CountDownLatch

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

案例（来自源码注释）

import java.util.concurrent.CountDownLatch;

public class Driver {

private final static int N =10;

public static void main(String[] args)throws Exception {

CountDownLatch startSignal =new CountDownLatch(1);

CountDownLatch doneSignal =new CountDownLatch(N);

for (int i =0; i

            new Thread(new Worker(startSignal, doneSignal)).start();

}

startSignal.countDown();// let all threads proceed

        System.out.println("-----比赛开始--------");

doneSignal.await();// wait for all to finish

        System.out.println("均已到终点，比赛结束");

}

static class Workerimplements Runnable {

final CountDownLatchstartSignal;

final CountDownLatchdoneSignal;

public Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {

this.startSignal = startSignal;

this.doneSignal = doneSignal;

}

@Override

        public void run() {

try {

startSignal.await();

System.out.println(Thread.currentThread().getName() +"加速，冲刺");

System.out.println(Thread.currentThread().getName() +"到终点");

doneSignal.countDown();

}catch (InterruptedException e) {

e.printStackTrace();

}

}

}

}