控制两个线程的启动顺序

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：

public class RunA implements Runnable {
    private CountDownLatch cdl;
    public RunA(CountDownLatch cdl){
        this.cdl = cdl;
    }
    @Override
    public void run() {
        // TODO Auto-generated method stub
        try {
            Thread.sleep(1000);
            cdl.countDown();
            System.out.println("A run over");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

public class RunB implements Runnable {
    private CountDownLatch cdl;
    public RunB(CountDownLatch cdl){
        this.cdl = cdl;
    }
    @Override
    public void run() {
        // TODO Auto-generated method stub
        try {
            cdl.await(3000, TimeUnit.MILLISECONDS);
            System.out.println("B run over");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

public class Test {
    public static void main(String[] args) {
        CountDownLatch cdl = new CountDownLatch(1);
        Thread ta = new Thread(new RunA(cdl));
        Thread tb = new Thread(new RunB(cdl));
        ta.start();
        tb.start();
    }
}