Java并发工具类(闭锁CountDownLatch)

内容纲要

闭锁是一种同步工具类,可以延迟线程的进度直到其到达终止状态。
CountDownLatch是一种灵活的闭锁实现,它可以使一个或者多个线程等待一组事件的发生。
闭锁状态包含一个计数器,该计数器被初始化为一个正数,表示需要等待的事件数量。countDown方法递减计数器,表示已经有一个事件已经发生了。而await方法等待计数器达到0,这表示所有需要等待的事件都已经发生。如果计数器的值非0,那么await会一直阻塞直到计数器为0,或者等待中的线程中断或者超时。
下面,我们以经典的运动员赛跑举例:

我们在这里设置了两个门,一个是开始门,一个是结束门。

开始门: 所有运动员处于准备状态,等待教练的枪声。这时候运动员为n个,枪响只需要一声,等待的这一声枪响到了,开始门也就打开了,所有运动员开始跑。
结束门: 教练等待所有运动员,当最后一个运动员也冲破底线,教练才能宣布所有人到达终点,这时候是教练等待n个运动员,直到n为0。

下面我们根据具体的代码来演示CountDownLatch的用法:

package concurrency;

import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

class Runner implements Runnable {
    private static int counter = 0;
    private final int id = counter++;
    private static Random rand= new Random(47);
    private final CountDownLatch start_latch;
    private final CountDownLatch end_latch;

    public Runner(CountDownLatch start_latch, CountDownLatch end_latch) {
        this.start_latch = start_latch;
        this.end_latch = end_latch;
    }

    @Override
    public void run() {
        try {
            start_latch.await();  //所有运动员都在准备状态中,等待教练释放开始门
            try {
                doWork();  //每个人跑步的时间不同
                end_latch.countDow n();  //跑完后,告诉教练跑完了
            } catch (InterruptedException e) {
                System.out.println("Interrupted Runner" + id);
            }
        } catch (InterruptedException e) {
            System.out.println("Interrupted Runner" + id);
        }
    }

    public void doWork() throws InterruptedException {
        TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000));
        System.out.println(this + "completed");
    }

    @Override
    public String toString() {
        return String.format("%1$-3d", id);
    }
}

class Coach implements Runnable {
    private final CountDownLatch start_latch;
    private final CountDownLatch end_latch;

    public Coach(CountDownLatch start_latch, CountDownLatch end_latch) {
        this.start_latch = start_latch;
        this.end_latch = end_latch;
    }

    @Override
    public void run() {
        start_latch.countDown();  //教练释放了开始门,运动员们都开始跑
        System.out.println("Coach say: Ready!!!!  Go!!!!");
        try {
            end_latch.await();  //当结束门的count down减为0时,教练宣布所有人都跑完了。
            System.out.println("All runner passed the end point");
        } catch (InterruptedException ex) {
            System.out.println(this + " interrupted");
        }
    }
}

public class TestRunner {
    private static final int SIZE = 10;
    public static void main(String[] args) {
        ExecutorService exec = Executors.newCachedThreadPool();
        CountDownLatch startGate = new CountDownLatch(1);
        CountDownLatch endGate = new CountDownLatch(SIZE);
        for (int i = 0; i < SIZE; i++) {
            exec.execute(new Runner(startGate, endGate));
        }
        exec.execute(new Coach(startGate, endGate));
        exec.shutdown();
    }
}

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,Coach线程是裁判,10个Runner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startGate)。10个人谁跑到终点了,countdown一下,直到10个人全部到达,裁判喊停(这是第二次同步,对应endGate)。
最后运行结果如下:
Coach say: Ready!!!! Go!!!! 7 completed 9 completed 5 completed 8 completed 2 completed 0 completed 6 completed 4 completed 1 completed 3 completed All runner passed the end point

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