java.util.concurrent 之三： 出发--等待--汇合--再出发

    在实际应用中，有时候需要多个线程同时工作以完成同一件事情，而且在完成过程中，往往会等待其他线程都完成某一阶段后再执行，等所有线程都到达某一个阶段后再统一执行。

    比如有几个旅行团需要途经深圳、广州、韶关、长沙最后到达武汉。旅行团中有自驾游的，有徒步的，有乘坐旅游大巴的；这些旅行团同时出发，并且每到一个目的地，都要等待其他旅行团到达此地后再同时出发，直到都到达终点站武汉。

  这时候 CyclicBarrier 就可以派上用场。CyclicBarrier最重要的属性就是参与者个数，另外最要方法是await()。当所有线程都调用了await()后，就表示这些线程都可以继续执行，否则就会等待。

package test;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TestCyclicBarrier{
	// 徒步需要的時間
	private static int[] timeWalk = { 5, 8, 15, 15, 10 };
	// 自駕遊
	private static int[] timeSelf = { 1, 3, 4, 4, 5 };
	// 旅遊大巴
	private static int[] timeBus = { 2,4,6,6,7 };
	
	static String now(){
		SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
		return sdf.format(new Date()) + ": ";
	}
	static class Tour implements Runnable{
		private int[] times;
		private CyclicBarrier barrier;
		private String tourName;
		public Tour(CyclicBarrier barrier, String tourName, int[] times){
			this.times=times;
			this.tourName=tourName;
			this.barrier=barrier;
		}
		public void run(){
			try{
				Thread.sleep(times[0]*1000);
				System.out.println(now() + tourName + " Reached Shenzhen");
				barrier.await();
				Thread.sleep(times[1]*1000);
				System.out.println(now() + tourName + " Reached Guangzhou");
				barrier.await();
				Thread.sleep(times[2]*1000);
				System.out.println(now() + tourName + " Reached Shaoguan");
				barrier.await();
				Thread.sleep(times[3]*1000);
				System.out.println(now() + tourName + " Reached Changsha");
				barrier.await();
				Thread.sleep(times[4]*1000);
				System.out.println(now() + tourName + " Reached Wuhan");
				barrier.await();
		}catch(InterruptedException e){
		}catch(BrokenBarrierException e){
		}
	}
}
public static void main(String[] args){
	// 三個旅行團
	CyclicBarrier barrier = new CyclicBarrier(3);
	ExecutorService exec = Executors.newFixedThreadPool(3);
	exec.submit(new Tour(barrier, "WalkTour", timeWalk));
	exec.submit(new Tour(barrier, "SelfTour", timeSelf));
	//
	exec.submit(new Tour(barrier, "BusTour", timeBus));
	exec.shutdown();
  }
}

可能的执行结果：

21:47:18: SelfTour Reached Shenzhen
21:47:19: BusTour Reached Shenzhen
21:47:22: WalkTour Reached Shenzhen
21:47:25: SelfTour Reached Guangzhou
21:47:26: BusTour Reached Guangzhou
21:47:30: WalkTour Reached Guangzhou
21:47:34: SelfTour Reached Shaoguan
21:47:36: BusTour Reached Shaoguan
21:47:45: WalkTour Reached Shaoguan
21:47:49: SelfTour Reached Changsha
21:47:51: BusTour Reached Changsha
21:48:00: WalkTour Reached Changsha
21:48:05: SelfTour Reached Wuhan
21:48:07: BusTour Reached Wuhan
21:48:10: WalkTour Reached Wuhan