为什么 ForkJoinPool 没有运行所有任务？答案

【问题标题】：Why is ForkJoinPool not running all tasks?为什么 ForkJoinPool 没有运行所有任务？
【发布时间】：2014-11-26 07:28:09
【问题描述】：

我试图理解 ForkJoinPools，所以我做了以下简单的测试类。不幸的是，它没有达到我的预期。谁能指出我哪里出错了？

import java.util.*;
import java.util.concurrent.*;

public class UtilsShowcase {

    public static void main(String[] args){
            final UtilsShowcase us = new UtilsShowcase();
            us.run();
    }

    public void run(){
            forkJoinPoolDemo();
    }

    public void forkJoinPoolDemo(){
            final ForkJoinPool forkJoinPool = new ForkJoinPool();

            Future<String> result = forkJoinPool.submit(new MyTask(8));
            try {
                    result.get();
            } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
            } catch (ExecutionException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
            }
    }       
   private class MyTask extends RecursiveTask<String> {
            final long num;

            MyTask(long number){
                    this.num = number;
            }

            protected String compute(){
                    System.out.println("In compute with number of "+num);
                    if (num > 1){
                            System.out.println("Generating two more tasks with number of "+num/2);

                            (new MyTask(num/2)).fork();
                            (new MyTask(num/2)).fork();

                    }
                    return "Returning from a number "+num;
            }
    }
}

我希望它从数字 8 开始，然后为数字 4 启动两个任务，为数字 2 启动 4 个任务，为数字 1 启动 8 个任务。相反，它打印出：

In compute with number of 8
Generating two more tasks with number of 4
In compute with number of 4
Generating two more tasks with number of 2
In compute with number of 4
Generating two more tasks with number of 2
In compute with number of 2
Generating two more tasks with number of 1
In compute with number of 2
Generating two more tasks with number of 1
In compute with number of 1
In compute with number of 2
In compute with number of 1

它的一部分是正确的 - 它打印出 2 个数字 4 任务，但只有 3 个数字 2（而不是 4 个）和 2 个数字 1（而不是 8 个！）

非常感谢！

【问题讨论】：

子任务被分叉，但从未加入...
使用join结果更有趣ideone.com/BIjFjQ
谢谢！是的，我只是想先让我的头绕过叉子。现在也在查看连接部分。

标签： java concurrency

【解决方案1】：

啊，在这里回答我自己的问题。

意识到主线程可能在其他线程完成所有工作之前就退出了，并且由于 ForkJoinPool 是一个守护进程，它只是默默地结束了。如果我添加

            try {
                    Thread.sleep(10000);
            } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
            }

在 forkJoinPoolDemo 方法结束时，我得到了我期望的结果：

In compute with number of 8
Generating two more tasks with number of 4
In compute with number of 4
Generating two more tasks with number of 2
In compute with number of 2
Generating two more tasks with number of 1
In compute with number of 4
In compute with number of 1
Generating two more tasks with number of 2
In compute with number of 2
Generating two more tasks with number of 1
In compute with number of 2
Generating two more tasks with number of 1
In compute with number of 1
In compute with number of 1
In compute with number of 1
In compute with number of 1
In compute with number of 1
In compute with number of 2
Generating two more tasks with number of 1
In compute with number of 1
In compute with number of 1

恢复理智 :-)

【讨论】：