快速学习mongodb的mapreduce例子

map和reduce是十分有用的操作,特别是在NOSQL中.本文简单小结下
在mongodb中对mapreduce的操作,以及在JAVA中如何操作.

1 启动mongodb
   mongo启动即可

2 建立db
   use test

3 加点记录
   > book1 = {name : "Understanding JAVA", pages : 100}
> book2 = {name : "Understanding JSON", pages : 200}
   > db.books.save(book1)
> db.books.save(book2)
  继续加
  > book = {name : "Understanding XML", pages : 300}
> db.books.save(book)
> book = {name : "Understanding Web Services", pages : 400}
> db.books.save(book)
> book = {name : "Understanding Axis2", pages : 150}
> db.books.save(book)

4  先来做MAP,这里是先归类,按页数去划分分类,如下:

> var map = function() {  var category;  if ( this.pages >= 250 )   category = 'Big Books';  else   category = "Small Books";  emit(category, {name: this.name});  };

5 然后再按reduce来统计个数

> var reduce = function(key, values) {  var sum = 0;  values.forEach(function(doc) {  sum += 1;  });  return {books: sum};  };

6 然后再查看下,结果显示为:
  > var count  = db.books.mapReduce(map, reduce, {out: "book_results"});
> db[count.result].find()

{ "_id" : "Big Books", "value" : { "books" : 2 } }
{ "_id" : "Small Books", "value" : { "books" : 3 } }

7 换用JAVA去实现之,注意下载mongodb的驱动,代码如下:

import com.mongodb.BasicDBObject;  import com.mongodb.DB;  import com.mongodb.DBCollection;  import com.mongodb.DBObject;  import com.mongodb.MapReduceCommand;  import com.mongodb.MapReduceOutput;  import com.mongodb.Mongo;    public class MongoClient {     /**    * @param args    */   public static void main(String[] args) {      Mongo mongo;        try {     mongo = new Mongo("localhost", 27017);     DB db = mongo.getDB("library");       DBCollection books = db.getCollection("books");       BasicDBObject book = new BasicDBObject();     book.put("name", "Understanding JAVA");     book.put("pages", 100);     books.insert(book);          book = new BasicDBObject();       book.put("name", "Understanding JSON");     book.put("pages", 200);     books.insert(book);          book = new BasicDBObject();     book.put("name", "Understanding XML");     book.put("pages", 300);     books.insert(book);          book = new BasicDBObject();     book.put("name", "Understanding Web Services");     book.put("pages", 400);     books.insert(book);        book = new BasicDBObject();     book.put("name", "Understanding Axis2");     book.put("pages", 150);     books.insert(book);          String map = "function() { "+                "var category; " +                 "if ( this.pages >= 250 ) "+                 "category = 'Big Books'; " +               "else " +               "category = 'Small Books'; "+                 "emit(category, {name: this.name});}";          String reduce = "function(key, values) { " +                              "var sum = 0; " +                              "values.forEach(function(doc) { " +                              "sum += 1; "+                              "}); " +                              "return {books: sum};} ";          MapReduceCommand cmd = new MapReduceCommand(books, map, reduce,       null, MapReduceCommand.OutputType.INLINE, null);       MapReduceOutput out = books.mapReduce(cmd);       for (DBObject o : out.results()) {      System.out.println(o.toString());     }    } catch (Exception e) {     // TODO Auto-generated catch block     e.printStackTrace();    }   }  }