CountDownLatch

一.使用

1个线程等待多个线程执行完成后再执行。

public class CountDownLatch1 {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(5);
        for (int i = 0; i < 5; i++) {
            new Thread(new Runnable() {
                @SneakyThrows
                @Override
                public void run() {
                    Thread.sleep(3000);
                    System.out.println("countDown");
                    countDownLatch.countDown();
                }
            }).start();
        }
        System.out.println("await");
        countDownLatch.await();
        System.out.println("---------------");
    }
}
输出：
await
countDown
countDown
countDown
countDown
countDown
---------------

多个线程等待1个线程执行完成后再执行。

public class CountDownLatch2 {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(1);
        for (int i = 0; i < 5; i++) {
            new Thread(new Runnable() {
                @SneakyThrows
                @Override
                public void run() {
                    System.out.println("await");
                    countDownLatch.await();
                    System.out.println("hello");
                }
            }).start();
        }
        Thread.sleep(3000);
        System.out.println("countDown");
        countDownLatch.countDown();
    }
}
输出：
await
await
await
await
await
countDown
hello
hello
hello
hello
hello

二.源码

1.构造函数

public CountDownLatch(int count) {
    this.sync = new Sync(count);
}
private static final class Sync extends AbstractQueuedSynchronizer {
    Sync(int count) {
        setState(count);
    }
}

2.await

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}
public final void acquireSharedInterruptibly(int arg)
    throws InterruptedException {
    // 初始时state不是0
    if (tryAcquireShared(arg) < 0)
        // 没拿到锁，入队阻塞
        doAcquireSharedInterruptibly(arg);
}
protected int tryAcquireShared(int acquires) {
    return (getState() == 0) ? 1 : -1;
}

3.countDown

public void countDown() {
    sync.releaseShared(1);
}
public final boolean releaseShared(int arg) {
    // state为0
    if (tryReleaseShared(arg)) {
        // 唤醒阻塞的线程
        doReleaseShared();
        return true;
    }
    // state不为0
    return false;
}
protected boolean tryReleaseShared(int releases) {
    for (;;) {
        int c = getState();
        if (c == 0)
            return false;
        int nextc = c-1;
        if (compareAndSetState(c, nextc))
            // 执行到这
            return nextc == 0;
    }
}