mongodb的MapReduce主要包含兩個方法：map和reduce。

舉個例子，假設現在有下面3條記錄

{ "_id" : ObjectId("4e5ff893c0277826074ec533"), "x" : [ "a", "b" ] }{ "_id" : ObjectId("4e5ff893c0277826074ec534"), "x" : [ "b", "c" ] }{ "_id" : ObjectId("4e5ff893c0277826074ec535"), "x" : [ "c", "d" ] }

map方法調用emit方法，返回鍵值對，鍵是x[i]的值，比如a; 值是1。

function () {    for (var i = 0; i < this.x.length; i++) {        emit(this.x[i], 1);    }}

reduce方法負責把鍵值對進行統計。

function (key, values) {    var sum = 0;    for (var i = 0; i < values.length; i++)        sum += values[i];    return sum;}

執行結果如下

{ "_id" : "a", "value" : 1 }{ "_id" : "b", "value" : 2 }{ "_id" : "c", "value" : 2 }{ "_id" : "d", "value" : 1 }

假設map和reduce方法分別存放在map.js和reduce.js中，可以通過下面的代碼獲得mapreduce結果。

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js", ValueObject.class);for (ValueObject valueObject : results) {  System.out.println(valueObject);}

輸出是

ValueObject [id=a, value=1.0]ValueObject [id=b, value=2.0]ValueObject [id=c, value=2.0]ValueObject [id=d, value=1.0]

ValueObject代碼

public class ValueObject {  private String id;  private float value;  public String getId() {    return id;  }  public float getValue() {    return value;  }  public void setValue(float value) {    this.value = value;  }  @Override  public String toString() {    return "ValueObject [id=" + id + ", value=" + value + "]";  }}

可以增加一個MapReduceOptions來增加一些map－reduce的options，下面把結果存入collection。

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js",                                                                      new MapReduceOptions().outputCollection("jmr1_out"), ValueObject.class);

MapReduceOptions也可以通過靜態方法options()來實例化。

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js",                                                                      options().outputCollection("jmr1_out"), ValueObject.class);

還可以加入一個query來對統計進行過濾，下面是把包含a和b的document排除在外。

Query query = new Query(where("x").ne(new String[] { "a", "b" }));MapReduceResults<ValueObject> results = mongoOperations.mapReduce(query, "jmr1", "classpath:map.js", "classpath:reduce.js",                                                                      options().outputCollection("jmr1_out"), ValueObject.class);