Servlet 3特性：异步Servlet
jopen
10年前
理解异步Servlet之前，让我们试着理解为什么需要它。假设我们有一个Servlet需要很多的时间来处理，类似下面的内容：
LongRunningServlet.java
package com.journaldev.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet("/LongRunningServlet") public class LongRunningServlet extends HttpServlet { private static final long serialVersionUID = 1L; protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); System.out.println("LongRunningServlet Start::Name=" + Thread.currentThread().getName() + "::ID=" + Thread.currentThread().getId()); String time = request.getParameter("time"); int secs = Integer.valueOf(time); // max 10 seconds if (secs > 10000) secs = 10000; longProcessing(secs); PrintWriter out = response.getWriter(); long endTime = System.currentTimeMillis(); out.write("Processing done for " + secs + " milliseconds!!"); System.out.println("LongRunningServlet Start::Name=" + Thread.currentThread().getName() + "::ID=" + Thread.currentThread().getId() + "::Time Taken=" + (endTime - startTime) + " ms."); } private void longProcessing(int secs) { // wait for given time before finishing try { Thread.sleep(secs); } catch (InterruptedException e) { e.printStackTrace(); } } } 如果我们的URL是：http://localhost:8080/AsyncServletExample/LongRunningServlet?time=8000
得到响应为“Processing done for 8000 milliseconds! !“。现在，如果你会查看服务器日志，会得到以下记录:
LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103 LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103::Time Taken=8002 ms.
所以Servlet线程实际运行超过 8秒，尽管大多数时间用来处理其它Servlet请求或响应。
这可能导致线程饥饿——因为我们的Servlet线程被阻塞，直到所有的处理完成。如果服务器的请求得到了很多过程，它将达到最大Servlet线程限制和进一步的请求会拒绝连接错误。
Servlet 3.0之前，这些长期运行的线程容器特定的解决方案，我们可以产生一个单独的工作线程完成耗时的任务，然后返回响应客户。Servlet线程返回Servlet池后启动工作线程。Tomcat 的 Comet、WebLogic FutureResponseServlet 和 WebSphere Asynchronous Request Dispatcher都是实现异步处理的很好示例。
容器特定解决方案的问题在于，在不改变应用程序代码时不能移动到其他Servlet容器。这就是为什么在Servlet3.0提供标准的方式异步处理Servlet的同时增加异步Servlet支持。
实现异步Servlet
让我们看看步骤来实现异步Servlet，然后我们将提供异步支持Servlet上面的例子：
- 首先Servlet,我们提供异步支持 Annotation @WebServlet 的属性asyncSupported 值为true。
- 由于实际实现委托给另一个线程，我们应该有一个线程池实现。我们可以一个通过Executors framework 创建线程池和使用servlet context listener来初始化线程池。
- 通过ServletRequest.startAsync方法获取AsyncContext的实例。AsyncContext提供方法让ServletRequest和ServletResponse对象引用。它还提供了使用调度方法将请求转发到另一个 dispatch() 方法。
- 编写一个可运行的实现,我们将进行重处理，然后使用AsyncContext对象发送请求到另一个资源或使用ServletResponse编写响应对象。一旦处理完成，我们通过AsyncContext.complete()方法通知容器异步处理完成。
- 添加AsyncListener实现AsyncContext对象实现回调方法，我们可以使用它来提供错误响应客户端装进箱的错误或超时，而异步线程处理。在这里我们也可以做一些清理工作。
一旦我们将完成我们的项目对于异步Servlet示例，项目结构看起来会像下面的图片：
在监听中初始化线程池
package com.journaldev.servlet.async; import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; import javax.servlet.ServletContextEvent; import javax.servlet.ServletContextListener; import javax.servlet.annotation.WebListener; @WebListener public class AppContextListener implements ServletContextListener { public void contextInitialized(ServletContextEvent servletContextEvent) { // create the thread pool ThreadPoolExecutor executor = new ThreadPoolExecutor(100, 200, 50000L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<Runnable>(100)); servletContextEvent.getServletContext().setAttribute("executor", executor); } public void contextDestroyed(ServletContextEvent servletContextEvent) { ThreadPoolExecutor executor = (ThreadPoolExecutor) servletContextEvent .getServletContext().getAttribute("executor"); executor.shutdown(); } } 实现很直接，如果你不熟悉ThreadPoolExecutor 框架请读线程池的ThreadPoolExecutor 。关于listeners 的更多细节，请阅读教程Servlet Listener。
工作线程实现
package com.journaldev.servlet.async; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.AsyncContext; public class AsyncRequestProcessor implements Runnable { private AsyncContext asyncContext; private int secs; public AsyncRequestProcessor() { } public AsyncRequestProcessor(AsyncContext asyncCtx, int secs) { this.asyncContext = asyncCtx; this.secs = secs; } @Override public void run() { System.out.println("Async Supported? " + asyncContext.getRequest().isAsyncSupported()); longProcessing(secs); try { PrintWriter out = asyncContext.getResponse().getWriter(); out.write("Processing done for " + secs + " milliseconds!!"); } catch (IOException e) { e.printStackTrace(); } //complete the processing asyncContext.complete(); } private void longProcessing(int secs) { // wait for given time before finishing try { Thread.sleep(secs); } catch (InterruptedException e) { e.printStackTrace(); } } } 注意：在请求和响应时使用AsyncContext对象，然后在完成时调用 asyncContext.complete() 方法。
