UDAF实现一个自定义的求和函数

转自：http://richie-hu.blog.sohu.com/141308974.html

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Hive进行UDAF开发，相对要比UDF复杂一些，不过也不是很难。

请看一个例子

package org.hrj.hive.udf;

import org.apache.hadoop.hive.ql.exec.UDAFEvaluator;
import org.apache.hadoop.hive.serde2.io.DoubleWritable;

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public class UDAFSum_Sample extends NumericUDAF {
??? public static class Evaluator implements UDAFEvaluator {
??????? private boolean mEmpty;
??????? private double mSum;
??????? public Evaluator() {
??????????? super();
??????????? init();
??????? }

??????? public void init() {
??????????? mSum = 0;
??????????? mEmpty = true;
??????? }

??????? public boolean iterate(DoubleWritable o) {
??????????? if (o != null) {
??????????????? mSum += o.get();
??????????????? mEmpty = false;
??????????? }
??????????? return true;
??????? }

??????? public DoubleWritable terminatePartial() {
??????????? // This is SQL standard - sum of zero items should be null.
??????????? return mEmpty ? null : new DoubleWritable(mSum);
??????? }

??????? public boolean merge(DoubleWritable o) {
??????????? if (o != null) {
??????????????? mSum += o.get();
??????????????? mEmpty = false;
??????????? }
??????????? return true;
??????? }

??????? public DoubleWritable terminate() {
??????????? // This is SQL standard - sum of zero items should be null.
??????????? return mEmpty ? null : new DoubleWritable(mSum);
??????? }
??? }
}

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1.将java文件编译成Sum_Sample.jar

2.进入hive

hive> add jar Sum_sample.jar;

hive> create temporary function sum_test as 'com.hrj.hive.udf.UDAFSum_Sample';

hive> select sum_test(t.num) from t;

hive> drop temporary function sum_test;

hive> quit;

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关于UDAF开发注意点：

1.需要import org.apache.hadoop.hive.ql.exec.UDAF以及org.apache.hadoop.hive.ql.exec.UDAFEvaluator,这两个包都是必须的

2.函数类需要继承UDAF类，内部类Evaluator实现UDAFEvaluator接口

3.Evaluator需要实现 init、iterate、terminatePartial、merge、terminate这几个函数

??? 1）init函数类似于构造函数，用于UDAF的初始化

????2）iterate接收传入的参数，并进行内部的轮转。其返回类型为boolean

????3）terminatePartial无参数，其为iterate函数轮转结束后，返回乱转数据，iterate和terminatePartial类似于hadoop的Combiner

????4）merge接收terminatePartial的返回结果，进行数据merge操作，其返回类型为boolean

????5）terminate返回最终的聚集函数结果