学步园

本文主要讲怎么在mongodb中使用mapreduce，而不涉及Spring与Mongodb整合等内容，如想了解这些，请参考我的其它文章。

这个例子主要目的是：统计每个名字有多少人重名。

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.FileSystemXmlApplicationContext;
import org.springframework.data.mongodb.core.MongoTemplate;

import com.mongodb.DBCollection;
import com.mongodb.DBCursor;
import com.mongodb.MapReduceOutput;
import com.zolcorp.mahout.model.Student;
import com.zolcorp.mahout.service.StudentService;

/**
 * @author hadoop
 * 
 */
public class MapReduceMain {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		ApplicationContext context = new FileSystemXmlApplicationContext(
				new String[] { "classpath:applicationContext-bean.xml",
						"classpath:applicationContext-datasource.xml" });
		StudentService studentService = context.getBean("studentService", StudentService.class);
		studentService.add(new Student("ming", 25));
		studentService.add(new Student("gang", 24));
		studentService.add(new Student("ming", 24));
		MongoTemplate mongoTemplate = context.getBean("mongoTemplate",
				MongoTemplate.class);
		DBCollection coll = mongoTemplate.getCollection("student");
		String map = "function() { emit(this.name, {count:1});}";
		String reduce = "function(key, values) {var total = 0;for(var i=0;i<values.length;i++){total += values[i].count;}return {count:total};}";
		String result = "resultCollection";
		MapReduceOutput mapReduceOutput = coll.mapReduce(map,
				reduce.toString(), result, null);
		DBCollection resultColl = mapReduceOutput.getOutputCollection();
		DBCursor cursor = resultColl.find();
		while (cursor.hasNext()) {
			System.out.println(cursor.next());
		}
	}

}

我们在student这个集合中，加入了三条数据。Student.java如下：

import org.springframework.data.mongodb.core.mapping.Document;

/**
 * @author hadoop
 *
 */
@Document(collection="student")
public class Student {
	
	private String id;
	
	private String name;
	
	private int age;
	
	public Student()
	{
		this("", 0);
	}
	
	public Student(String name, int age)
	{
		this.name = name;
		this.age = age;
	}

	public String getId() {
		return id;
	}

	public void setId(String id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}
}

插入数据后，库中的内容如下：

程序运行结果如下：

{ "_id" : "gang" , "value" : { "count" : 1.0}}
{ "_id" : "ming" , "value" : { "count" : 2.0}}

mapreduce的函数都是用javasript编写，还有如果想了解mapreduce的话，请参考其它的文档，这篇文档只是告诉大家，在mongodb中使用mapreduce的方法。如果读者用过hadoop的话的，应该能发现mangodb的mapreduce有很多优点，先不做阐述，以后再补充吧。