CountDownLatch

闭锁： 

阻塞几个线程，直到某些线程的任务执行完成后，才会唤醒被阻塞线程，

 

await 方法的实现:

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

　　1)  AQS中的 acquireSharedInterruptibly 方法

    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())    // 如果当前线程的中断状态为true, 抛出异常
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

　　　2）Count中的 Sync的  tryAcquireShared 方法

 　　　　protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;    // state的值不等于0才会返回 -1
        }

　　  3）AQS 中的  doAcquireSharedInterruptibly  方法

private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED); // 入队   共享锁
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);  // 当state为0时，返回1，否则返回-1；
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())    // 阻塞（挂起）当前线程
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

　　AQS中的

  private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        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.
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

countDown的实现：

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

　　1) AQS 中的 releaseShared 方法。

    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {    // 执行 该方法后，state的值正好为0 时，才为true;(也就是当前state的值为1时，减1后，会返回true)
            doReleaseShared();
            return true;
        }
        return false;
    }

　　2）CountDown中的 tryReleaseShared 方法

  　　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;    // 将state的值减1
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }

　　　3)  AQS 中的 doReleaseShared 方法

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) {    // 阻塞队列（CLH）中有node结点
                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;
        }
    }