Using CountDownLatch

https://www.cnblogs.com/evasean/p/7999582.html

面试中碰到这样一道题：有线程A和线程B，怎样控制线程B在线程A启动3秒后或者线程A运行结束后启动？

题目说明了线程B的启动时间是要满足两个条件：

1. 线程A启动3秒后

2. 线程A运行结束后

也就是说只要上面两个条件有一个满足，线程B就要启动。

用CountDownLatch控制调用时序，代码如下：

 1 public class RunA implements Runnable {
 2     private CountDownLatch cdl;
 3     public RunA(CountDownLatch cdl){
 4         this.cdl = cdl;
 5     }
 6     @Override
 7     public void run() {
 8         // TODO Auto-generated method stub
 9         try {
10             Thread.sleep(1000);
11             cdl.countDown();
12             System.out.println("A run over");
13         } catch (InterruptedException e) {
14             // TODO Auto-generated catch block
15             e.printStackTrace();
16         }
17     }
18 }

 1 public class RunB implements Runnable {
 2     private CountDownLatch cdl;
 3     public RunB(CountDownLatch cdl){
 4         this.cdl = cdl;
 5     }
 6     @Override
 7     public void run() {
 8         // TODO Auto-generated method stub
 9         try {
10             cdl.await(3000, TimeUnit.MILLISECONDS);
11             System.out.println("B run over");
12         } catch (InterruptedException e) {
13             // TODO Auto-generated catch block
14             e.printStackTrace();
15         }
16     }