Fork/Join的使用

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一、概述

Fork/Join框架是Java7提供的一个用于并行执行任务的框架，是一个把大任务分割成若干个小任务，最终汇总每个小任务结果后得到大任务结果的框架。（http://www.infoq.com/cn/articles/fork-join-introduction）

二、使用举例

计算从1加到1亿的总和。当然这里只是用来说明Fork/Join框架的使用方法，从最后的运行结果中也可以看出就这个任务采用Fork/Join框架并没有提高性能，性能反而低于单个线程。原因是这个任务太简单了，采用Fork/Join框架的好处远远低于创建线程和线程调度切换的开销。

package com.my.test9;import java.util.concurrent.ForkJoinPool;import java.util.concurrent.Future;import java.util.concurrent.RecursiveTask;/** * Title: <br/> * Intention: <br/> * <p> * Class Name: com.my.test9.SumTest<br/> * Create Date: 2017/9/3 16:56 <br/> * Project Name: MyTest <br/> * Company:  All Rights Reserved. <br/> * Copyright © 2017 <br/> * </p> * <p> * author: GaoWei <br/> * 1st_examiner: <br/> * 2nd_examiner: <br/> * </p> * * @version 1.0 * @since JDK 1.7 */public class SumTask extends RecursiveTask<Long>{private Long start;private Long end;private static Long threshHold = 10000000L;public SumTask(Long start, Long end) {this.start = start;this.end = end;}@Overrideprotected Long compute() {Long sum = 0L;if (end - start <= threshHold) {for (long i = start; i <= end; i++) {sum += i;}} else {long middle = (end + start)/2;SumTask taskLeft = new SumTask(start, middle);SumTask taskRight = new SumTask(middle + 1, end);taskLeft.fork();taskRight.fork();Long leftResult = taskLeft.join();Long rightResult = taskRight.join();sum = leftResult + rightResult;}return sum;}public static void main(String[] args) throws Exception{Long num = 100000000L;long start = System.currentTimeMillis();ForkJoinPool forkJoinPool = new ForkJoinPool();SumTask task = new SumTask(1L, num);Future<Long> future = forkJoinPool.submit(task);System.out.println("result = "+future.get());long end = System.currentTimeMillis();System.out.println("fork join 耗时="+(end - start));start = System.currentTimeMillis();long sum = 0L;for(long i=1;i<=num;i++){sum += i;}end = System.currentTimeMillis();System.out.println("result = "+sum +", for 循环耗时="+(end - start));}}

运行结果：

result = 5000000050000000
fork join 耗时=577
result = 5000000050000000, for 循环耗时=43

参考：

1、http://www.infoq.com/cn/articles/fork-join-introduction

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