CountDownLatch源码分析

本节一起学习CountDownLatch类的源码

1.类变量信息

    private final Sync sync;

2.构造函数

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

3.Sync内部类，继承AQS，实现同步

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

        Sync(int count) {
            setState(count);//设置线程个数
        }

        int getCount() {
            return getState();//获取线程个数
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;//尝试获取共享锁
        }

        protected boolean tryReleaseShared(int releases) {//释放共享锁
            // 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;
            }
        }
    }

4.常用的方法

    public void countDown() {//一个线程处理完成后，锁个数减1
        sync.releaseShared(1);
    }

    public void await() throws InterruptedException {//等待所有线程处理完成
        sync.acquireSharedInterruptibly(1);
    }

AQS:

    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {//如果所有线程处理完成
            doReleaseShared();//唤醒等待队列中的节点
            return true;
        }
        return false;
    }

    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);//唤醒节点的后继者
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

    /**
     * 唤醒节点
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);//设置状态为初始状态
        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);//唤醒线程
    }

    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())//线程中断
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)//如果线程未处理完
            doAcquireSharedInterruptibly(arg);
    }

    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);//封装节点，加入等待队列
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);//查询线程是否都处理完成
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);//设置head
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())//中断线程
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

总结：

CountDownLatch类底层通过AQS控制，首先初始化计数器，countDown时，计数器减1，当计数器等于0时，会唤醒AQS等待队列中的线程。

await方法，如果计数器等于0，则返回true，否则会加入到等待队列中，如果等待队列再次查询计数器还是不等于0，则会中断线程，等待被唤醒。