JUC源码分析(四)-CountDownLatch源码分析

背景介绍

CountDownLatch是JUC下的一个类,通过调用继承了AQS的内部类完成同步操作.
CountDownLatch的语义是一个计数器，你可以向CountDownLatch对象设置一个初始的数字作为计数值，任何调用这个对象上的await()方法都会阻塞，直到这个计数器的计数值被其他的线程减为0为止。

源码分析

CountDownLatch.await()

先看下源码

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

可知是调用可中断且共享的acquire(),可中断与不可中断的区别见JUC-ReentrantLock源码分析,我们一起看下共享和独占的区别

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

        //此方法非常简单,仅判断state是否为0
        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        //nextWaiter为Node.SHARED
        final Node node = addWaiter(Node.SHARED);
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        //重点
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }

可以看出与非共享的acquire相比只有两处不同:

1. 生成Node.SHARED模式的Node(非共享的acquire生成Node.EXCLUSIVE模式的Node)
2. 成功获取锁资源后,调用setHeadAndPropagate(node, r);(非共享的acquire只会setHead而不会向后传播)

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

    private void doReleaseShared() {
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!h.compareAndSetWaitStatus(Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !h.compareAndSetWaitStatus(0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

在doReleaseShared()中有个死循环,还有一句unparkSuccessor(h),可知是通过unparkSuccessor()进行线程的唤醒.比较奇怪的地方是for循环的break条件是loop if head changed,那什么时候head 才会 changed呢?
在上述代码中,被挂起的第一个线程唤醒了被挂起的第二个线程,此时第二个线程便会执行与第一个线程相似的代码,即setHeadAndPropagate()–>doReleaseShared(),调用setHead()时便修改了head.

CountDownLatch.countDown()

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

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                //使用CAS将state原子减1
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c - 1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }

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

可以发现在tryReleaseShared()将state减1,减至0时还是执行doReleaseShared()唤醒被挂起的线程

ReentrantLock与CountDownLatch语义分析

ReentrantLock

在ReentrantLock中

• 初始时state为0
• lock()–>若state为0或已获得锁资源,state加1;否则,挂起
• unlock()–>state减一,若state为0,则唤醒AQS队列的第一个线程

CountDownLatch

在CountDownLatch中

• state初始为一正整数
• await()–>若state不为0,挂起
• countDown()–>state减1,若state为0,唤醒AQS队列的所有线程

Semphore 允许多个线程同时访问

• state初始为一正整数
• acquire()–>若state小于0,挂起
• release()–>释放许可,若有被挂起的线程,则唤醒线程

总结

CountDownLatch使用AQS的共享模式,在release时唤醒AQS队列上所有线程

参考