuration elements is that most of them must be used in the context of the element. There are a few exceptions to this rule, but when it comes to declaring beans as aspects, you’ll always start with . In , you may declare one or more advisers, aspects, or pointcuts. In listing 4.9, you declare a single aspect using the element. The ref attri- bute references the POJO bean that will be used to supply the functionality of the aspect—in this case, audience. The bean that’s referenced by the ref attribute will supply the methods called by any advice in the aspect. It’s worth noting that the referenced advice bean can be any type that provides methods to be called at the designated pointcuts. This makes Spring’s XML configura- tion for AOP a handy way to use types defined in third-party libraries as advice, even though you can’t annotate them with AspectJ aspects. The aspect has four different bits of advice. The two elements define before advice that will call the takeSeats() and silenceCellPhones() meth- ods (declared by the method attribute) of the Audience bean before any methods matching the pointcut are executed. The element defines after-returning advice to call the applause() method after the pointcut. Meanwhile, the element defines an after-throwing advice to call the demandRefund() method if any exceptions are thrown. Figure 4.8 shows how the advice logic is woven into the business logic. In all advice elements, the pointcut attribute defines the pointcut where the advice will be applied. The value given to the pointcut attribute is a pointcut defined in AspectJ’s pointcut expression syntax. Listing 4.9 Annotation-free Audience class, declared in XML as an aspect Reference audience bean Before performance After performance After bad performance 120 CHAPTER 4 Aspect-oriented Spring You’ve probably noticed that the value of the pointcut attribute is the same for all the advice elements. That’s because all the advice is being applied to the same point- cut. When you found the same kind of duplication in your AspectJ-annotated advice, you eliminated it by using the @Pointcut annotation. For XML-based aspect declara- tions, however, you’ll need to use the element. The following XML shows how to extract the common pointcut expression into a single pointcut declara- tion that can be used across all advice elements. Audience aspect try { audience.takeSeats(); audience.turnOffCellPhones(); audience.applause(); } catch (Exception e) { audience.demandRefund(); } Advice logic performance.perform(); Business logic Figure 4.8 The Audience aspect includes four bits of advice that weave advice logic around methods that match the aspect’s pointcut. Define pointcut Reference pointcut 121Declaring aspects in XML method="applause"/> Now the pointcut is defined in a single location and is referenced across multiple advice elements. The element defines the pointcut to have an id of performance. Meanwhile, all the advice elements have been changed to reference the named pointcut with the pointcut-ref attribute. As used in listing 4.10, the element defines a pointcut that can be referenced by all advice in the same element. But you can also define pointcuts that can be used across multiple aspects by placing the elements within the scope of the element. 4.4.2 Declaring around advice The current implementation of Audience works great. But basic before and after advice have some limitations. Specifically, it’s tricky to share information between before advice and after advice without resorting to storing that information in mem- ber variables. For example, suppose that in addition to putting away cell phones and applauding at the end, you also want the audience to keep their eyes on their watches and report how long the performance takes. The only way to accomplish this with before and after advice is to note the start time in before advice and report the length of time in after advice. But you’d have to store the start time in a member variable. Because Audience is a singleton, it wouldn’t be thread-safe to retain state like that. Around advice has an advantage over before and after advice in this regard. With around advice, you can accomplish the same thing you could with distinct before and after advice, but you can do it in a single method. Because the entire set of advice takes place in a single method, there’s no need to retain state in a member variable. For example, consider the new annotation-free Audience class with a single watchPerformance() method. package concert; import org.aspectj.lang.ProceedingJoinPoint; public class Audience { public void watchPerformance(ProceedingJoinPoint jp) { try { System.out.println("Silencing cell phones"); System.out.println("Taking seats"); Listing 4.11 Providing around advice with the watchPerformance() method Reference pointcut Before performance 122 CHAPTER 4 Aspect-oriented Spring jp.proceed(); System.out.println("CLAP CLAP CLAP!!!"); } catch (Throwable e) { System.out.println("Demanding a refund"); } } } In the case of the audience aspect, the watchPerformance() method contains all the functionality of the previous four advice methods. But all of it is contained in this sin- gle method, and this method is responsible for its own exception handling. Declaring around advice isn’t dramatically different from declaring other types of advice. All you need to do is use the element, as shown next. As with the other advice XML elements, is given a pointcut and the name of an advice method. Here you’re using the same pointcut as before, but you set the method attribute to point to the new watchPerformance() method. 4.4.3 Passing parameters to advice In section 4.3.3, you used AspectJ annotations to create an aspect that kept a running count of the number of times tracks were played on a CompactDisc. Now that you’re configuring your aspects in XML, let’s see how you can accomplish the same thing. First, let’s strip all the @AspectJ annotations out of the TrackCounter. package soundsystem; import java.util.HashMap; import java.util.Map; public class TrackCounter { private Map trackCounts = new HashMap(); public void countTrack(int trackNumber) { int currentCount = getPlayCount(trackNumber); Listing 4.12 Declaring around advice in XML with the element Listing 4.13 Annotation-free TrackCounter Proceed to advised method After performance After bad performance Declare around advice Method to be declared as before advice 123Declaring aspects in XML trackCounts.put(trackNumber, currentCount + 1); } public int getPlayCount(int trackNumber) { return trackCounts.containsKey(trackNumber) ? trackCounts.get(trackNumber) : 0; } } Without the AspectJ annotations, TrackCounter seems kind of bare. And as it stands now, TrackCounter won’t count any tracks unless you explicitly call the count- Track() method. But with a little XML Spring configuration, you can reinstate TrackCounter’s status as an aspect. The following listing shows the complete Spring configuration that declares both the TrackCounter bean and the BlankDisc bean and enables TrackCounter as an aspect. Sgt. Pepper's Lonely Hearts Club Band With a Little Help from My Friends Lucy in the Sky with Diamonds Getting Better Fixing a Hole Listing 4.14 Configuring TrackCounter as a parameterized aspect in XML TrackCounter bean BlankDisc bean Declare TrackCounter as an aspect 124 CHAPTER 4 Aspect-oriented Spring As you can see, you’re using the same XML elements from the aop namespace as before; they declare a POJO to be treated as an aspect. The only significant difference is that your pointcut expression now includes a parameter to be passed into the advice method. If you compare this expression with the one from listing 4.6, you’ll see that they’re almost identical. The only real difference is that here you use the and keyword instead of && (because ampersands are interpreted as the beginning of an entity in XML). Now that you’ve exercised Spring’s aop namespace to declare a few basic aspects in XML, let’s see how the aop namespace can help you declare introduction aspects. 4.4.4 Introducing new functionality with aspects Earlier, in section 4.3.4, I showed you how to use AspectJ’s @DeclareParents annota- tion to magically introduce a new method into an advised bean. But AOP introduc- tions aren’t exclusive to AspectJ. Using the element from Spring’s aop namespace, you can do similar magic in XML. The following snippet of XML is equivalent to the AspectJ-based introduction you created earlier: As its name implies, declares that the beans it advises will have new parents in its object hierarchy. Specifically, in this case you’re saying that the beans whose type matches the Performance interface (per the types-matching attri- bute) should have Encoreable in their parentage (per the implement-interface attribute). The final matter to settle is where the implementation of the Encoreable’s methods will come from. There are two ways to identify the implementation of the introduced interface. In this case, you’re using the default-impl attribute to explicitly identify the implemen- tation by its fully qualified class name. Alternatively, you could identify it using the delegate-ref attribute: The delegate-ref attribute refers to a Spring bean as the introduction delegate. This assumes that a bean with an ID of encoreableDelegate exists in the Spring con- text: The difference between directly identifying the delegate using default-impl and indirectly using delegate-ref is that the latter will be a Spring bean that itself may be injected, advised, or otherwise configured through Spring. 4.5 Injecting AspectJ aspects Although Spring AOP is sufficient for many applications of aspects, it’s a weak AOP solution when contrasted with AspectJ. AspectJ offers many types of pointcuts that aren’t possible with Spring AOP. Constructor pointcuts, for example, are convenient when you need to apply advice on the creation of an object. Unlike constructors in some other object-oriented lan- guages, Java constructors are different from normal methods. This makes Spring’s proxy-based AOP woefully inadequate for advising the creation of an object. For the most part, AspectJ aspects are independent of Spring. Although they can be woven into any Java-based application, including Spring applications, there’s little involvement on Spring’s part in applying AspectJ aspects. But any well-designed and meaningful aspect will likely depend on other classes to assist in its work. If an aspect depends on one or more classes when executing its advice, you can instantiate those collaborating objects with the aspect itself. Or, better yet, you can use Spring’s dependency injection to inject beans into AspectJ aspects. To illustrate, let’s create a new aspect for performances. Specifically, let’s create an aspect that plays the role of a critic who watches a performance and provides a critical review afterward. CriticAspect is such an aspect. package concert; public aspect CriticAspect { public CriticAspect() {} pointcut performance() : execution(* perform(..)); afterReturning() : performance() { System.out.println(criticismEngine.getCriticism()); } private CriticismEngine criticismEngine; public void setCriticismEngine(CriticismEngine criticismEngine) { this.criticismEngine = criticismEngine; } Listing 4.15 Implementing a performance critic using AspectJ Inject CriticismEngine 126 CHAPTER 4 Aspect-oriented Spring } The chief responsibility for CriticAspect is to comment on a performance after the performance has completed. The performance() pointcut in listing 4.15 matches the perform() method. When it’s married with the afterReturning() advice, you get an aspect that reacts to the completion of a performance. What makes listing 4.15 interesting is that the critic doesn’t make commentary on its own. Instead, CriticAspect collaborates with a CriticismEngine object, calling its getCriticism() method, to produce critical commentary after a performance. To avoid unnecessary coupling between CriticAspect and CriticismEngine, Critic- Aspect is given a reference to CriticismEngine through setter injection. This rela- tionship is illustrated in figure 4.9. CriticismEngine is an interface that declares a simple getCriticism() method. The next listing shows the implementation of CriticismEngine. package com.springinaction.springidol; public class CriticismEngineImpl implements CriticismEngine { public CriticismEngineImpl() {} public String getCriticism() { int i = (int) (Math.random() * criticismPool.length); return