Java 并发编程之 AQS CountDownLatch 源码解析

init

// java.util.concurrent.CountDownLatch#CountDownLatch
public CountDownLatch(int count) {
    if (count < 0) throw new IllegalArgumentException("count < 0");
    this.sync = new Sync(count);
}

// java.util.concurrent.CountDownLatch.Sync
private static final class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 4982264981922014374L;

    Sync(int count) {
        setState(count);
    }
}

初始化时将AQS的state设置为构造方法中的值，不能为负数，但可以为0。

await

// java.util.concurrent.CountDownLatch#await()
public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

// java.util.concurrent.locks.AbstractQueuedSynchronizer#acquireSharedInterruptibly
public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}

// java.util.concurrent.CountDownLatch.Sync#tryAcquireShared
protected int tryAcquireShared(int acquires) {
    return (getState() == 0) ? 1 : -1;
}

tryAcquireShared直接判断的state的值是否为0，等于0表示不用等待，继续运行。

此处的doAcquireSharedInterruptibly和doAcquireSharedInterruptibly与ReentrantReadWriteLock#ReadLock和Semaphore实现都相似。

共享锁解锁时下一个节点会传递去唤醒。

countDown

// java.util.concurrent.CountDownLatch#countDown
public void countDown() {
    sync.releaseShared(1);
}

// java.util.concurrent.locks.AbstractQueuedSynchronizer#releaseShared
public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        doReleaseShared();
        return true;
    }
    return false;
}

// java.util.concurrent.CountDownLatch.Sync#tryReleaseShared
protected boolean tryReleaseShared(int releases) {
    // Decrement count; signal when transition to zero
    for (;;) {
        int c = getState();
        if (c == 0)
            return false;
        int nextc = c - 1;
        if (compareAndSetState(c, nextc))
            return nextc == 0;
    }
}

countDown每次将计数减去1，state为0了，执行释放锁的流程。