结果却一直没有出现

Question

嗨，我是新来的Java并发，我正在尝试通过分叉连接将列表内容加倍，并将任务分成多个部分。任务完成，但结果从未到达。

package com.learning;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;
import java.util.concurrent.TimeUnit;

class DoubleNumbers extends RecursiveTask<List<Integer>> {
    private final List<Integer> listToDo;
    public DoubleNumbers(List<Integer> list) {
        System.out.println("Cons Called"+list.get(0));
        this.listToDo = list;
    }

    @Override
    protected List<Integer> compute() {
        List<DoubleNumbers> doubleNumbersList= new ArrayList<>();
        System.out.println(Thread.currentThread().toString());
        for (int i = 0; i < listToDo.size(); i++) {
            listToDo.set(i, listToDo.get(i) * 2);
        }
        return listToDo;
    }
}

public class FJPExample {
    public static void main(String[] args) {
        List<Integer> arrayList = new ArrayList<>();
        for (int i = 0; i < 149; i++) {
            arrayList.add(i, i);
        }
        ForkJoinPool forkJoinPool = new ForkJoinPool(4);

        System.out.println(forkJoinPool.getParallelism());
        DoubleNumbers doubleNumbers = new DoubleNumbers(arrayList.subList(0, 49));
        DoubleNumbers doubleNumbers50ToNext = new DoubleNumbers(arrayList.subList(50, 99));
        DoubleNumbers doubleNumbers100ToNext = new DoubleNumbers(arrayList.subList(100, 149));
        forkJoinPool.submit(doubleNumbers);
        forkJoinPool.execute(doubleNumbers50ToNext);
        forkJoinPool.execute(doubleNumbers100ToNext);
        do {
            System.out.println("Parallel " + forkJoinPool.getParallelism());
            System.out.println("isWorking" + forkJoinPool.getRunningThreadCount());
            System.out.println("isQSubmission" + forkJoinPool.getQueuedSubmissionCount());
            try {
                TimeUnit.SECONDS.sleep(1000);
            } catch (InterruptedException e) {
                //
            }
        } while ((!doubleNumbers.isDone()) || (!doubleNumbers50ToNext.isDone()) || (!doubleNumbers100ToNext.isDone()));
        forkJoinPool.shutdown(); // Line 56

        arrayList.addAll(doubleNumbers.join());  
        arrayList.addAll(doubleNumbers50ToNext.join());
        arrayList.addAll(doubleNumbers100ToNext.join());
        System.out.println(arrayList.size());
        arrayList.forEach(System.out::println);
    }
}

如果我调试我的任务，那么我可以发现数字增加了一倍，但结果从未到达第56行

Answer 1

问题在于代码arrayList。addAll（doubleNumber.join（）），第54、55和56行，因为这可能会导致ConcurrentModificationException。所以，你可以做的是，用下面的行替换这些行，它就会工作（这会工作，因为你在第36行使用了arrayList.subList，它由同一个arrayList支持，请阅读它的javadoc以获取更多信息）

doubleNumbers.join();
doubleNumbers50ToNext.join();
doubleNumbers100ToNext.join();