criticismPool[i]; } // injected private String[] criticismPool; public void setCriticismPool(String[] criticismPool) { this.criticismPool = criticismPool; } } CriticismEngineImpl implements the CriticismEngine interface by randomly choosing a critical comment from a pool of injected criticisms. This class can be declared as a Spring using the following XML: Listing 4.16 CriticismEngine to be injected into CriticAspect JudgeAspectPerformer CriticismEngine CriticismEngineImpl Advises getCriticism() Injected into Figure 4.9 Aspects need injection, too. Spring can inject AspectJ aspects with dependencies just as if they were another bean. 127Summary Worst performance ever! I laughed, I cried, then I realized I was at the wrong show. A must see show! So far, so good. You now have a CriticismEngine implementation to give to Critic- Aspect. All that’s left is to wire CriticismEngineImpl into CriticAspect. Before I show you how to do the injection, you should know that AspectJ aspects can be woven into your application without involving Spring at all. But if you want to use Spring’s dependency injection to inject collaborators into an AspectJ aspect, you’ll need to declare the aspect as a in Spring’s configuration. The following declaration injects the criticismEngine bean into CriticAspect: For the most part, this declaration isn’t much different from any other you may find in Spring. The big difference is the use of the factory-method attribute. Normally, Spring beans are instantiated by the Spring container, but AspectJ aspects are created by the AspectJ runtime. By the time Spring gets a chance to inject CriticismEngine into CriticAspect, CriticAspect has already been instantiated. Because Spring isn’t responsible for the creation of CriticAspect, it isn’t possible to declare CriticAspect as a bean in Spring. Instead, you need a way for Spring to get a handle to the CriticAspect instance that has already been created by AspectJ so that you can inject it with a CriticismEngine. Conveniently, all AspectJ aspects provide a static aspectOf() method that returns the singleton instance of the aspect. So to get an instance of the aspect, you must use factory-method to invoke the aspectOf() method instead of trying to call CriticAspect’s constructor. In short, Spring doesn’t use the declaration from earlier to create an instance of the CriticAspect—it has already been created by the AspectJ runtime. Instead, Spring retrieves a reference to the aspect through the aspectOf() factory method and then performs dependency injection on it as prescribed by the element. 4.6 Summary AOP is a powerful complement to object-oriented programming. With aspects, you can group application behavior that was once spread throughout your applications into reusable modules. You can then declare exactly where and how this behavior is applied. This reduces code duplication and lets your classes focus on their main functionality. 128 CHAPTER 4 Aspect-oriented Spring Spring provides an AOP framework that lets you insert aspects around method exe- cutions. You’ve learned how to weave advice before, after, and around a method invo- cation, as well as to add custom behavior for handling exceptions. You have several choices in how you can use aspects in your Spring applications. Wiring advice and pointcuts in Spring is much easier with the addition of @AspectJ annotation support and a simplified configuration schema. Finally, there are times when Spring AOP isn’t enough, and you must turn to AspectJ for more powerful aspects. For those situations, we looked at how to use Spring to inject dependencies into AspectJ aspects. At this point, we’ve covered the basics of the Spring Framework. You’ve seen how to configure the Spring container and how to apply aspects to Spring-managed objects. These core techniques offer you a great opportunity to create applications composed of loosely coupled objects. Now we’ll move past the essentials and look at what it takes to build real applica- tions in Spring. Starting in the next chapter, you’ll see how to build web applications using Spring. Part 2 Spring on the web Spring is often used to develop web applications. Therefore, in part 2 you’ll see how to use Spring’s MVC framework to add a web front end to your application. In chapter 5, “Building Spring web applications,” you’ll learn the basics of Spring MVC, a web framework built on the principles of the Spring Framework. You’ll discover how to write controllers to handle web requests and see how to transparently bind request parameters and payload to your business objects while providing validation and error handling at the same time. Chapter 6, “Rendering web views,” continues what chapter 5 started by show- ing you how to take model data produced in Spring MVC controllers and render it as HTML to be served to a user’s browser. This chapter includes discussions of JavaServer Pages (JSP), Apache Tiles, and Thymeleaf templates. In Chapter 7, “Advanced Spring MVC,” you’ll learn a few more advanced techniques to use when building web applications, including custom Spring MVC configuration options, handling multipart file uploads, dealing with excep- tions, and passing data across requests using flash attributes. Chapter 8, “Working with Spring Web Flow,” will show you how to build con- versation, flow-based web applications using the Spring Web Flow framework. As security is an important aspect of many applications, chapter 9, “Securing Spring,” will show you how to use Spring Security to secure your web application and protect the information it serves. 131 Building Spring web applications As an enterprise Java developer, you’ve likely developed a web-based application or two. For many Java developers, web-based applications are their primary focus. If this is your experience, then you’re well aware of the challenges that come with these systems. Specifically, state management, workflow, and validation are all important features that need to be addressed. None of these is made any easier given the HTTP protocol’s stateless nature. Spring’s web framework is designed to help you address these concerns. Based on the Model-View-Controller (MVC) pattern, Spring MVC helps you build web- based applications that are as flexible and as loosely coupled as the Spring Frame- work itself. In this chapter, we’ll explore the essentials of Spring’s MVC web framework. We’ll focus on using annotations to create controllers that handle various kinds of web requests, parameters, and form input. Before we go too deep with the specifics This chapter covers  Mapping requests to Spring controllers  Transparently binding form parameters  Validating form submissions 132 CHAPTER 5 Building Spring web applications of Spring MVC, let’s start with a high-level view and set up the basic plumbing needed to make Spring MVC work. 5.1 Getting started with Spring MVC Have you ever seen the children’s game Mousetrap? It’s crazy. The goal is to send a small steel ball through a series of wacky contraptions in order to trigger a mousetrap. The ball navigates all kinds of intricate gadgets, from rolling down a curvy ramp to springing off a teeter-totter to spinning on a miniature Ferris wheel to being kicked out of a bucket by a rubber boot. It goes through all this to spring a trap on a poor, unsuspecting plastic mouse. At first glance, you may think that Spring’s MVC framework is a lot like Mousetrap. Instead of moving a ball through various ramps, teeter-totters, and wheels, Spring moves requests between a dispatcher servlet, handler mappings, controllers, and view resolvers. But don’t draw too strong a comparison between Spring MVC and the Rube Goldberg-esque game of Mousetrap. Each of the components in Spring MVC per- forms a specific purpose. And it’s really not that complex. Let’s take a look at how a request makes its way from the client through the com- ponents in Spring MVC, ultimately resulting in a request that goes back to the client. 5.1.1 Following the life of a request Every time a user clicks a link or submits a form in their web browser, a request goes to work. A request’s job description is that of a courier. Just like a postal carrier or a FedEx delivery person, a request lives to carry information from one place to another. The request is a busy creature. From the time it leaves the browser until it returns with a response, it makes several stops, each time dropping off a bit of information and picking up some more. Figure 5.1 shows all the stops the request makes as it trav- els through Spring MVC. When the request leaves the browser B, it carries information about what the user is asking for. At the least, the request will be carrying the requested URL. But it may also carry additional data, such as the information submitted in a form by the user. bRequest DispatcherServlet Handler mapping Controller View Model and logical view name ViewResolver Response c d e f g H Figure 5.1 A request couriers information to several stops on its way to producing the desired results. 133Getting started with Spring MVC The first stop in the request’s travels is at Spring’s DispatcherServlet. Like most Java- based web frameworks, Spring MVC funnels requests through a single front controller servlet. A front controller is a common web application pattern where a single servlet delegates responsibility for a request to other components of an application to per- form actual processing. In the case of Spring MVC, DispatcherServlet is the front controller. The DispatcherServlet’s job is to send the request on to a Spring MVC controller. A controller is a Spring component that processes the request. But a typical application may have several controllers, and DispatcherServlet needs some help deciding which controller to send the request to. So the DispatcherServlet consults one or more handler mappings C to figure out where the request’s next stop will be. The handler mapping pays particular attention to the URL carried by the request when making its decision. Once an appropriate controller has been chosen, DispatcherServlet sends the request on its merry way to the chosen controller D. At the controller, the request drops off its payload (the information submitted by the user) and patiently waits while the controller processes that information. (Actually, a well-designed controller per- forms little or no processing itself and instead delegates responsibility for the business logic to one or more service objects.) The logic performed by a controller often results in some information that needs to be carried back to the user and displayed in the browser. This information is referred to as the model. But sending raw information back to the user isn’t suffi- cient—it needs to be formatted in a user-friendly format, typically HTML. For that, the information needs to be given to a view, typically a JavaServer Page (JSP). One of the last things a controller does is package up the model data and identify the name of a view that should render the output. It then sends the request, along with the model and view name, back to the DispatcherServlet E. So that the controller doesn’t get coupled to a particular view, the view name passed back to DispatcherServlet doesn’t directly identify a specific JSP. It doesn’t even necessarily suggest that the view is a JSP. Instead, it only carries a logical name that will be used to look up the actual view that will produce the result. The DispatcherServlet consults a view resolver F to map the logical view name to a spe- cific view implementation, which may or may not be a JSP. Now that DispatcherServlet knows which view will render the result, the request’s job is almost over. Its final stop is at the view implementation G, typically a JSP, where it delivers the model data. The request’s job is finally done. The view will use the model data to render output that will be carried back to the client by the (not- so-hardworking) response object H. As you can see, a request goes through several steps along its way to producing a response for the client. Most of these steps take place within the Spring MVC frame- work, in the components shown in figure 5.1. Although the bulk of this chapter will focus on writing controllers, let’s take a moment to set up the essential components of Spring MVC. 134 CHAPTER 5 Building Spring web applications 5.1.2 Setting up Spring MVC Based on figure 5.1, it looks like there are a lot of moving parts to be configured. For- tunately, thanks