Java并发-CountDownLatch

CountDownLatch

CountDownLatch允许一个或多个线程等待其他线程完成操作。

CountDownLatch的构造函数接受一个int类型的参数作为计数器。

当我们调用COuntDownLatch的countDown()时，N就会减1，CountDownLatch的await()会阻塞当前线程，知道N变成0。

CountDownLatch的实现分析

CountDownLatch内部依赖Sync实现，而Sync继承AQS。

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

sync是CountDownLatch的一个内部类

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

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

        int getCount() {
            return getState();
        }

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

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == )
                    return false;
                int nextc = c-;
                if (compareAndSetState(c, nextc))
                    return nextc == ;
            }
        }
    }
           

都是采用共享锁实现的，state就是当前需要等待的线程数

await()使当前线程在state减为0前一直等待，这个方法是相应中断的

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

调用AQS的acquireSharedInterruptibly

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

然后在CountDownLatch内部类Sync中重写了tryAcquireShared

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

getState(),state是一个用volatile修饰的变量

private volatile int state;
protected final int getState() {
        return state;
    }
           

doAcquireSharedInterruptibly这个方法是一个自旋方法会尝试一直去获取同步状态

private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                //计数器部不等于0，那么r会一直小于0
                    int r = tryAcquireShared(arg);
                    if (r >= ) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
           

CountDown()递减在构造函数中设置的计数，如果计数为0，则释放所有等待的线程。

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

内部调用AQS的releaseShared来释放共享锁同步状态

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

tryReleaseShared在CountDownLatch中被重写了

protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == )
                    return false;
                int nextc = c-;
                if (compareAndSetState(c, nextc))
                    return nextc == ;
            }
        }
           

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, ))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws ==  &&
                         !compareAndSetWaitStatus(h, , Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }