CountDownLatch与CyclicBarrier 通熟易懂

http://aaron-han.iteye.com/blog/1591755

1.CountDownLatch减计数，CyclicBarrier加计数。
2.CountDownLatch是一次性的，CyclicBarrier可以重用。

1. 有五个人，一个裁判。这五个人同时跑，裁判开始计时，五个人都到终点了，裁判喊停，然后统计这五个人从开始跑到最后一个撞线用了多长时间。

import java.util.concurrent.CountDownLatch;

public class Race {

    public static void main(String[] args) {
        final int num = 5;
        final CountDownLatch begin = new CountDownLatch(1);
        final CountDownLatch end = new CountDownLatch(num);

        for (int i = 0; i < num; i++) {
            new Thread(new AWorker(i, begin, end)).start();
        }

        // judge prepare...
        try {
            Thread.sleep((long) (Math.random() * 5000));
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }

        System.out.println("judge say : run !");
        begin.countDown();
        long startTime = System.currentTimeMillis();

        try {
            end.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            long endTime = System.currentTimeMillis();
            System.out.println("judge say : all arrived !");
            System.out.println("spend time: " + (endTime - startTime));
        }

    }

}

class AWorker implements Runnable {
    final CountDownLatch begin;
    final CountDownLatch end;
    final int id;

    public AWorker(final int id, final CountDownLatch begin,
            final CountDownLatch end) {
        this.id = id;
        this.begin = begin;
        this.end = end;
    }

    @Override
    public void run() {
        try {
            System.out.println(this.id + " ready !");
            begin.await();
            // run...
            Thread.sleep((long) (Math.random() * 10000));
        } catch (Throwable e) {
            e.printStackTrace();
        } finally {
            System.out.println(this.id + " arrived !");
            end.countDown();
        }
    }

}

CountDownLatch强调的是一个线程（或多个）需要等待另外的n个线程干完某件事情之后才能继续执行。 上述例子，main线程是裁判，5个AWorker是跑步的。运动员先准备，裁判喊跑，运动员才开始跑（这是第一次同步，对应begin）。5个人谁跑到终点了，countdown一下，直到5个人全部到达，裁判喊停（这是第二次同步，对应end），然后算时间。

2. 继续，还是这五个人（这五个人真无聊..），这次没裁判。规定五个人只要都跑到终点了，大家可以喝啤酒。但是，只要有一个人没到终点，就不能喝。 这里也没有要求大家要同时起跑(当然也可以，加latch)。

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class Beer {

    public static void main(String[] args) {
        final int count = 5;
        final CyclicBarrier barrier = new CyclicBarrier(count, new Runnable() {
            @Override
            public void run() {
                System.out.println("drink beer!");
            }
        });

        // they do not have to start at the same time...
        for (int i = 0; i < count; i++) {
            new Thread(new Worker(i, barrier)).start();
        }
    }

}

class Worker implements Runnable {
    final int id;
    final CyclicBarrier barrier;

    public Worker(final int id, final CyclicBarrier barrier) {
        this.id = id;
        this.barrier = barrier;
    }

    @Override
    public void run() {
        try {
            System.out.println(this.id + "starts to run !");
            Thread.sleep((long) (Math.random() * 10000));
            System.out.println(this.id + "arrived !");
            this.barrier.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (BrokenBarrierException e) {
            e.printStackTrace();
        }
    }
}

CyclicBarrier强调的是n个线程，大家相互等待，只要有一个没完成，所有人都得等着。正如上例，只有5个人全部跑到终点，大家才能开喝，否则只能全等着。

再强调下，CountDownLatch强调一个线程等多个线程完成某件事情。CyclicBarrier是多个线程互等，等大家都完成。