to some advancements in the most recent versions of Spring, it’s easy to get started with Spring MVC. For now, you’ll take the simplest approach to configur- ing Spring MVC: you’ll do just enough configuring to be able to run the controllers you create. In chapter 7, we’ll look at some additional setup options. CONFIGURING DISPATCHERSERVLET DispatcherServlet is the centerpiece of Spring MVC. It’s where the request first hits the framework, and it’s responsible for routing the request through all the other components. Historically, servlets like DispatcherServlet have been configured in a web.xml file that’s carried in the web application’s WAR file. Certainly that’s one option for con- figuring DispatcherServlet. But thanks to recent advances in the Servlet 3 specifica- tion and in Spring 3.1, it’s not the only option. And it’s not the option we’ll go with in this chapter. Instead of a web.xml file, you’re going to use Java to configure DispatcherServlet in the servlet container. The following listing shows the Java class you’ll need. package spittr.config; import org.springframework.web.servlet.support. AbstractAnnotationConfigDispatcherServletInitializer; public class SpittrWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer { @Override protected String[] getServletMappings() { return new String[] { "/" }; } @Override protected Class[] getRootConfigClasses() { return new Class[] { RootConfig.class }; } @Override protected Class[] getServletConfigClasses() { return new Class[] { WebConfig.class }; } } Before we dive into the details of listing 5.1, you may wonder what the word spittr has to do with anything. The class is named SpittrWebAppInitializer, and it’s in a pack- age named spittr.config. I’ll explain that in a moment (in section 5.1.3), but for now, suffice it to say that the application you’ll create is named Spittr. Listing 5.1 Configuring DispatcherServlet Map DispatcherServlet to / Specify configuration class 135Getting started with Spring MVC To understand how listing 5.1 works, it’s probably sufficient to know that any class that extends AbstractAnnotationConfigDispatcherServletInitializer will auto- matically be used to configure DispatcherServlet and the Spring application con- text in the application’s servlet context. Even though its name is extremely long, AbstractAnnotationConfigDispatcher- ServletInitializer is a snap to use. Looking at listing 5.1, you can see that SpittrWebAppInitializer overrides three methods. The first method, getServletMappings(), identifies one or more paths that DispatcherServlet will be mapped to. In this case, it’s mapped to /, indicating that it will be the application’s default servlet. It will handle all requests coming into the application. In order to understand the other two methods, you must first understand the rela- tionship between DispatcherServlet and a servlet listener known as ContextLoader- Listener. A TALE OF TWO APPLICATION CONTEXTS When DispatcherServlet starts up, it creates a Spring application context and starts loading it with beans declared in the configuration files or classes that it’s given. With the getServletConfigClasses() method in listing 5.1, you’ve asked that Dispatcher- Servlet load its application context with beans defined in the WebConfig configura- tion class (using Java configuration). But in Spring web applications, there’s often another application context. This other application context is created by ContextLoaderListener. Whereas DispatcherServlet is expected to load beans containing web components such as controllers, view resolvers, and handler mappings, ContextLoaderListener is expected to load the other beans in your application. These beans are typically the middle-tier and data-tier components that drive the back end of the application. AbstractAnnotationConfigDispatcherServletInitializer exposed If you insist on the more detailed explanation, here it is. In a Servlet 3.0 environment, the container looks for any classes in the classpath that implement the javax.servlet .ServletContainerInitializer interface; if any are found, they’re used to config- ure the servlet container. Spring supplies an implementation of that interface called SpringServlet- ContainerInitializer that, in turn, seeks out any classes that implement Web- ApplicationInitializer and delegates to them for configuration. Spring 3.2 introduced a convenient base implementation of WebApplicationInitializer called AbstractAnnotationConfigDispatcherServletInitializer. Because your SpittrWebAppInitializer extends AbstractAnnotationConfigDispatcher- ServletInitializer (and thus implements WebApplicationInitializer), it will be automatically discovered when deployed in a Servlet 3.0 container and be used to configure the servlet context. 136 CHAPTER 5 Building Spring web applications Under the covers, AbstractAnnotationConfigDispatcherServletInitializer cre- ates both a DispatcherServlet and a ContextLoaderListener. The @Configuration classes returned from getServletConfigClasses() will define beans for Dispatcher- Servlet’s application context. Meanwhile, the @Configuration class’s returned get- RootConfigClasses() will be used to configure the application context created by ContextLoaderListener. In this case, your root configuration is defined in RootConfig, whereas Dispatcher- Servlet’s configuration is declared in WebConfig. You’ll see what those two configura- tion classes look like in a moment. It’s important to realize that configuring DispatcherServlet via Abstract- AnnotationConfigDispatcherServletInitializer is an alternative to the tradi- tional web.xml file. Although you can include a web.xml file alongside a subclass of AbstractAnnotationConfigDispatcherServletInitializer if you like, it’s not necessary. The only gotcha with configuring DispatcherServlet in this way, as opposed to in a web.xml file, is that it will only work when deploying to a server that supports Servlet 3.0, such as Apache Tomcat 7 or higher. The Servlet 3.0 specification has been final since December 2009, and the odds are good that you’ll be deploying your appli- cations to a servlet container that supports Servlet 3.0. If you’re not yet working with a Servlet 3.0-capable server, then configuring DispatcherServlet in a subclass of AbstractAnnotationConfigDispatcherServlet- Initializer won’t work for you. You’ll have no choice but to configure Dispatcher- Servlet in web.xml. We’ll look at web.xml and other configuration options in chapter 7. For now, though, let’s look at WebConfig and RootConfig, the two configu- ration classes referred to in listing 5.1, and see how to enable Spring MVC. ENABLING SPRING MVC Just as there are several ways of configuring DispatcherServlet, there’s more than one way to enable Spring MVC components. Historically, Spring has been configured using XML, and there’s an element that you can use to enable annotation-driven Spring MVC. We’ll talk about , among other Spring MVC configura- tion options, in chapter 7. But for now, you’ll keep your Spring MVC setup simple and Java-based. The very simplest Spring MVC configuration you can create is a class annotated with @EnableWebMvc: package spittr.config; import org.springframework.context.annotation.Configuration; import org.springframework.web.servlet.config.annotation.EnableWebMvc; @Configuration @EnableWebMvc public class WebConfig { } 137Getting started with Spring MVC This will work, and it will enable Spring MVC. But it leaves a lot to be desired:  No view resolver is configured. As such, Spring will default to using Bean- NameViewResolver, a view resolver that resolves views by looking for beans whose ID matches the view name and whose class implements the View interface.  Component-scanning isn’t enabled. Consequently, the only way Spring will find any controllers is if you declare them explicitly in the configuration.  As it is, DispatcherServlet is mapped as the default servlet for the application and will handle all requests, including requests for static resources, such as images and stylesheets (which is probably not what you want in most cases). Therefore, you need to add a bit more configuration in WebConfig on top of this bare minimum Spring MVC configuration to make it useful. The new WebConfig in the next listing addresses these concerns. package spittr.config; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.Configuration; import org.springframework.web.servlet.ViewResolver; import org.springframework.web.servlet.config.annotation. DefaultServletHandlerConfigurer; import org.springframework.web.servlet.config.annotation.EnableWebMvc; import org.springframework.web.servlet.config.annotation. WebMvcConfigurerAdapter; import org.springframework.web.servlet.view. InternalResourceViewResolver; @Configuration @EnableWebMvc @ComponentScan("spitter.web") public class WebConfig extends WebMvcConfigurerAdapter { @Bean public ViewResolver viewResolver() { InternalResourceViewResolver resolver = new InternalResourceViewResolver(); resolver.setPrefix("/WEB-INF/views/"); resolver.setSuffix(".jsp"); resolver.setExposeContextBeansAsAttributes(true); return resolver; } @Override public void configureDefaultServletHandling( DefaultServletHandlerConfigurer configurer) { configurer.enable(); } } Listing 5.2 A minimal yet useful configuration for Spring MVC Enable Spring MVC Enable component-scanning Configure a JSP view resolver Configure static content handling 138 CHAPTER 5 Building Spring web applications The first thing to notice in listing 5.2 is that WebConfig is now annotated with @ComponentScan so that the spitter.web package will be scanned for components. As you’ll soon see, the controllers you write will be annotated with @Controller, which will make them candidates for component-scanning. Consequently, you won’t have to explicitly declare any controllers in the configuration class. Next, you add a ViewResolver bean. More specifically, it’s an Internal- ResourceViewResolver. We’ll talk more about view resolvers in chapter 6. For now, just know that it’s configured to look for JSP files by wrapping view names with a spe- cific prefix and suffix (for example, a view name of home will be resolved as /WEB-INF/ views/home.jsp). Finally, this new WebConfig class extends WebMvcConfigurerAdapter and overrides its configureDefaultServletHandling() method. By calling enable() on the given DefaultServletHandlerConfigurer, you’re asking DispatcherServlet to forward requests for static resources to the servlet container’s default servlet and not to try to handle them itself. With WebConfig settled, what about RootConfig? Because this chapter is focused on web development, and web configuration is done in the application context cre- ated by DispatcherServlet, you’ll keep RootConfig relatively simple for now: package spittr.config; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.ComponentScan.Filter; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.FilterType; import org.springframework.web.servlet.config.annotation.EnableWebMvc; @Configuration @ComponentScan(basePackages={"spitter"}, excludeFilters={ @Filter(type=FilterType.ANNOTATION, value=EnableWebMvc.class) }) public class RootConfig { } The only significant thing to note in RootConfig is that it’s annotated with @Component- Scan. There will be plenty of opportunities throughout this book to flesh out Root- Config with non-web components. You’re almost ready to start building a web application with Spring MVC. The big question at this point is what application you’ll build. 5.1.3 Introducing the Spittr application In an attempt to get in on the online social networking game, you’re going to develop a simple microblogging application. In many ways, your application will be much like the original microblogging application, Twitter. You’ll add some little twists on the idea along the way. And, of course, you’ll develop it using Spring. 139Writing a simple controller Borrowing some ideas from Twitter and implementing them in Spring gives the application a working title: Spitter. Taking it a step further and applying a naming pat- tern that’s popular with sites like Flickr, let’s drop the e and call the app Spittr. This name will also be helpful in differentiating the application name from a domain type you’ll create called Spitter. The Spittr application has two essential domain concepts: spitters (the users of the application) and spittles (the brief status updates that users publish). We’ll draw pri- marily on these two domain concepts throughout this book as we flesh out the func- tionality of the Spittr application. Initially, in this chapter, you’ll build out the web layer of the application, create controllers that display spittles, and process forms where users register as spitters. The stage is now set. You’ve configured DispatcherServlet, enabled essential Spring MVC components, and established a target application. Let’s turn to the meat of the chapter: handling web requests with Spring MVC controllers. 