java并发编程十六 CountdownLatch介绍

CountdownLatch

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

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();
        }
    });
}

输出

18:52:25.831 c.TestCountDownLatch [pool-1-thread-3] - begin... 
18:52:25.831 c.TestCountDownLatch [pool-1-thread-1] - begin... 
18:52:25.831 c.TestCountDownLatch [pool-1-thread-2] - begin... 
18:52:25.831 c.TestCountDownLatch [pool-1-thread-4] - waiting... 
18:52:26.835 c.TestCountDownLatch [pool-1-thread-1] - end...2 
18:52:27.335 c.TestCountDownLatch [pool-1-thread-2] - end...1 
18:52:27.835 c.TestCountDownLatch [pool-1-thread-3] - end...0 
18:52:27.835 c.TestCountDownLatch [pool-1-thread-4] - 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);
    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

18:52:25.831 c.TestCountDownLatch [pool-1-thread-3] - begin... 
18:52:25.831 c.TestCountDownLatch [pool-1-thread-1] - begin... 
18:52:25.831 c.TestCountDownLatch [pool-1-thread-2] - begin... 
18:52:25.831 c.TestCountDownLatch [pool-1-thread-4] - waiting... 
18:52:26.835 c.TestCountDownLatch [pool-1-thread-1] - end...2 
18:52:27.335 c.TestCountDownLatch [pool-1-thread-2] - end...1 
18:52:27.835 c.TestCountDownLatch [pool-1-thread-3] - end...0 
18:52:27.835 c.TestCountDownLatch [pool-1-thread-4] - wait end... 

应用之同步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();

中间输出

[t0(52%), t1(47%), t2(51%), t3(40%), t4(49%), t5(44%), t6(49%), t7(52%), t8(46%), t9(46%)] 

最后输出

[t0(100%), t1(100%), t2(100%), t3(100%), t4(100%), t5(100%), t6(100%), t7(100%), t8(100%), 
t9(100%)] 
游戏开始...