标签：style blog color os io for ar cti
mongodb的MapReduce主要包含两个方法：map和reduce。
举个例子，假设现在有下面3条记录
{ "_id" : ObjectId("4e5ff893c0277826074ec533"), "x" : [ "a", "b" ] }
{ "_id" : ObjectId("4e5ff893c0277826074ec534"), "x" : [ "b", "c" ] }
{ "_id" : ObjectId("4e5ff893c0277826074ec535"), "x" : [ "c", "d" ] }
map方法调用emit方法，返回键值对，键是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方法负责把键值对进行统计。
function (key, values) { var sum = 0; for (var i = 0; i < values.length; i++) sum += values[i]; return sum; }
执行结果如下
{ "_id" : "a", "value" : 1 }
{ "_id" : "b", "value" : 2 }
{ "_id" : "c", "value" : 2 }
{ "_id" : "d", "value" : 1 }
假设map和reduce方法分别存放在map.js和reduce.js中，可以通过下面的代码获得mapreduce结果。
MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js", ValueObject.class);
for (ValueObject valueObject : results) {
System.out.println(valueObject);
}
输出是
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代码
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 + "]"; } }
可以增加一个MapReduceOptions来增加一些map－reduce的options，下面把结果存入collection。
MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js",
new MapReduceOptions().outputCollection("jmr1_out"), ValueObject.class);
MapReduceOptions也可以通过静态方法options()来实例化。
MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js",
options().outputCollection("jmr1_out"), ValueObject.class);
还可以加入一个query来对统计进行过滤，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);
[Spring Data MongoDB]学习笔记－－MapReduce,布布扣,bubuko.com
[Spring Data MongoDB]学习笔记－－MapReduce
标签：style blog color os io for ar cti
原文地址：http://www.cnblogs.com/lemonbar/p/3895000.html
