主要用来解决一个线程等待多个线程的场景，计数器不能循环利用

public class CountDownLatchDemo {

    public static void main(String[] args) throws InterruptedException {

        CountDownLatch countDownLatch = new CountDownLatch(6);

        for (int i = 1; i <= 6; i++) {

            new Thread(() -> {

                System.out.println(Thread.currentThread().getName() + " 上完自习，离开教室");

                countDownLatch.countDown();

            }, String.valueOf(i)).start();

        }

        countDownLatch.await();

        System.out.println(Thread.currentThread().getName() + " 班长最后关门走人");

    }

}

是一组线程之间互相等待，计数器可以循环利用。