Java CountDownLatch 不解锁？为什么最后一行不打印？

Question

countDownLatch 不会在 countDown() 之后等待直到零。但是程序被锁定了。最后一次打印永远不会输出。

public static void main(String[] args) throws Exception{
    CountDownLatch countDownLatch = new CountDownLatch(1);
    countDownLatch.await();

    new Thread(new Runnable() {
        @Override
        public void run() {
            countDownLatch.countDown();
            countDownLatch.countDown();
        }
    }).start();

    System.out.println("------should print-------");
}

如果 await 方法在一个线程(不是主线程)中，它会起作用。为什么？

public static void main(String[] args) throws Exception {
    CountDownLatch countDownLatch = new CountDownLatch(1);
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }).start();


    new Thread(new Runnable() {
        @Override
        public void run() {
            countDownLatch.countDown();
            countDownLatch.countDown();
        }
    }).start();

    System.out.println("------should print-------");
}

Answer 1

在启动子线程之前调用 await(并停止线程):

public static void main(String[] args) throws Exception{
    CountDownLatch countDownLatch = new CountDownLatch(1);

    new Thread(new Runnable() {
        @Override
        public void run() {
            countDownLatch.countDown();
            countDownLatch.countDown();
        }
    }).start();
    countDownLatch.await();
    System.out.println("------should print-------");
}