java - 并行线程需要到达某个点，等待另一个线程，然后继续执行-最佳方法？

Question

这是(简化的)场景:

我有并行运行的线程A，B，C。

所有3个线程都到达其执行的第7步后，它们必须等待。 (每个线程将在不同的时间到达步骤7，这无法提前知道)。

然后，线程D启动并运行完成。

只有在D完成之后，A，B和C才能恢复并运行到完成。

有什么好的工具或设计可以解决此问题？
到目前为止，我看过的并发和信号量示例似乎只处理2个线程，或者假设并行线程在做类似的事情，但只共享一个var或消息传递。我还没有找到适合上述方案的任何方法。我将继续研究该问题，并将根据发现结果进行更新。
如果CountDownLatch不需要退出其他线程，则可能会起作用。或者，如果它可以反向工作，则使 n线程等待直到线程D退出。
我对并发还比较陌生(大学类(class)不会在需要的任何时间提供它)，所以请多多包涵。如果有人对此问题有类似的疑问，请将此链接放在这里: HowToDoInJava - Concurrency

Answer 1

我会用Phaser

import java.util.concurrent.Phaser;

class Scratch {
    public static class ThreadABCWorker implements Runnable {
        String threadName;
        Phaser phaser;

        public ThreadABCWorker(String threadName, Phaser phaser) {
            this.threadName =  threadName;
            this.phaser = phaser;
        }

        @Override
        public void run() {
            System.out.println("Thread " + threadName + " has started");
            // do steps 1-7 as part of phase 0
            phaser.arriveAndAwaitAdvance();
            // All the work for phase 1 is done in Thread D, so just arrive again and wait for D to do its thing
            phaser.arriveAndAwaitAdvance();
            System.out.println("Continue Thread" + threadName);
        }
    }


    public static void main(String[] args) {
        var phaser = new Phaser(4);
        var threadA = new Thread(new ThreadABCWorker("A", phaser));
        var threadB = new Thread(new ThreadABCWorker("B", phaser));
        var threadC = new Thread(new ThreadABCWorker("C", phaser));
        var threadD = new Thread(() -> {
            phaser.arriveAndAwaitAdvance(); // D shouldn't start doing its thing until phase 1
            System.out.println("Thread D has started");

            try {
                System.out.println("sleep 100");
                Thread.sleep(100);
                System.out.println("Thread D has finished");
                phaser.arriveAndDeregister(); // All done, let ths other threads continue
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        threadA.start();
        threadB.start();
        threadC.start();
        threadD.start();
    }
}

输出示例:

Thread A has started
Thread C has started
Thread B has started
Thread D has started
sleep 100
Thread D has finished
Continue ThreadB
Continue ThreadA
Continue ThreadC