快速例子学习mongodb的mapreduce

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();
  }
 }
}