CountDownLatch源码分析

CountDownLatch的作用，就是将多个线程阻塞，等待某一事件触发后，再同时将阻塞的多个线程放行。
如果你对ReentrantLock，ReentrantReadWriteLock足够了解的话，通过CountDownLatch的功能，不难想到，CountDownLatch要阻塞多个线程并做到同时放行，所以他的await肯定是使用的共享锁进行阻塞。而创建CountDownLatch的时候，指定的count数量，肯定保存在AQS的state中，await的时候，如果state不为0，将将线程阻塞挂起。当调用countDown时候，就将state减1，当state为0的时候，就释放共享锁，共享锁的释放是一个链式反应，就会将阻塞的线程全部释放掉。

CountDownLatch是通过共享锁实现的，如果对ReentrantReadWriteLock不熟悉的，请先看ReentrantReadWriteLock源码分析。弄明白了共享锁，理解CountDownLatch就很简单了。

CountDownLatch创建

public CountDownLatch(int count) {
    if (count < 0) throw new IllegalArgumentException("count < 0");
    this.sync = new Sync(count);
}
//设置state的值为count
Sync(int count) {
    setState(count);
}

CountDownLatch.await

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

//只要state不等于，就会返回-1，将线程阻塞
protected int tryAcquireShared(int acquires) {
    return (getState() == 0) ? 1 : -1;
}

//就是将线程挂起，如果一旦一个线程获取锁成功，在setHeadAndPropagate会紧接着唤醒后续的线程
//中断异常会通过throws往外抛
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);
    }
}

CountDownLatch.countDown

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

//释放共享锁，就是对state进行减法操作，如果state为0了，就返回true、返回true将会唤醒被挂起的线程
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;
    }
}

//释放被挂起的第一个线程，当这个线程获取锁之后，在doAcquireSharedInterruptibly中的setHeadAndPropagate中会继续释放下一个线程。
private void doReleaseShared() {
    /*
     * Ensure that a release propagates, even if there are other
     * in-progress acquires/releases.  This proceeds in the usual
     * way of trying to unparkSuccessor of head if it needs
     * signal. But if it does not, status is set to PROPAGATE to
     * ensure that upon release, propagation continues.
     * Additionally, we must loop in case a new node is added
     * while we are doing this. Also, unlike other uses of
     * unparkSuccessor, we need to know if CAS to reset status
     * fails, if so rechecking.
     */
    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;
    }