CountDownLatch的原理

上次大概说了CountDownLatch的使用，今天说下实现的原理，CountDownLatch的使用效果和Join差不多，实现起来也比较简单。

大体的思路就是一个死循环阻塞，等到某个条件满足后就跳出循环，继续执行后面的代码。执行逻辑如下：

源码分析

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我们下面分析下CountDownLatch的源码:

创建CountDownLatch对象

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

创建CountDownLatch 对象很简单，就是创建一个Sync对象。 Sync的对应的代码

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

protected final void setState(int newState) {
   state = newState;
}

这个可以确定给state设置了一个值。

await 方法 阻塞等待

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


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

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


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);
        }
    }

doAcquireSharedInterruptibly 这个方法中有一个for(;;),这个是一个死循环, 直到tryAcquireShared 返回的r>=0.也就是state==0。

countDown 方法

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

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



     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;
            }
        }


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) {
                    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;
        }
    }

compareAndSetWaitStatus(h, 0, Node.PROPAGATE)这个是用CAS的方法改变state的值。