Fork/Join的使用

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;
	}

	@Override
	protected 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