CountDownLatch解析

逃离我推掉我的手 2022-03-17 06:18 370阅读 0赞

### 1.CountDownLatch的await方法 ###

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

然后调用

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

再调用

protected int tryAcquireShared(int acquires) {
        return (getState() == 0) ? 1 : -1;
    }

countdown的state数量（也是CountDownLatch的count数量）不为0 所以返回 -1，然后调用doAcquireSharedInterruptibly，将当前线程加入到阻塞的线程并将当前线程阻塞

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) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

然后把当前线程加到阻塞的队列中，并把当前线程给 阻塞掉

private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

## 2.CountDownLatch的countdown方法 ##

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

调用

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

首先调用，tryReleaseShared采用cas 循环自旋的方式将当前的state 减去1，如果已经被别的线程减到到0了，返回false（防止多个线程都重复唤醒被阻塞的线程），首次减到0的时候返回true，否则也返回fasle

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;
        }
    }

tryReleaseShared 返回true的时候开始调用 doReleaseShared()方法，将队列中被阻塞的线程唤醒

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;
        }
    }