JAVA并发-CountDownLatch

源码：

内部类Sync

private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        //调用AQS类的setState设置状态位
        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();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

CountDownLatch初始化

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

可以设置AQS中的state为count

阻塞分析

await

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

acquireSharedInterruptibly

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

具体如下：

1、检测中断标志位

2、调用tryAcquireShared方法来检查AQS标志位state是否等于0，如果state等于0，则说明不需要等待，立即返回，否则进行3

3、调用doAcquireSharedInterruptibly方法进入AQS同步队列进行等待，并不断的自旋检测是否需要唤醒

doAcquireSharedInterruptibly

  private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);//加入队列尾部
        boolean failed = true;//是否成功标志
        try {
            for (;;) {
                final Node p = node.predecessor();//前驱
                //如果到head的下一个，因为head是拿到资源的线程，此时node被唤醒，很可能是head用完资源来唤醒自己的
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) { //如果大于零，则说明需要唤醒
                        setHeadAndPropagate(node, r);//将head指向自己，还有剩余资源可以再唤醒之后的线程
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                //判断状态，寻找安全点，进入waiting状态，等着被unpark()或interrupt()
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

addWaiter(Node.SHARED),这里将会新增两个node

第一轮循环创建一个new Node(),空节点，线程也为空

第二轮将Node.SHARED加入到队列中，prev指向head

在最后一次release之前，tryAcquireShared会为-1

shouldParkAfterFailedAcquire(Node, Node)

1、源码：
    /**
     * Checks and updates status for a node that failed to acquire.
     * Returns true if thread should block. This is the main signal
     * control in all acquire loops.  Requires that pred == node.prev.
     *
     * @param pred node's predecessor holding status
     * @param node the node
     * @return {@code true} if thread should block
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus; // 获取前驱结点的状态值
        if (ws == Node.SIGNAL) // 若前驱结点的状态为SIGNAL状态的话，那么该结点就不要想事了，直接返回true准备休息
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            // 若前驱结点的状态为CANCELLED状态的话，那么就一直向前遍历，直到找到一个不为CANCELLED状态的结点
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
             // 剩下的结点状态，则设置其为SIGNAL状态，然后返回false标志等外层循环再次判断
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

2、shouldParkAfterFailedAcquire主要是检测前驱结点状态，前驱结点为SIGNAL的话，则新结点可以安安心心休息了；
   如果前驱结点大于零，说明前驱结点处于CANCELLED状态，那么则以入参pred前驱为起点，一直往前找，直到找到最近一个正常等待状态的结点；
   如果前驱结点小于零，那么就将前驱结点设置为SIGNAL状态，然后返回false依赖acquireQueued的自旋再次判断是否需要进行休息；

第一次进入，ws为0

compareAndSetWaitStatus将pred的waitStatus设置为Node.SIGNAL

第二次进入就直接return true

parkAndCheckInterrupt()

1、源码：
    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this); // 阻塞等待
        return Thread.interrupted(); // 被唤醒后查看是否有被中断过否？
    }

2、parkAndCheckInterrupt首先调用park让线程进入等待状态，然后当park阻塞被唤醒后，再次检测是否曾经被中断过；
   而被唤醒有两种情况，一个是利用unpark唤醒，一个是利用interrupt唤醒；

main线程会进入上述代码，阻塞main线程。

释放分析

countDown

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

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

Sync内部类中的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;
            }
        }

可见只有最后一次release的时候才会进入到doReleaseShared

doReleaseShared

    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;
        }
    }

进入上述代码时，head的waitStatus在上述shouldParkAfterFailedAcquire中设置为了Node.SIGNAL

所以这里会调用unparkSuccessor(h)

unparkSuccessor

 private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

这里s=node.next,node为head,s的线程为main线程，这里释放main线程。

流程

参考：

【JUC】JDK1.8源码分析之CountDownLatch（五）