特点：计数器的值减为0才往下执行。

    1. import java.util.concurrent.CountDownLatch;
    3. // 计数器
    4. public class CountDownLatchDemo {
    5.     public static void main(String[] args) throws InterruptedException {
    6.         // 总数是6，必须要执行任务的时候，再使用！
    7.         CountDownLatch countDownLatch = new CountDownLatch(6);
    9.         for (int i = 1; i <=6 ; i++) {
    10.             new Thread(()->{
    11.                 System.out.println(Thread.currentThread().getName()+" Go out");
    12.                 countDownLatch.countDown(); // 数量-1
    13.             },String.valueOf(i)).start();
    14.         }
    16.         countDownLatch.await(); // 等待计数器归零，然后再向下执行
    18.         System.out.println("Close Door");
    20.     }
    21. }