import java.io.IOException;

import java.util.Random;

import java.util.concurrent.BrokenBarrierException;

import java.util.concurrent.CyclicBarrier;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

 public class R {  

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

        //如果将参数改为4，但是下面只加入了3个选手，这永远等待下去

        //Waits until all parties have invoked await on this barrier.

        CyclicBarrier barrier = new CyclicBarrier();  

        ExecutorService executor = Executors.newFixedThreadPool();

        executor.submit(new Thread(new Runner(barrier, "1号选手")));

        executor.submit(new Thread(new Runner(barrier, "2号选手")));

        executor.submit(new Thread(new Runner(barrier, "3号选手")));  

        executor.shutdown();

    }

}  

class Runner implements Runnable {

    // 一个同步辅助类，它允许一组线程互相等待，直到到达某个公共屏障点 (common barrier point)

    private CyclicBarrier barrier;  

    private String name;  

    public Runner(CyclicBarrier barrier, String name) {

        super();

        this.barrier = barrier;

        this.name = name;

    }  

    @Override

    public void run() {

        try {

            Thread.sleep( * (new Random()).nextInt());

            System.out.println(name + " 准备好了...");  //

            // barrier的await方法，在所有参与者都已经在此 barrier 上调用 await 方法之前，将一直等待。

            //barrier.await();  关闭此处，所有线程各自执行会不影响

            //让barrier中所有线程 做完 barrier.await()之前的任务，

            //比如 "System.out.println(name + " 准备好了..."); "   然后才继续下面的任务

            barrier.await(); 

        } catch (InterruptedException e) {

            e.printStackTrace();

        } catch (BrokenBarrierException e) {

            e.printStackTrace();

         }

        System.out.println(name + " 起跑！");

    }

}