java.util.concurrent.CountDownLatch类是用来做减法计数器的
Demo如下：

public class CountDownLatchDemo {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(8);
        for (int i = 1; i <= 8; i++) {
            new Thread(()->{
                System.out.println(Thread.currentThread().getName()+"走了");
                //数量-1
                countDownLatch.countDown();
            },String.valueOf(i)).start();
        }
        /* 等待计数器归零,然后再向下执行 */
        countDownLatch.wait();
        System.out.println("close door");
    }
}

• new CountDownLatch(8)意思是从8开始递减；
• countDownLatch.wait()是监听递减值变化，如果到0了，则开始向下的操作