AbstractQueuedSynchronizer理解之二（CountDownLatch）

本文分析一下CountDownLatch是如何运用AQS的

CountDownLatch是什么

CountDownLatch顾名思义它是一个Latch（门闩），它是用一个计数器实现的，初始状态计数器的数值等于线程数，每当有线程完成任务后，计数器就会减一。当state为0时，锁就会被释放，凡是以前因抢占锁而等待的线程这时候就会被唤醒继续抢占锁。

CountDownLatch小栗子

public static void main(String[] args) throws InterruptedException{
    int threadSize = 3;
    CountDownLatch doneSignal = new CountDownLatch(threadSize);

    for (int i = 1; i <= threadSize; i++) {
        final int threadNum = i;
        new Thread(() -> {
            System.out.println("thread" + threadNum + ":start");

            try {
                Thread.sleep(1000 * threadNum);
            } catch (InterruptedException e) {
                System.out.println("thread" + threadNum + ":exception");
            }

            doneSignal.countDown();
            System.out.println("thread" + threadNum + ":complete");
        }).start();
    }

    System.out.println("main thread:await");
    doneSignal.await();
    System.out.println("main thread:go on");
}

例子中主线程启动了三条子线程，睡眠一段时间，此时主线程在等待全部子线程结束后才会继续执行下去；
看一下输出结果：

main thread:await
thread1:start
thread2:start
thread3:start
thread1:complete
thread2:complete
thread3:complete
main thread:go on

Process finished with exit code 0

CountDownLatch原理分析

既然CountDownLatch也是AQS的一种使用方式，咱们看一下它的内部类Syc是怎么实现AQS的：

private static final class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 4982264981922014374L;
    
    //构造函数，初始化同步状态state的值，即线程个数
    Sync(int count) {
        setState(count);
    }

    int getCount() {
        return getState();
    }

    //这里重写了方法，在共享模式下，告诉调用者是否能够抢占state锁了，正数表明能够，负数表明否认；当state为0时返回正数
    protected int tryAcquireShared(int acquires) {
        return (getState() == 0) ? 1 : -1;
    }

    //共享模式下释放锁
    protected boolean tryReleaseShared(int releases) {
        // Decrement count; signal when transition to zero
        for (;;) {
            int c = getState();
            //state为0时说明没有什么可释放
            if (c == 0)
                return false;
            int nextc = c-1;
            if (compareAndSetState(c, nextc))
                //CAS对state操做成功后返回state值是否为0，为0则释放成功
                return nextc == 0;
        }
    }
}

看完了重写的AQS同步器后，咱们了解了CountDownLatch对state锁的描述。接下来先看主线程调用的await方法，在await方法里调用了AQS的acquireSharedInterruptibly：

//在共享模式下尝试抢占锁
public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    //线程中断抛出异常
    if (Thread.interrupted())
        throw new InterruptedException();
    //尝试抢占前先查询一下是否能够抢占，若是返回值大于0程序往下执行，小于0则等待
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}


private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    //在Reentrant解析中咱们看过，往队列中新增node(共享模式)
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {
                //若是当前node的前继时head，立刻尝试抢占锁
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    //若是state==0即容许往下执行，从新设置head并往下传播信号
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    //获得往下执行的容许
                    return;
                }
            }
            //如下都跟Reentrant同样
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private void setHeadAndPropagate(Node node, int propagate) {
    Node h = head; // Record old head for check below
    //将当前node设置为head，清空node的thread、prev
    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.
     */
    //若是propagate大于0，或者原来head的等待状态小于0或者如今head的等待状态小于0
    if (propagate > 0 || h == null || h.waitStatus < 0 ||
        (h = head) == null || h.waitStatus < 0) {
        Node s = node.next;
        //准备唤醒下一个节点
        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;
            if (ws == Node.SIGNAL) {
                //若是head的状态为SIGNAL，更改状态为0
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                //唤醒后继节点
                unparkSuccessor(h);
            }
            //若是head状态为0，更改状态为PROPAGATE
            else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        //若是head没有改变，结束当前loop，若是遇到head被别的线程改变，继续loop
        if (h == head)                   // loop if head changed
            break;
    }
}

释放锁的信号一直向后传播，直到全部node被唤醒并继续执行，那第一个信号时什么时候发起的呢？咱们来看一下CountDownLatch的countDown方法，该方法调用了sync的releaseShared方法：

public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        //若是同步状态state为0时，调用doReleaseShared，在这里就发出了第一个唤醒全部等待node的信号，而后信号自动日后传播
        doReleaseShared();
        return true;
    }
    return false;
}

总结

CountDownLatch在调用await的时候判断state释放为0，若是大于0则阻塞当前线程，将当前线程的node添加到队列中等待；在调用countDown时当遇到state减到0时，发出释放共享锁的信号，从头节点的后记节点开始日后传递信号，将队列等待的线程逐个唤醒并继续往下执行；
在这里state跟Reentrant的state独占锁含义不一样，state的含义是由AQS的子类去描述的。