JUC包下的线程协作计数CountDownLatch及CyclicBarrier

CountDownLatch

概述

用来进行线程同步协作,等待所有线程完成倒计时。

其中构造参数用来初始化等待计数值,await() 用来等待计数归零,countDown() 用来让计数减一

CountDownLatch不能重用,只能减不能加

public static void main(String[] args) throws InterruptedException {
    CountDownLatch latch = new CountDownLatch(3);
    new Thread(() -> {
        log.debug("begin...");
        sleep(1);
        latch.countDown();
        log.debug("end...{}", latch.getCount());
    }).start();
    new Thread(() -> {
        log.debug("begin...");
        sleep(2);
        latch.countDown();
        log.debug("end...{}", latch.getCount());
    }).start();
    new Thread(() -> {
        log.debug("begin...");
        sleep(1.5);
        latch.countDown();
        log.debug("end...{}", latch.getCount());
    }).start();
    log.debug("waiting...");
    latch.await();
    log.debug("wait end...");
}

输出

18:44:00.778 c.TestCountDownLatch [main] - waiting...
18:44:00.778 c.TestCountDownLatch [Thread-2] - begin...
18:44:00.778 c.TestCountDownLatch [Thread-0] - begin...
18:44:00.778 c.TestCountDownLatch [Thread-1] - begin...
18:44:01.782 c.TestCountDownLatch [Thread-0] - end...2
18:44:02.283 c.TestCountDownLatch [Thread-2] - end...1
18:44:02.782 c.TestCountDownLatch [Thread-1] - end...0
18:44:02.782 c.TestCountDownLatch [main] - wait end... 

可以配合线程池使用,改进如下

public static void main(String[] args) throws InterruptedException {
    CountDownLatch latch = new CountDownLatch(3);
    ExecutorService service = Executors.newFixedThreadPool(4);
    service.submit(() -> {
        log.debug("begin...");
        sleep(1);
        latch.countDown();
        log.debug("end...{}", latch.getCount());
    });
    service.submit(() -> {
        log.debug("begin...");
        sleep(1.5);
        latch.countDown();
        log.debug("end...{}", latch.getCount());
    });
    service.submit(() -> {
        log.debug("begin...");
        sleep(2);
        latch.countDown();
        log.debug("end...{}", latch.getCount());
    });
    service.submit(()->{
        try {
            log.debug("waiting...");
            latch.await();
            log.debug("wait end...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    });
}

应用

等待多线程准备完毕

AtomicInteger num = new AtomicInteger(0);
ExecutorService service = Executors.newFixedThreadPool(10, (r) -> {
    return new Thread(r, "t" + num.getAndIncrement());
});
CountDownLatch latch = new CountDownLatch(10);
String[] all = new String[10];
Random r = new Random();
for (int j = 0; j < 10; j++) {
    int x = j;
    service.submit(() -> {
        for (int i = 0; i <= 100; i++) {
            try {
                Thread.sleep(r.nextInt(100));
            } catch (InterruptedException e) {
            }
            all[x] = Thread.currentThread().getName() + "(" + (i + "%") + ")";
            System.out.print("\r" + Arrays.toString(all));
        }
        latch.countDown();
    });
}
latch.await();
System.out.println("\n游戏开始...");
service.shutdown();

CyclicBarrier

循环栅栏,用来进行线程协作,等待线程满足某个计数。构造时设置『计数个数』,每个线程执
行到某个需要“同步”的时刻调用 await() 方法进行等待,当等待的线程数满足『计数个数』时,继续执行

构造方法:

  • public CyclicBarrier(int parties)
  • public CyclicBarrier(int parties, Runnable barrierAction)

parties是参与线程的个数,每次调用await等待的线程数加一,直到达到parties所有线程继续运行。Runnable 参数则是可以选择计数值达到parties要做的任务。

在计数值达到parties之后,等待的线程数清零,可以重用。

CyclicBarrier cb = new CyclicBarrier(2); // 个数为2时才会继续执行
new Thread(()->{
    System.out.println("线程1开始.."+new Date());
    try {
        cb.await(); // 当个数不足时,等待
    } catch (InterruptedException | BrokenBarrierException e) {
        e.printStackTrace();
    }
    System.out.println("线程1继续向下运行..."+new Date());
}).start();
new Thread(()->{
    System.out.println("线程2开始.."+new Date());
    try { Thread.sleep(2000); } catch (InterruptedException e) { }
    try {
        cb.await(); // 2 秒后,线程个数够2,继续运行
    } catch (InterruptedException | BrokenBarrierException e) {
        e.printStackTrace();
    }
    System.out.println("线程2继续向下运行..."+new Date());
}).start();

原创文章,作者:彭晨涛,如若转载,请注明出处:https://www.codetool.top/article/juc%e5%8c%85%e4%b8%8b%e7%9a%84%e7%ba%bf%e7%a8%8b%e5%8d%8f%e4%bd%9c%e8%ae%a1%e6%95%b0countdownlatch%e5%8f%8acyclicbarrier/