java - 当我在 CyclicBarrier 回调中调用 join 时，应用程序挂起

Question

我在启动线程时有以下方法测试:

public static void main(String[] args) throws InterruptedException {
        List<Thread> threads = new ArrayList<>();
        final CyclicBarrier cyclicBarrier = new CyclicBarrier(1);
        Thread thread = new Thread(new CallbackThread(cyclicBarrier, threads));
        threads.add(thread);
        thread.start();          
    }

回调线程如下所示:

class CallbackThread implements Runnable {
    CyclicBarrier cyclicBarrier;
    List<Thread> threads;

    CallbackThread(CyclicBarrier cyclicBarrier, List<Thread> threads) {
        this.cyclicBarrier = cyclicBarrier;
        this.threads = threads;
    }

    @Override
    public void run() {
        try {
            cyclicBarrier.await();
        } catch (InterruptedException e) {
            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
        } catch (BrokenBarrierException e) {
            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
        }
        System.out.println("Threads started");
        for (Thread thread1 : threads) {
            try {
                thread1.join();
            } catch (InterruptedException e) {
                e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
            }
        }
        System.out.println("Threads finished");

    }
}

当我运行应用程序时，我看到以下输出:

Threads started

并且应用程序挂起。

我不明白为什么。

如果将连接逻辑替换为主方法 - 一切都很好。

public static void main(String[] args) throws InterruptedException {
        List<Thread> threads = new ArrayList<>();
        final CyclicBarrier cyclicBarrier = new CyclicBarrier(1);
        Thread thread = new Thread(new CallbackThread(cyclicBarrier, threads));
        threads.add(thread);
        thread.start();
        for (Thread thread1 : threads) {
            try {
                thread1.join();
            } catch (InterruptedException e) {
                e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
            }
        }
        System.out.println("Threads finished");

    }

你能解释一下这种差异吗？
恕我直言，它应该工作相同。

Answer 1

第一个示例中的代码在其自己的线程上调用 join。您将其添加到列表中，线程会迭代列表并连接列表中的每个线程。