5.2 Writing a simple controller In Spring MVC, controllers are just classes with methods that are annotated with @RequestMapping to declare the kind of requests they’ll handle. Starting simple, let’s imagine a controller class that handles requests for / and ren- ders the application’s home page. HomeController, shown in the following listing, is an example of what might be the simplest possible Spring MVC controller class. package spittr.web; import static org.springframework.web.bind.annotation.RequestMethod.*; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RequestMethod; @Controller public class HomeController { @RequestMapping(value="/", method=GET) public String home() { return "home"; } } The first thing you’ll notice about HomeController is that it’s annotated with @Controller. Although it’s clear that this annotation declares a controller, the annota- tion has little to do with Spring MVC. @Controller is a stereotype annotation, based on the @Component annotation. Its purpose here is entirely for the benefit of component-scanning. Because Home- Controller is annotated with @Controller, the component scanner will automatically pick up HomeController and declare it as a bean in the Spring application context. Listing 5.3 HomeController: an example of an extremely simple controller Declared to be a controller Handle GET requests for / View name is home 140 CHAPTER 5 Building Spring web applications You could have annotated HomeController with @Component, and it would have had the same effect, but it would have been less expressive about what type of component HomeController is. HomeController’s only method, the home() method, is annotated with @Request- Mapping. The value attribute specifies the request path that this method will handle, and the method attribute details the HTTP method that it can handle. In this case, whenever an HTTP GET request comes in for /, the home() method will be called. As you can see, the home() method doesn’t do much: it returns a String value of “home”. This String will be interpreted by Spring MVC as the name of the view that will be rendered. DispatcherServlet will ask the view resolver to resolve this logical view name into an actual view. Given the way you configured InternalResourceViewResolver, the view name “home” will be resolved as a JSP at /WEB-INF/views/home.jsp. For now, you’ll keep the Spittr application’s home page rather basic, as shown next. <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %> <%@ page session="false" %> Spittr " >

Welcome to Spittr

">Spittles | ">Register There’s nothing noteworthy about this JSP. It merely welcomes the user to the applica- tion and offers two links: one to view a Spittle list and another to register with the application. Figure 5.2 shows what the home page looks like at this point. Before this chapter is complete, you’ll have implemented the controller methods to handle those requests. But for now, let’s throw some requests at this controller and see if it works. The obvious way to test a controller may be to build and deploy the applica- tion and poke at it with a web browser, but an automated test will give you quicker feed- back and more consistent hands-off results. So, let’s cover HomeController with a test. 5.2.1 Testing the controller Take another look at HomeController. If you squint really hard—so hard that you can’t see the annotations—you’ll see that what’s left is a simple POJO. And you know Listing 5.4 Spittr home page, defined as a simple JSP 141Writing a simple controller it’s easy to test POJOs. Therefore, you can test HomeController by writing a simple test like the following. package spittr.web; import static org.junit.Assert.assertEquals; import org.junit.Test; import spittr.web.HomeController; public class HomeControllerTest { @Test public void testHomePage() throws Exception { HomeController controller = new HomeController(); assertEquals("home", controller.home()); } } Although the test in listing 5.5 is straightforward, it only tests what happens in the home() method. It calls home() directly and asserts that a String containing the value “home” is returned. It completely fails to test what makes that method a Spring MVC controller method. Nothing about the test asserts that home() will be called when a GET request for / comes in. And just because it returns “home”, there’s nothing to truly test that home is the name of the view. Starting with Spring 3.2, however, you have a way to test Spring MVC controllers as controllers, not merely as POJOs. Spring now includes a mechanism for mocking all Listing 5.5 HomeControllerTest: tests HomeController Figure 5.2 The Spittr home page in action 142 CHAPTER 5 Building Spring web applications the mechanics of Spring MVC and executing HTTP requests against controllers. This will enable you to test your controllers without firing up a web server or web browser. To demonstrate proper testing of a Spring MVC controller, you can rewrite Home- ControllerTest to take advantage of the new Spring MVC testing features. The fol- lowing listing shows the new HomeControllerTest. package spittr.web; import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*; import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*; import static org.springframework.test.web.servlet.setup.MockMvcBuilders.*; import org.junit.Test; import org.springframework.test.web.servlet.MockMvc; import spittr.web.HomeController; public class HomeControllerTest { @Test public void testHomePage() throws Exception { HomeController controller = new HomeController(); MockMvc mockMvc = standaloneSetup(controller).build(); mockMvc.perform(get("/")) .andExpect(view().name("home")); } } Even though this new version of the test is a few lines longer than its predecessor, it more completely tests HomeController. Rather than call home() directly and test its return value, this test issues a GET request for / and asserts that the resulting view is named home. It starts by passing an instance of HomeController to MockMvcBuilders .standaloneSetup() and calling build() to set up the MockMvc instance. Then it asks the MockMvc instance to perform a GET request for / and sets an expectation for the view name. 5.2.2 Defining class-level request handling Now that you have a test around HomeController, you can do a bit of refactoring to be certain that nothing breaks. One thing you can do is split up @RequestMapping by placing the path-mapping portion of it at the class level. The next listing shows how this is done. package spittr.web; import static org.springframework.web.bind.annotation.RequestMethod.*; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.RequestMapping; Listing 5.6 Revised HomeControllerTest Listing 5.7 Splitting the @RequestMapping in HomeController Set up MockMvc Perform GET / Expect home view 143Writing a simple controller import org.springframework.web.bind.annotation.RequestMethod; @Controller @RequestMapping("/") public class HomeController { @RequestMapping(method=GET) public String home() { return "home"; } } In this new version of HomeController, the path has been moved up to a new class- level @RequestMapping, whereas the HTTP method is still mapped at the method level. Any time there’s a class-level @RequestMapping on a controller class, it applies to all handler methods in the controller. Then any @RequestMapping annotations on handler methods will complement the class-level @RequestMapping. In the case of HomeController, there’s only one handler method. Its @RequestMap- ping, when taken together with the class-level @RequestMapping, indicates that the home() method will handle GET requests for /. In other words, you really haven’t changed anything. You’ve moved a few things around, but HomeController still does the same thing as before. Because you have a test, you can be sure you haven’t broken anything along the way. While you’re tinkering with the @RequestMapping annotations, you can make another tweak to HomeController. The value attribute of @RequestMapping accepts an array of String. So far, you’ve only given it a single String value of “/”. But you can also map it to requests whose path is /homepage by changing the class-level @RequestMapping to look like this: @Controller @RequestMapping({"/", "/homepage"}) public class HomeController { ... } Now HomeController’s home() method is mapped to handle GET requests for both / and /homepage requests. 5.2.3 Passing model data to the view As it stands now, HomeController is a great example of how to write an extremely sim- ple controller. But most controllers aren’t this simple. In the Spittr application, you’ll need a page that displays a list of the most recent spittles that have been submitted. Therefore, you’ll need a new method to serve such a page. First you need to define a repository for data access. For decoupling purposes, and so you don’t get bogged down in database specifics, you’ll define the repository as an interface now and create an implementation of it later (in chapter 10). At the moment, you only need a repository that can fetch a list of the spittles. Spittle- Repository, as defined here, is a sufficient start: Map controller to / Handle GET requests View name is home 144 CHAPTER 5 Building Spring web applications package spittr.data; import java.util.List; import spittr.Spittle; public interface SpittleRepository { List findSpittles(long max, int count); } The findSpittles() method takes two parameters. The max parameter is a Spittle ID that represents the maximum ID of any Spittle that should be returned. As for the count parameter, it indicates how many Spittle objects to return. In order to get the 20 most recent Spittle objects, you can call findSpittles() like this: List recent = spittleRepository.findSpittles(Long.MAX_VALUE, 20); You’ll keep the Spittle class fairly simple for now, as shown next. It will have proper- ties to carry a message, a timestamp, and the latitude/longitude of the location from which the spittle was posted. package spittr; import java.util.Date; public class Spittle { private final Long id; private final String message; private final Date time; private Double latitude; private Double longitude; public Spittle(String message, Date time) { this(message, time, null, null); } public Spittle( String message, Date time, Double longitude, Double latitude) { this.id = null; this.message = message; this.time = time; this.longitude = longitude; this.latitude = latitude; } public long getId() { return id; } public String getMessage() { return message; } public Date getTime() { return time; } Listing 5.8 Spittle class: carries a message, a timestamp, and a location 145Writing a simple controller public Double getLongitude() { return longitude; } public Double getLatitude() { return latitude; } @Override public boolean equals(Object that) { return EqualsBuilder.reflectionEquals(this, that, "id", "time"); } @Override public int hashCode() { return HashCodeBuilder.reflectionHashCode(this, "id", "time"); } } For the most part, Spittle is a basic POJO data object—nothing complicated. The only thing to note is that you’re using Apache Commons Lang for easy implementa- tion of the equals() and hashCode() methods. Aside from the general utility value of those methods, they’ll be valuable in writing a test for the controller handler method. While we’re on the subject of testing, let’s go ahead and write a test for the new controller method. The following listing uses Spring’s MockMvc to assert the behavior you want in the new handler method. @Test public void shouldShowRecentSpittles() throws Exception { List expectedSpittles = createSpittleList(20); SpittleRepository mockRepository = mock(SpittleRepository.class); when(mockRepository.findSpittles(Long.MAX_VALUE, 20)) .thenReturn(expectedSpittles); SpittleController controller = new SpittleController(mockRepository); SpittleController controller = new SpittleController(mockRepository); MockMvc mockMvc = standaloneSetup(controller) .setSingleView( new