CountDownLatch源码分析

// 共享状态
private volatile int state;

// 头节点，head和tail表明这是一个双向队列
private transient volatile Node head;
// 尾节点
private transient volatile Node tail;

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

   private final Sync sync;

   public void await() throws InterruptedException {
   		 // 获取共享锁，响应中断
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

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

        int getCount() {
            return getState();
        }

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

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                // 当前线程调用getState返回1，不满足条件，继续往下之行
                if (c == 0)
                    return false;
                // 如果这时其他线程修改state变量为0，而此时c为1，nextc为0，statue变量为0，但是并不满足cap条件，当前线程执行下一次循环
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

  public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
           // 先判断当前线程的中断状态
        if (Thread.interrupted())
            throw new InterruptedException();
            // 由子类CountDownLatch实现，返回值固定为1或者－1，判断state状态，如果state等于0，则返回1，否则返回－1
        if (tryAcquireShared(arg) < 0)
        
            doAcquireSharedInterruptibly(arg);
    }
    
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        // 将当前线程节点插入到FIFO的末尾
        // 流程1
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
        // 注意for循环的作用域，只有当获取成功或者线程被中断才会跳出循环
        // 流程2
            for (;;) {
            
                final Node p = node.predecessor();
                if (p == head) {
                // 尝试获取
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                    // 获取成功，则前继节点出队，同时传播到后继节点
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                // 流程3
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

   public void countDown() {
   		 // 内部类Sync实现AQS
        sync.releaseShared(1);
    }

    public final boolean releaseShared(int arg) {
        tryReleaseShared由子类实现，CountDownLatch的实现见上文代码，通过CAS设置属性state的值，如果state为0，则返回true，否则返回false
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

    /**
     * Release action for shared mode -- signal successor and ensure
     * propagation. (Note: For exclusive mode, release just amounts
     * to calling unparkSuccessor of head if it needs signal.)
     */
    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;
        }
    }