CountDownLatch

CountDownLatch解读

• 是基于AQS原理，共享锁的一种实现，构造函数里的count值代表计数器，会将count赋值给state。
• CountDownLatch不可重入。
• 通过await()来阻塞线程。countDown使计数器-1，当count为0时候所有await的线程会同时获取锁。

实现原理

CountDownLatch有一个内部类Sync，该类继承了AQS,await()方法和countDown方法都是调用Sync的tryAcquireShared和tryReleaseShared

共享锁获取锁-tryAcquireShared

CountDownLatch掉用await()实际上是调用sync的acquireSharedInterruptibly，最终会调用AQS中的doAcquireSharedInterruptiblyState，在该方法中调用doAcquireSharedInterruptibly，在改方法中，会调用CountDownLatch实现的tryAcquireShared方法如果state==0返回1，否则-1，其中代码和逻辑如下：

private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    //将当前线程封装成SHARDED类型的节点，加入到链表中
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        for (;;) {
            //自旋来获取锁，获取前置节点，如果前置节点是头结点，尝试获取锁，将当前节点设置为头节点，开始传播为后置节点解锁
            final Node p = node.predecessor();
            if (p == head) {
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    /**自旋解锁：唤醒头结点的线程并且开始根据链表顺序传播解锁。
                    *   因为是公平锁，所以无论现在在哪个线程都会根据链表顺序从head节点开始唤醒
                    **/
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }

            //通独占锁，根据前置节点的情况判断是否应该休眠（前置节点是SINGAL，当前节点的线程会block，如果前置节点是cancel会跳过所有cancel的节点）
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private void setHeadAndPropagate(Node node, int propagate) {
    //当前的头节点当中间变量同时设置头结点
    Node h = head;
    setHead(node);
    /*
     * Try to signal next queued node if:
     *   Propagation was indicated by caller,
     *     or was recorded (as h.waitStatus either before
     *     or after setHead) by a previous operation
     *     (note: this uses sign-check of waitStatus because
     *      PROPAGATE status may transition to SIGNAL.)
     * and
     *   The next node is waiting in shared mode,
     *     or we don't know, because it appears null
     *
     * The conservatism in both of these checks may cause
     * unnecessary wake-ups, but only when there are multiple
     * racing acquires/releases, so most need signals now or soon
     * anyway.
     */
     //propagate>0（这里是state==0），或者头结点为空，或者头结点不是CANCELED状态，或者现在的头节点为空，或者头结点不是CANCELED状态
    if (propagate > 0 || h == null || h.waitStatus < 0 ||
        (h = head) == null || h.waitStatus < 0) {
        Node s = node.next;//把当前Node状态之后的节点Shared都唤醒
        if (s == null || s.isShared())
            doReleaseShared();
    }
}

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;
             //自旋解锁，如果头结点的状态是SIGNAL，因为刚才将当前节点置为了Head，实际就是当前节点，同时一般ws也是SINGAL
            if (ws == Node.SIGNAL) {
                //cas成功后将头结点的线程解锁
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                unparkSuccessor(h);
            }else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    //刚才的setHead()方法已经将Thread=null所以，如果修改成功了会跳出循环
                continue;                // loop on failed CAS
        }

        //上面修改成功了node是
        if (h == head)                   // loop if head changed
            break;
    }
}

共享锁释放-tryAcquireShared

CountDownLatch调用countDown()时候回调用sync.releaseShared(1)，实际是AQS的releaseShared(int args)方法，如果返回true即state==0,在该方法中会调用doReleaseShared()，获取锁和传播获取锁的流程

CountDownLatch中的Sync的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;
    }
}

AQS中的releaseShared代码

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