AsyncListener 实现
package com.journaldev.servlet.async; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.AsyncEvent; import javax.servlet.AsyncListener; import javax.servlet.ServletResponse; import javax.servlet.annotation.WebListener; @WebListener public class AppAsyncListener implements AsyncListener { @Override public void onComplete(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onComplete"); // we can do resource cleanup activity here } @Override public void onError(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onError"); //we can return error response to client } @Override public void onStartAsync(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onStartAsync"); //we can log the event here } @Override public void onTimeout(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onTimeout"); //we can send appropriate response to client ServletResponse response = asyncEvent.getAsyncContext().getResponse(); PrintWriter out = response.getWriter(); out.write("TimeOut Error in Processing"); } } 通知的实现在 Timeout()方法，通过它发送超时响应给客户端。
Async Servlet 实现
这是我们的异步Servlet实现，注意使用AsyncContext和ThreadPoolExecutor进行处理。
package com.journaldev.servlet.async; import java.io.IOException; import java.util.concurrent.ThreadPoolExecutor; import javax.servlet.AsyncContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet(urlPatterns = "/AsyncLongRunningServlet", asyncSupported = true) public class AsyncLongRunningServlet extends HttpServlet { private static final long serialVersionUID = 1L; protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); System.out.println("AsyncLongRunningServlet Start::Name=" + Thread.currentThread().getName() + "::ID=" + Thread.currentThread().getId()); request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true); String time = request.getParameter("time"); int secs = Integer.valueOf(time); // max 10 seconds if (secs > 10000) secs = 10000; AsyncContext asyncCtx = request.startAsync(); asyncCtx.addListener(new AppAsyncListener()); asyncCtx.setTimeout(9000); ThreadPoolExecutor executor = (ThreadPoolExecutor) request .getServletContext().getAttribute("executor"); executor.execute(new AsyncRequestProcessor(asyncCtx, secs)); long endTime = System.currentTimeMillis(); System.out.println("AsyncLongRunningServlet End::Name=" + Thread.currentThread().getName() + "::ID=" + Thread.currentThread().getId() + "::Time Taken=" + (endTime - startTime) + " ms."); } } Run Async Servlet
现在，当我们将上面运行servlet URL：
http://localhost:8080/AsyncServletExample/AsyncLongRunningServlet?time=8000
得到响应和日志：
AsyncLongRunningServlet Start::Name=http-bio-8080-exec-50::ID=124 AsyncLongRunningServlet End::Name=http-bio-8080-exec-50::ID=124::Time Taken=1 ms. Async Supported? true AppAsyncListener onComplete
如果运行时设置time=9999，在客户端超时以后会得到响应超时错误处理和日志：
AsyncLongRunningServlet Start::Name=http-bio-8080-exec-44::ID=117 AsyncLongRunningServlet End::Name=http-bio-8080-exec-44::ID=117::Time Taken=1 ms. Async Supported? true AppAsyncListener onTimeout AppAsyncListener onError AppAsyncListener onComplete Exception in thread "pool-5-thread-6" java.lang.IllegalStateException: The request associated with the AsyncContext has already completed processing. at org.apache.catalina.core.AsyncContextImpl.check(AsyncContextImpl.java:439) at org.apache.catalina.core.AsyncContextImpl.getResponse(AsyncContextImpl.java:197) at com.journaldev.servlet.async.AsyncRequestProcessor.run(AsyncRequestProcessor.java:27) at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918) at java.lang.Thread.run(Thread.java:680)
注意：Servlet线程执行完，很快就和所有主要的处理工作是发生在其他线程。
这是所有异步Servlet内容，希望你喜欢它。下载 AsyncServletExample 工程
原文链接： journaldev 翻译： ImportNew.com - 彭秦进译文链接： http://www.importnew.com/8864.html