InternalResourceView("/WEB-INF/views/spittles.jsp")) .build(); mockMvc.perform(get("/spittles")) .andExpect(view().name("spittles")) .andExpect(model().attributeExists("spittleList")) .andExpect(model().attribute("spittleList", hasItems(expectedSpittles.toArray()))); } ... Listing 5.9 Testing that SpittleController handles GET requests for /spittles Mock repository Mock Spring MVC GET /spittles Assert expectations 146 CHAPTER 5 Building Spring web applications private List createSpittleList(int count) { List spittles = new ArrayList(); for (int i=0; i < count; i++) { spittles.add(new Spittle("Spittle"+i,newDate())); } return spittles; } This test starts by creating a mock implementation of the SpittleRepository inter- face that will return a list of 20 Spittle objects from its findSpittles() method. It then injects that repository into a new SpittleController instance and sets up Mock- Mvc to use that controller. Notice that unlike HomeControllerTest, this test calls setSingleView() on the MockMvc builder. This is so the mock framework won’t try to resolve the view name coming from the controller on its own. In many cases, this is unnecessary. But for this controller method, the view name will be similar to the request’s path; left to its default view resolution, MockMvc will fail because the view path will be confused with the controller’s path. The actual path given when constructing the Internal- ResourceView is unimportant in this test, but you set it to be consistent with how you’ve configured InternalResourceViewResolver. The test wraps up by performing a GET request for /spittles and asserting that the view name is spittles and that the model has an attribute named spittleList with the expected contents. Of course, if you ran the test at this point, it would fail. It wouldn’t just fail to run; it would fail to compile. That’s because you haven’t yet written the Spittle- Controller. Let’s create a SpittleController so that it satisfies the expectations of the test in listing 5.9. Here’s an implementation of SpittleController that should satisfy the test. package spittr.web; import java.util.List; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RequestMethod; import spittr.Spittle; import spittr.data.SpittleRepository; @Controller @RequestMapping("/spittles") public class SpittleController { private SpittleRepository spittleRepository; @Autowired public SpittleController( SpittleRepository spittleRepository) { this.spittleRepository = spittleRepository; Listing 5.10 SpittleController: places a list of recent spittles in the model Inject SpittleRepository 147Writing a simple controller } @RequestMapping(method=RequestMethod.GET) public String spittles(Model model) { model.addAttribute( spittleRepository.findSpittles( Long.MAX_VALUE, 20)); return "spittles"; } } As you can see, SpittleController has a constructor that’s annotated with @Autowired to be given a SpittleRepository. That SpittleRepository is then used in the spittles() method to fetch a list of recent spittles. Notice that the spittles() method is given a Model as a parameter. This is so that spittles() can populate the model with the Spittle list it retrieves from the reposi- tory. The Model is essentially a map (that is, a collection of key-value pairs) that will be handed off to the view so that the data can be rendered to the client. When add- Attribute() is called without specifying a key, the key is inferred from the type of object being set as the value. In this case, because it’s a List, the key will be inferred as spittleList. The last thing spittles() does is return spittles as the name of the view that will render the model. If you’d prefer to be explicit about the model key, you’re welcome to specify it. For example, the following version of spittles() is equivalent to the one in listing 5.10: @RequestMapping(method=RequestMethod.GET) public String spittles(Model model) { model.addAttribute("spittleList", spittleRepository.findSpittles(Long.MAX_VALUE, 20)); return "spittles"; } Likewise, if you’d prefer to work with a non-Spring type, you can ask for a java .util.Map instead of Model. Here’s another version of spittles() that’s functionally equivalent to the others: @RequestMapping(method=RequestMethod.GET) public String spittles(Map model) { model.put("spittleList", spittleRepository.findSpittles(Long.MAX_VALUE, 20)); return "spittles"; } And while we’re on the subject of alternate implementations, here’s another way to write the spittles() method: @RequestMapping(method=RequestMethod.GET) public List spittles() { return spittleRepository.findSpittles(Long.MAX_VALUE, 20)); } Add spittles to model Return view name 148 CHAPTER 5 Building Spring web applications This version is quite a bit different from the others. Rather than return a logical view name and explicitly setting the model, this method returns the Spittle list. When a handler method returns an object or a collection like this, the value returned is put into the model, and the model key is inferred from its type (spittleList, as in the other examples). As for the logical view name, it’s inferred from the request path. Because this method handles GET requests for /spittles, the view name is spittles (chopping off the leading slash). No matter which way you choose to write the spittles() method, the result is the same. A list of Spittle objects is stored in the model with a key of spittleList and given to the view whose name is spittles. Given the way you’ve configured InternalResourceViewResolver, that view is a JSP at /WEB-INF/views/spittles.jsp. Now that there’s data in the model, how does the JSP access it? As it turns out, when the view is a JSP, the model data is copied into the request as request attributes. Therefore, the spittles.jsp file can use JavaServer Pages Standard Tag Library’s (JSTL) tag to render the list of spittles:
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  • Figure 5.3 will help you visualize how this might look in your web browser. Although SpittleController is simple, it’s still a step up from what you wrote in HomeController. One thing that neither HomeController nor SpittleController does, however, is handle any form of input. Let’s expand on SpittleController to take some input from the client. 5.3 Accepting request input Some web applications are read-only. Humans poke about on the website in their web browser, reading whatever content the server sends to the browser. The good news is that it doesn’t have to be that way. Many web applications give the user an opportunity to chime in and send data back to the server. Without this capability, the web would be a very different place. 149Accepting request input Spring MVC provides several ways that a client can pass data into a controller’s handler method. These include  Query parameters  Form parameters  Path variables You’ll see how to write controllers to handle input using all of these mechanisms. For a start, let’s look at handling requests with query parameters, the simplest and most straightforward way to send data from the client to the server. 5.3.1 Taking query parameters One thing that your Spittr application will need to do is display a paged list of spittles. As it is, SpittleController only displays the most recent spittles; it offers no way to page back through the history of the spittles that have been written. If you’re going to let users go through spittle history a page at a time, you’ll need to offer a way for them to pass in parameters that determine which set of spittles to display. In deciding how to do this, consider that if you’re viewing a page of spittles, it’s ordered with the most recent spittle first. Therefore, the first spittle on the next page should have an ID that is before the ID of the last spittle on the current page. So, in order to display the next page of spittles, you should be able to pass in a spittle ID that is just less than the ID of the last spittle on the current page. You can also pass in a parameter saying how many spittles to display. Figure 5.3 Spittle model data from a controller is made available as request parameters and rendered as a list on a web page. 150 CHAPTER 5 Building Spring web applications To implement this paging solution, you’ll need to write a handler method that accepts the following:  A before parameter (which indicates the ID of the Spittle that all Spittle objects in the results are before)  A count parameter (which indicates how many spittles to include in the result) To achieve this, let’s replace the spittles() method you created in listing 5.10 with a new spittles() method that works with the before and count parameters. You’ll start by adding a test to reflect the functionality you want to see from the new spittles() method. @Test public void shouldShowPagedSpittles() throws Exception { List expectedSpittles = createSpittleList(50); SpittleRepository mockRepository = mock(SpittleRepository.class); when(mockRepository.findSpittles(238900, 50)) .thenReturn(expectedSpittles); SpittleController controller = new SpittleController(mockRepository); MockMvc mockMvc = standaloneSetup(controller) .setSingleView( new InternalResourceView("/WEB-INF/views/spittles.jsp")) .build(); mockMvc.perform(get("/spittles?max=238900&count=50")) .andExpect(view().name("spittles")) .andExpect(model().attributeExists("spittleList")) .andExpect(model().attribute("spittleList", hasItems(expectedSpittles.toArray()))); } The key difference between this test method and the one in listing 5.9 is that it per- forms a GET request against /spittles, passing in values for the max and count parame- ters. This tests the handler method when those parameters are present; the other test method tests for when those parameters are absent. With both tests in place, you can be assured that no matter what changes you make to the controller, it will still be able to handle both kinds of requests: @RequestMapping(method=RequestMethod.GET) public List spittles( @RequestParam("max") long max, @RequestParam("count") int count) { return spittleRepository.findSpittles(max, count); } If the handler method in SpittleController is going to handle requests with or with- out the max and count parameters, you’ll need to change it to accept those parameters Listing 5.11 New method to test for a paged list of spittles Expect max and count parameters Pass max and count parameters 151Accepting request input but still default to Long.MAX_VALUE and 20 if those parameters are absent on the request. The defaultValue attribute of @RequestParam will do the trick: @RequestMapping(method=RequestMethod.GET) public List spittles( @RequestParam(value="max", defaultValue=MAX_LONG_AS_STRING) long max, @RequestParam(value="count", defaultValue="20") int count) { return spittleRepository.findSpittles(max, count); } Now, if the max parameter isn’t specified, it will default to the maximum value of Long. Because query parameters are always of type String, the defaultValue attribute requires a String value. Therefore, Long.MAX_VALUE won’t work. Instead, you can cap- ture Long.MAX_VALUE in a String constant named MAX_LONG_AS_STRING: private static final String MAX_LONG_AS_STRING = Long.toString(Long.MAX_VALUE); Even though the defaultValue is given as a String, it will be converted to a Long when bound to the method’s max parameter. The count parameter will default to 20 if the request doesn’t have a count parameter. Query parameters are a common way to pass information to a controller in a request. Another way that’s popular, especially in a discussion of building resource- oriented controllers, is to pass parameters as part of the request path. Let’s see how to use path variables to take input as part of the request path. 5.3.2 Taking input via path parameters Let’s say your application needs to support the display of a single Spittle, given its ID. One option you have is to write a handler method that accepts the ID as a query parameter using @RequestParam: @RequestMapping(value="/show", method=RequestMethod.GET) public String showSpittle( @RequestParam("spittle_id") long spittleId, Model model) { model.addAttribute(spittleRepository.findOne(spittleId)); return "spittle"; } This handler method would handle requests such as /spittles/show?spittle_id=12345. Although this could be made to work, it’s not ideal from a resource-orientation per- spective. Ideally, the resource being identified (the Spittle) would be identified by the URL path, not by query parameters. As a general