如何使用 runAsync 等待完全完成的未来？答案

【问题标题】：How to wait for full completion of a completable future with runAsync?如何使用 runAsync 等待完全完成的未来？
【发布时间】：2019-10-20 21:28:22
【问题描述】：

此测试失败：

package com.stackoverflow.demo;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ForkJoinPool;

import org.junit.Assert;
import org.junit.Test;

public class AsyncTest {

    @Test
    public void test1() {
        Assert.assertTrue("please run this test in a machine with 2 or more cores", ForkJoinPool.getCommonPoolParallelism() > 1);

        CompletableFuture<String> cf = CompletableFuture.completedFuture("ok");
        ConcurrentLinkedQueue<String> out = new ConcurrentLinkedQueue<>();

        cf.thenRunAsync(() -> {
            out.add("one");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            out.add("two");
        }, ForkJoinPool.commonPool());

        cf.join();

        Assert.assertEquals(2, out.size());
    }

}

我很惊讶，因为我预计 cf.join() 会考虑所有附加任务。我确信它在文档中的某处说join 只等待初始任务，但不知何故我错过了它。

如何获得我想要的行为：等待 CompletableFuture 及其所有附加子任务完成？

【问题讨论】：

标签： java java.util.concurrent completable-future

【解决方案1】：

在校对我的帖子时修复它：

public class AsyncTest {

    @Test
    public void test1() {
        Assert.assertTrue("please run this test in a machine with 2 or more cores", ForkJoinPool.getCommonPoolParallelism() > 1);

        CompletableFuture<String> cf = CompletableFuture.completedFuture("ok");
        ConcurrentLinkedQueue<String> out = new ConcurrentLinkedQueue<>();

        CompletableFuture<Void> cf2 = cf.thenRunAsync(() -> {
            out.add("one");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            out.add("two");
        }, ForkJoinPool.commonPool());

        cf2.join();

        Assert.assertEquals(2, out.size());
    }

}

【讨论】：