CountDownLatch解析-蒲公英云

CountDownLatch解析

1.CountDownLatch的await方法

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

然后调用

public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}

再调用

protected int tryAcquireShared(int acquires) {
    return (getState() == 0) ? 1 : -1;
}

countdown的state数量（也是CountDownLatch的count数量）不为0 所以返回 -1，然后调用doAcquireSharedInterruptibly，将当前线程加入到阻塞的线程并将当前线程阻塞

private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

然后把当前线程加到阻塞的队列中，并把当前线程给阻塞掉

private final boolean parkAndCheckInterrupt() {
    LockSupport.park(this);
    return Thread.interrupted();
}

2.CountDownLatch的countdown方法

public void countDown() {
    sync.releaseShared(1);
}

调用

public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        doReleaseShared();
        return true;
    }
    return false;
}

首先调用，tryReleaseShared采用cas 循环自旋的方式将当前的state 减去1，如果已经被别的线程减到到0了，返回false（防止多个线程都重复唤醒被阻塞的线程），首次减到0的时候返回true，否则也返回fasle

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;
    }
}

tryReleaseShared 返回true的时候开始调用 doReleaseShared()方法，将队列中被阻塞的线程唤醒

private void doReleaseShared() {
    for (;;) {
        Node h = head;
        if (h != null && h != tail) {
            int ws = h.waitStatus;
            if (ws == Node.SIGNAL) {
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                unparkSuccessor(h);
            }
            else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed
            break;
    }
}