rule, query parameters should not be used to identify a resource. A GET request for /spittles/12345 is better than one for /spittles/show?spittle_id=12345. The former identifies a resource to be retrieved. The latter describes an operation with a parameter—essentially RPC over HTTP. 152 CHAPTER 5 Building Spring web applications With the goal of resource-oriented controllers in mind, let’s capture this require- ment in a test. The following listing shows a new test method to assert resource- oriented request handling in SpittleController. @Test public void testSpittle() throws Exception { Spittle expectedSpittle = new Spittle("Hello", new Date()); SpittleRepository mockRepository = mock(SpittleRepository.class); when(mockRepository.findOne(12345)).thenReturn(expectedSpittle); SpittleController controller = new SpittleController(mockRepository); MockMvc mockMvc = standaloneSetup(controller).build(); mockMvc.perform(get("/spittles/12345")) .andExpect(view().name("spittle")) .andExpect(model().attributeExists("spittle")) .andExpect(model().attribute("spittle", expectedSpittle)); } As you can see, this test sets up a mock repository, a controller, and MockMvc, much like the other tests you’ve written in this chapter. The most important part of the test is in the last few lines, where it performs a GET request for /spittles/12345 and asserts that the view name is spittle and that the expected Spittle object is placed in the model. Because you haven’t yet implemented the handler method for that kind of request, the request will fail. But you can fix that by adding a new method to Spittle- Controller. Up to this point, all of your controller methods have been mapped (via @Request- Mapping) to a statically defined path. But if you’re going to make this test pass, you’ll need to write an @RequestMapping that has a variable portion of the path that repre- sents the Spittle ID. To accommodate these path variables, Spring MVC allows for placeholders in an @RequestMapping path. The placeholders are names surrounded by curly braces ({ and }). Although all the other parts of the path need to match exactly for the request to be handled, the placeholder can carry any value. Here’s a handler method that uses placeholders to accept a Spittle ID as part of the path: @RequestMapping(value="/{spittleId}", method=RequestMethod.GET) public String spittle( @PathVariable("spittleId") long spittleId, Model model) { model.addAttribute(spittleRepository.findOne(spittleId)); return "spittle"; } For example, it can handle requests for /spittles/12345, the path being tested for in listing 5.12. Listing 5.12 Testing a request for a Spittle with ID specified in a path variable Request resource via path 153Accepting request input As you can see, spittle() has a spittleId parameter that is annotated with @PathVariable("spittleId"). This indicates that whatever value is at the place- holder position in the request path will be passed into the handler method’s spittleId parameter. If the request is a GET request for /spittles/54321, then 54321 will be passed in as the value of spittleId. Notice that the phrase spittleId is repeated a few times in the example: in the @RequestMapping path, as the value attribute of @PathVariable, and again as a method parameter name. Because the method parameter name happens to be the same as the placeholder name, you can optionally omit the value parameter on @PathVariable: @RequestMapping(value="/{spittleId}", method=RequestMethod.GET) public String spittle(@PathVariable long spittleId, Model model) { model.addAttribute(spittleRepository.findOne(spittleId)); return "spittle"; } If no value attribute is given for @PathVariable, it assumes the placeholder’s name is the same as the method parameter name. This can make the code a little cleaner by not duplicating the placeholder name any more than necessary. But be cautioned: if you decide to rename the parameter, you must also change the placeholder name to match. The spittle() method will pass the parameter along to the findOne() method on the SpittleRepository to find a single Spittle object and will add that Spittle to the model. The model key will be spittle, inferred by the type passed in to add- Attribute(). The data in the Spittle object can then be rendered in the view by referring to the request attribute whose key is spittle (the same as the model key). Here’s a snip- pet of a JSP view that renders the Spittle:
    There’s nothing flashy about this view, as you can see from the screenshot in figure 5.4. Query parameters and path parameters are fine for passing small amounts of data on a request. But often you need to pass a lot of data (perhaps data coming from a form submission), and query parameters are too awkward and limited for that. Let’s see how you can write controller methods that handle form submissions. 154 CHAPTER 5 Building Spring web applications 5.4 Processing forms Web applications typically do more than just push content out to the user. Most also let users participate in the conversation by filling out forms and submitting data back into the application. Spring MVC controllers are well-suited for form processing as well as serving content. There are two sides to working with forms: displaying the form and processing the data the user submits from the form. In the Spittr application, you’ll need a form for new users to register with the application. SpitterController is a new controller with a single request-handling method for displaying the registration form. package spittr.web; import static org.springframework.web.bind.annotation.RequestMethod.*; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Controller; import org.springframework.ui.Model; import org.springframework.web.bind.annotation.PathVariable; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RequestMethod; import spittr.Spitter; import spittr.data.SpitterRepository; @Controller @RequestMapping("/spitter") public class SpitterController { Listing 5.13 SpitterController: displays a form for users to sign up with the app Figure 5.4 Displaying a spittle in the browser 155Processing forms @RequestMapping(value="/register", method=GET) public String showRegistrationForm() { return "registerForm"; } } The showRegistrationForm() method’s @RequestMapping annotation, along with the class-level @RequestMapping annotation, declares that it will handle HTTP GET requests for /spitter/register. It’s a simple method, taking no input and only returning a logi- cal view named registerForm. Given how you’ve configured InternalResourceView- Resolver, that means the JSP at /WEB-INF/views/registerForm.jsp will be called on to render the registration form. As simple as showRegistrationForm() is, it still deserves to be covered by a test. Because it’s a simple method, its test will be equally simple. @Test public void shouldShowRegistration() throws Exception { SpitterController controller = new SpitterController(); MockMvc mockMvc = standaloneSetup(controller).build(); mockMvc.perform(get("/spitter/register")) .andExpect(view().name("registerForm")); } This test method is very similar to the test for the home page controller method. It performs a GET request for /spitter/register and then asserts that the resulting view is named registerForm. Now let’s get back to the view. Because the view name is registerForm, you’ll need a JSP named registerForm.jsp. This JSP must include an HTML
    where the user will enter information to sign up with the application. Here’s the JSP you’ll use for now. <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %> <%@ page session="false" %> Spittr " >

    Register

    First Name:
    Last Name:
    Username:
    Password:
    Listing 5.14 Testing a form-displaying controller method Listing 5.15 JSP to render a registration form Handle GET requests for /spitter/register Set up MockMvc Assert registerForm view 156 CHAPTER 5 Building Spring web applications
    As you can see, this JSP is fairly basic. It has HTML form fields to capture the user’s first name, last name, a username, and a password, as well as a button to submit the form. Rendered in the browser, it looks a little something like figure 5.5. Notice that the
    tag doesn’t have an action parameter set. Because of that, when this form is submitted, it will be posted back to the same URL path that displayed it. That is, it will be posted back to /spitters/register. That means you’ll need something back on the server to handle the HTTP POST request. Let’s add another method to SpitterController to handle form submission. 5.4.1 Writing a form-handling controller When processing the POST request from the registration form, the controller needs to accept the form data and save the form data as a Spitter object. Finally, in order to prevent a duplicate submission (such as might happen if the user clicked their browser’s Refresh button), it should redirect the browser to the newly created user’s profile page. This behavior is captured and tested in shouldProcessRegistration(). Figure 5.5 The registration page offers a form that will be processed by SpitterController to add a new user to the application. 157Processing forms @Test public void shouldProcessRegistration() throws Exception { SpitterRepository mockRepository = mock(SpitterRepository.class); Spitter unsaved = new Spitter("jbauer", "24hours", "Jack", "Bauer"); Spitter saved = new Spitter(24L, "jbauer", "24hours", "Jack", "Bauer"); when(mockRepository.save(unsaved)).thenReturn(saved); SpitterController controller = new SpitterController(mockRepository); MockMvc mockMvc = standaloneSetup(controller).build(); mockMvc.perform(post("/spitter/register") .param("firstName", "Jack") .param("lastName", "Bauer") .param("username", "jbauer") .param("password", "24hours")) .andExpect(redirectedUrl("/spitter/jbauer")); verify(mockRepository, atLeastOnce()).save(unsaved); } Clearly, this test is more involved than the test for displaying the registration form. After setting up a mock implementation of SpitterRepository and creating a control- ler and MockMvc setup to execute against, shouldProcessRegistration() performs a POST request against /spitter/register. As part of that POST request, user information is passed as parameters on the request to simulate a form being submitted. When handling a POST request, it’s usually a good idea to send a redirect after the POST has completed processing so that a browser refresh won’t accidentally submit the form a second time. This test expects that the request will end in a redirect to /spitter /jbauer, the URL path of the new user’s profile page. Finally, the test verifies that the mocked SpitterRepository was actually used to save the data coming in on the form. Now let’s implement the controller method that will handle this form submission test. shouldProcessRegistration() may have left you with the impression that a chunk of work is required to satisfy the test. But as you can see in the new Spitter- Controller in this listing, there’s not much to it. package spittr.web; import static org.springframework.web.bind.annotation.RequestMethod.*; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Controller; import org.springframework.ui.Model; import org.springframework.web.bind.annotation.PathVariable; import org.springframework.web.bind.annotation.RequestMapping; Listing 5.16 Testing form-handling controller methods Listing 5.17 Handling form submission to register a new user Set up mock repository Set up MockMvc Perform request Verify save 158 CHAPTER 5 Building Spring web applications import spittr.Spitter; import spittr.data.SpitterRepository; @Controller @RequestMapping("/spitter") public class SpitterController { private SpitterRepository spitterRepository; @Autowired public SpitterController( SpitterRepository spitterRepository) { this.spitterRepository = spitterRepository; } @RequestMapping(value="/register", method=GET) public String showRegistrationForm() { return "registerForm"; } @RequestMapping(value="/register", method=POST) public String processRegistration(Spitter spitter) { spitterRepository.save(spitter); return "redirect:/spitter/" + spitter.getUsername(); } } The showRegistrationForm() method is still in place. But notice the new process- Registration() method: it’s given a Spitter object as a parameter. This object has firstName, lastName, username, and password properties that will be populated from the request parameters of the same name. Once it’s called with the Spitter object, processRegistration() calls the save() method on the SpitterRepository that is now injected into SpitterController in the constructor. The last thing that processRegistration() does is return a String specifying the view. But this view specification is different from what you’ve seen before. Rather than just return a view name and let the view resolver sort it out, here you’re returning a redirect specification. When InternalResourceViewResolver sees the redirect: prefix on the view spec- ification, it knows to interpret it as a redirect specification instead of as a view name. In this case, it will redirect to the path for a user’s profile page. For example, if the Spitter .username property is jbauer, then the view will redirect to /spitter/jbauer. It’s worth noting that in addition to redirect:, InternalResourceViewResolver also recognizes the forward: prefix. When it sees a view specification prefixed with forward:, the request is forwarded to the given URL path instead of redirected. Perfect! At this point, the test in listing 5.16 should pass. But you’re not finished yet. Because you’re redirecting to the user’s profile page, you should probably add a handler method to SpitterController to handle requests for the profile page. Here’s a showSpitterProfile() method that will do the trick: @RequestMapping(value="/{username}", method=GET) Inject SpitterRepository Save a Spitter Redirect to profile page 159Processing forms public String showSpitterProfile( @PathVariable String username, Model model) { Spitter spitter = spitterRepository.findByUsername(username); model.addAttribute(spitter); return "profile"; } showSpitterProfile() fetches a Spitter object from the SpitterRepository by the username. It adds the Spitter to the model and then returns profile, the logical view name for the profile view. Like all the other views presented in this chapter, you’ll keep the profile view simple for now:

    Your Profile


    Figure 5.6 shows the profile page as rendered in a web browser. What will happen if the form doesn’t send a username or password parameter? Or what if the firstName or lastName value is empty or too long? Let’s look at how to add validation to the form submission to prevent inconsistencies in the data presented. 5.4.2 Validating forms If the user were to leave the username or password field empty when submitting the form, it could result in the creation of a new Spitter object whose username and password were empty Strings. At the very least, this is odd behavior. But left Figure 5.6 The Spittr profile page displays a user’s information, as populated into the model by SpitterController. 160 CHAPTER 5 Building Spring web applications unchecked, it could present a security concern where anyone could sign in to the application by submitting an empty login form. Also, you should take steps to prevent the user from submitting an empty first- Name and/or lastName in an effort to maintain some level of anonymity. And it’s prob- ably a good idea to limit the length of the values given in those fields, keeping them at a reasonable size and avoiding misuse of the fields. One way to handle validation, albeit naive, is to add code to the process- Registration() method to check for invalid values and send the user back to the reg- istration form unless the data is valid. It’s a short method, so tossing in a few extra if statements won’t do much harm. Right? Rather than litter your handler methods with validation logic, however, you can take advantage of Spring’s support for the Java Validation API (a.k.a. JSR-303). Starting with Spring 3.0, Spring supports the Java Validation API in Spring MVC. No extra con- figuration is required to make Java Validation work in Spring MVC. You just need to make sure an implementation of the Java API, such as Hibernate Validator, is in the project’s classpath. The Java Validation API defines several annotations that you can put on properties to place constraints on the values of those properties. All of these annotations are in the javax.validation.constraints package. Table 5.1 lists these validation annotations. Table 5.1 Validation annotations provided by the Java Validation API Annotation Description @AssertFalse The annotated element must be a Boolean type and be false. @AssertTrue The annotated element must be a Boolean type and be true. @DecimalMax The annotated element must be a number whose value is less than or equal to a given BigDecimalString value. @DecimalMin The annotated element must be a number whose value is greater than or equal to a given BigDecimalString value. @Digits The annotated element must be a number whose value has a specified num- ber of digits. @Future The value of the annotated element must be a date in the future. @Max The annotated element must be a number whose value is less than or equal to a given value. @Min The annotated element must be a number whose value is greater than or equal to a given value. @NotNull The value of the annotated element must not be null. @Null The value of the annotated element must be null. @Past The value of the annotated element must be a date in the past. @Pattern The value of the annotated element must match a given regular expression. 161Processing forms In addition to the annotations in table 5.1, Java Validation API implementations may provide additional validation annotations. And it’s also possible to define your own constraints. But for our purposes, we’ll focus on a couple of the core constraint valida- tions from the table. Thinking over the constraints you need to apply to the fields in Spitter, it seems you’ll probably need the @NotNull and @Size annotations. All you need to do is toss those annotations around on the properties of Spitter. The next listing shows Spitter with its properties annotated for validation. package spittr; import javax.validation.constraints.NotNull; import javax.validation.constraints.Size; import org.apache.commons.lang3.builder.EqualsBuilder; import org.apache.commons.lang3.builder.HashCodeBuilder; public class Spitter { private Long id; @NotNull @Size(min=5, max=16) private String username; @NotNull @Size(min=5, max=25) private String password; @NotNull @Size(min=2, max=30) private String firstName; @NotNull @Size(min=2, max=30) private String lastName; ... } All the properties of Spitter are now annotated with @NotNull to ensure that they aren’t left null. Similarly, the @Size annotation is placed on the properties to con- strain them between minimum and maximum lengths. What this means in the Spittr @Size The value of the annotated element must be either a String, a collection, or an array whose length fits within the given range. Listing 5.18 SpittleForm: carries only fields submitted in a SpittlePOST request Table 5.1 Validation annotations provided by the Java Validation API (continued) Annotation Description Not null, from 5 to 16 characters Not null, from 5 to 25 characters Not null, from 2 to 30 characters Not null, from 2 to 30 characters 162 CHAPTER 5 Building Spring web applications application is that the user must completely fill out the registration form with values that fit within the size constraints. Now that you have annotated Spitter with validation constraints, you need to change the processRegistration() method to apply validation. The new validation- enabled processRegistration() is shown next. @RequestMapping(value="/register", method=POST) public String processRegistration( @Valid Spitter spitter, Errors errors) { if (errors.hasErrors()) { return "registerForm"; } spitterRepository.save(spitter); return "redirect:/spitter/" + spitter.getUsername(); } A lot has changed since the original processRegistration() in listing 5.17. The Spitter parameter is now annotated with @Valid to indicate to Spring that the com- mand object has validation constraints that should be enforced. Just having validation constraints on the Spitter’s properties won’t prevent the form from being submitted. Even if the user fails to fill in a field on the form or gives a value whose length exceeds the maximum length, the processRegistration() method will still be called. This gives you a chance to deal with the validation prob- lems however you see fit in processRegistration(). If there are any validation errors, they’re available in the Errors object that you’re now asking for as a parameter to processRegistration(). (Note that it’s important that the Errors parameter immediately follow the @Valid-annotated parameter that’s being validated.) The first thing processRegistration() does is call Errors.has- Errors() to check for any errors. If there are errors, Errors.hasErrors() returns registerForm, the view name for the registration form. This will take the user’s browser back to the registration form so they can correct any problems and try again. For now, the blank form will be dis- played, but in the next chapter, you’ll adapt the form to show the values that were originally submitted and communicate validation problems to the user. If there are no errors, the Spitter is saved via the repository, and the controller redirects to the profile page as before. 5.5 Summary In this chapter, you’ve made a good start on the web portion of your application. As you’ve seen, Spring comes with a powerful and flexible web framework. Employing annotations, Spring MVC offers a near-POJO development model, making simple work of developing controllers that handle requests and are easy to test. Listing 5.19 processRegistration(): ensures that data submitted is valid Validate Spitter input Return to form on validation errors 163Summary When it comes to writing controller handler methods, Spring MVC is extremely flexible. As a rule of thumb, if your handler method needs something, then it should ask for that object as a parameter. Likewise, anything it doesn’t need should be left out of the parameter list. This leads to infinite possibilities in request handling, while maintaining a simple programming model. Although much of this chapter focused on request handling with controllers, response rendering is also important. We briefly looked at how to write views for your controllers using JSPs. But there’s more to Spring MVC views than the basic JSPs you wrote in this chapter. Coming up in chapter 6, we’ll dig deeper into Spring views, expanding on how you can take advantage of Spring tag libraries in JSP. You’ll also see how to add consistent layouts to your views using Apache Tiles. And we’ll look at Thymeleaf, an exciting alternative to JSP that comes with built-in Spring support. 164 Rendering web views In the previous chapter, we primarily focused on writing the controllers that handle web requests. You also created some simple views to render the model data produced by those controllers, but we didn’t spend too much time discussing the views or what happens between the time a controller finishes handling a request and the time the results are displayed in the user’s web browser. That’s the topic of this chapter. 6.1 Understanding view resolution None of the methods in the controllers you wrote in chapter 5 directly produce the HTML that is rendered in the browser. Instead, they populate the model with some data and then pass the model off to a view for rendering. Those methods return a String value that is the logical name of the view but that doesn’t directly refer to a specific view implementation. Although you wrote a few simple JavaServer Page (JSP) views, nothing in the controllers is aware of that fact. This chapter covers  Rendering model data as HTML  Using JSP views  Defining view layout with tiles  Working with Thymeleaf views 165Understanding view resolution Decoupling request-handling logic in the controller from the view-rendering of a view is an important feature of Spring MVC. If the controller methods were directly responsible for producing HTML, it would be difficult to maintain and update the view without getting your hands dirty in request-handling logic. At most, the control- ler methods and view implementations should agree on the contents of the model; apart from that, they should keep an arms-length distance from each other. But if the controller only knows about the view by a logical view name, how does Spring determine which actual view implementation it should use to render the model? That’s a job for Spring’s view resolvers. In chapter 5, you used a view resolver known as InternalResourceViewResolver. It was configured to apply a prefix of /WEB-INF/views/ and a suffix of .jsp to a view name to arrive at the physical location of the JSP that would render the model. Now let’s take a step back and look at view resolution in general and some of the other view resolvers that Spring offers. Spring MVC defines an interface named ViewResolver that looks a little some- thing like this: public interface ViewResolver { View resolveViewName(String viewName, Locale locale) throws Exception; } The resolveViewName() method, when given a view name and a Locale, returns a View instance. View is another interface that looks like this: public interface View { String getContentType(); void render(Map model, HttpServletRequest request, HttpServletResponse response) throws Exception; } The View interface’s job is to take the model, as well as the servlet request and response objects, and render output into the response. It looks simple enough. All you need to do is start writing implementations of ViewResolver and View to render content into the response to be displayed in your users’ browsers. Right? Not necessarily. Although you can write your own custom implementations of ViewResolver and View, and although there are some special cases where that’s neces- sary, typically you needn’t worry yourself with these interfaces. I only mention them to give you some insight into how view resolution works. Fortunately, Spring provides sev- eral out-of-the-box implementations, listed in table 6.1, that fit most circumstances. 166 CHAPTER 6 Rendering web views Note that all the view resolvers in table 6.1 are available in both Spring 4 and Spring 3.2. And all but one of them (the Tiles 3 TilesViewResolver) are supported by Spring 3.1. We don’t have room in this book to cover all 13 view resolvers offered by Spring. But that’s okay, because there are only a handful of them that you’ll ever need in most applications. For the most part, each of the view resolvers in table 6.1 corresponds to a specific view technology available for Java web applications. InternalResourceViewResolver is typically used for JSP, TilesViewResolver is for Apache Tiles views, and FreeMarker- ViewResolver and VelocityViewResolver map to FreeMarker and Velocity template views respectively. In this chapter, we’ll focus our attention on the view technologies that are most rel- evant to the majority of Java developers. Because most Java web applications use JSP, Table 6.1 Spring comes with 13 view resolvers to translate logical view names into physical view implementations. View resolver Description BeanNameViewResolver Resolves views as beans in the Spring application context whose ID is the same as the view name. ContentNegotiatingViewResolver Resolves views by considering the content type desired by the client and delegating to another view resolver that can produce that type. FreeMarkerViewResolver Resolves views as FreeMarker templates. InternalResourceViewResolver Resolves views as resources internal to the web applica- tion (typically JSPs). JasperReportsViewResolver Resolves views as JasperReports definitions. ResourceBundleViewResolver Resolves views from a resource bundle (typically a proper- ties file). TilesViewResolver Resolves views as Apache Tile definitions, where the tile ID is the same as the view name. Note that there are two different TilesViewResolver implementations, one each for Tiles 2.0 and Tiles 3.0. UrlBasedViewResolver Resolves views directly from the view name, where the view name matches the name of a physical view definition. VelocityLayoutViewResolver Resolves views as Velocity layouts to compose pages from different Velocity templates. VelocityViewResolver Resolves views as Velocity templates. XmlViewResolver Resolves views as bean definitions from a specified XML file. Similar to BeanNameViewResolver. XsltViewResolver Resolves views to be rendered as the result of an XSLT transformation. 167Creating JSP views we’ll start by looking at InternalResourceViewResolver, the view resolver that’s typically used to resolve JSP views. Then we’ll try out TilesViewResolver to achieve layout control over JSP pages. To wrap up this chapter, we’ll look at a view-resolver option that isn’t listed in table 6.1. Thymeleaf is a compelling alternative to JSP that offers a view resolver for working with Thymeleaf’s natural templates: templates that have more in common with the HTML they produce than with the Java code that drives them. Thymeleaf is such an exciting view option that I wouldn’t blame you if you flipped a few pages ahead to section 6.4 to see how to use it with Spring. If you’re still on this page, it’s probably because you know that JSP has been, and still is, the dominant view technology for Java. You’ve probably used JSP on several projects before and are likely to need it again. So let’s start with a look at how you can use JSP views with Spring MVC. 6.2 Creating JSP views Believe it or not, JavaServer Pages has been the go-to view technology for Java-based web applications for almost 15 years. Although it started out as an ugly, Java-centric twist on similar templating technologies (such as Microsoft’s Active Server Pages), JSP has evolved over the years to include support for an expression language and custom tag libraries. Spring supports JSP views in two ways:  InternalResourceViewResolver can be used to resolve view names into JSP files. Moreover, if you’re using JavaServer Pages Standard Tag Library (JSTL) tags in your JSP pages, InternalResourceViewResolver can resolve view names into JSP files fronted by JstlView to expose JSTL locale and resource bundle variables to JSTL’s formatting and message tags.  Spring provides two JSP tag libraries, one for form-to-model binding and one providing general utility features. Whether or not you use JSTL or intend to use Spring’s JSP tag libraries, it’s important to configure a view resolver to resolve JSP views. Although a few of Spring’s other view resolvers could be used to map view names to JSP files, InternalResourceView- Resolver is the simplest and most commonly used view resolver for this task. We touched on configuring InternalResourceViewResolver in chapter 5. But that was done in haste just so you could exercise your controllers in a web browser. Let’s take a closer look at InternalResourceViewResolver and see how to tweak it to do your bidding. 6.2.1 Configuring a JSP-ready view resolver Whereas some view resolvers, such as ResourceBundleViewResolver, directly map a logical view name to a specific implementation of the View interface, Internal- ResourceViewResolver takes a more indirect approach. It follows a convention 168 CHAPTER 6 Rendering web views whereby a prefix and a suffix are attached to the view name to determine the physical path to a view resource in the same web application. As an example, consider the simple case where the logical view name is home. It’s a common practice to place JSP files under the web application’s WEB-INF folder to prevent direct access. If you were to keep all your JSP files in /WEB-INF/views/, and if your home page JSP is named home.jsp, then you could derive the physical view path by prefixing the logical view name home with /WEB-INF/views/ and adding a suffix of .jsp. This is illustrated in figure 6.1. You can configure InternalResourceViewResolver to apply this convention when resolving views by configuring it with this @Bean-annotated method: @Bean public ViewResolver viewResolver() { InternalResourceViewResolver resolver = new InternalResourceViewResolver(); resolver.setPrefix("/WEB-INF/views/"); resolver.setSuffix(".jsp"); return resolver; } Optionally, if you prefer to use Spring’s XML-based configuration, you can configure InternalResourceViewResolver like this: With this configuration of InternalResourceViewResolver in place, you can expect it to resolve logical view names into JSP files such as this:  home resolves to /WEB-INF/views/home.jsp  productList resolves to /WEB-INF/views/productList.jsp  books/detail resolves to /WEB-INF/views/books/detail.jsp Let me call particular attention to that last example. When a logical view name has a slash in it, that slash is carried over into the resource path name. Therefore, it maps to a JSP file that’s in a subdirectory of whatever directory is referenced by the prefix property. This offers a handy way of organizing your view templates under a hierarchy of directories rather than keeping them all in a single directory. RESOLVING JSTL VIEWS So far you’ve configured the basic, garden-variety InternalResourceViewResolver. It ultimately resolves logical view names into instances of InternalResourceView that /WEB-INF/views/home.jsp Prefix Logical view name Suffix Figure 6.1 InternalResourceViewResolver resolves views by adding a prefix and a suffix to the view name. 169Creating JSP views reference JSP files. But if those JSP files are using JSTL tags for formatting or messages, then you may want to configure InternalResourceViewResolver to resolve a JstlView instead. JSTL’s formatting tags need a Locale to properly format locale-specific values such as dates and money. And its message tags can use a Spring message source and a Locale to properly choose messages to render in HTML. By resolving JstlView, the JSTL tags will be given the Locale and any message source configured in Spring. All that’s needed to have InternalResourceViewResolver resolve JstlView instead of InternalResourceView is to set its viewClass property: @Bean public ViewResolver viewResolver() { InternalResourceViewResolver resolver = new InternalResourceViewResolver(); resolver.setPrefix("/WEB-INF/views/"); resolver.setSuffix(".jsp"); resolver.setViewClass( org.springframework.web.servlet.view.JstlView.class); return resolver; } Again, you can accomplish the same thing with XML: Whether you use Java configuration or XML, this will ensure that JSTL’s formatting and message tags will get the Locale and message sources configured in Spring. 6.2.2 Using Spring’s JSP libraries Tag libraries are a powerful way to bring functionality to a JSP template without resort- ing to writing Java code directly in scriptlet blocks. Spring offers two JSP tag libraries to help define the view of your Spring MVC web views. One tag library renders HTML form tags that are bound to a model attribute. The other has a hodgepodge of utility tags that come in handy from time to time. You’re likely to find the form-binding tag library to be the more useful of the two tag libraries. So that’s where you’ll start with Spring’s JSP tags. You’ll see how to bind the Spittr application’s registration form to the model so that the form will be prepop- ulated and validation errors can be displayed after a failed form submission. BINDING FORMS TO THE MODEL Spring’s form-binding JSP tag library includes 14 tags, most of which render HTML form tags. But what makes these different from the raw HTML tags is that they’re bound to an object in the model and can be populated with values from the model 170 CHAPTER 6 Rendering web views object’s properties. The tag library also includes a tag that can be used to communi- cate errors to the user by rendering them into the resulting HTML. To use the form-binding tag library, you’ll need to declare it in the JSP pages that will use it: <%@ taglib uri="http://www.springframework.org/tags/form" prefix="sf" %> Notice that I specified a prefix of sf, but it’s also common to use a prefix of form. You may specify any prefix you’d like. I chose sf because it’s succinct, easy to type, and an abbreviation for Spring forms. Throughout this book, I’ll assume a prefix of sf when- ever the form-binding library is used. Once you declare the form-binding tag library, you’re afforded 14 tags. These are listed in table 6.2. It would be hard to cook up an example to demonstrate all of these tags, and any attempt would certainly be contrived. For the Spittr example, you’ll only use the tags that are fitting for the Spittr application’s registration form. Specifically, you’ll start by using , , and . Applying those tags to the regis- tration JSP, you get the following: Table 6.2 Spring’s form-binding tag library includes tags to bind model objects to and from rendered HTML forms. JSP tag Description Renders an HTML tag with type set to checkbox. Renders multiple HTML tags with type set to checkbox. Renders field errors in an HTML tag. Renders an HTML tag and exposed binding path to inner tags for data-binding. Renders an HTML tag with type set to hidden. Renders an HTML tag with type set to text. Renders an HTML