java并发编程学习之再谈CountDownLatch

以前讲了AQS的独占锁的源码，这边，讲一下另一个锁的实现，共享锁，以CountDownLatch为例。

源码分析

构造方法

Sync方法是内部内的方法，跟以前ReentrantLock同样。构造方法须要传入一个不小于0的整数，用于赋值给state。当其余线程调用countDown方法的时候，state的值就减一。减到0的时候，其余调用await方法将被唤醒。await方法能够被多个线程调用，调用的时候，就进入了阻塞状态，直至state为0。后面重点讲countDown和await方法。

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

await

咱们先看看阻塞的方法，此时要等state为0的时候，才被唤醒。

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}
public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
    if (Thread.interrupted())//中断状况，抛出异常
        throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);//state不为0的状况下
}
protected int tryAcquireShared(int acquires) {
    return (getState() == 0) ? 1 : -1;//当前状态为0，返回1
}
private void doAcquireSharedInterruptibly(int arg)//获取共享锁，而且可被中断
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);//这个跟以前不一样的是Node.SHARED，加入到队列，不在说明
    boolean failed = true;
    try {
        for (;;) {//自旋
            final Node p = node.predecessor();//获取前面节点
            if (p == head) {
                int r = tryAcquireShared(arg);//尝试获取锁
                if (r >= 0) {//state为0的状况下
                    setHeadAndPropagate(node, r);//这个方法下面讲
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())//这部分同以前，挂起
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

countDown

public void countDown() {
    sync.releaseShared(1);
}
public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        doReleaseShared();//为true，唤醒
        return true;
    }
    return false;
}
//用自旋使status-1
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() {
    for (;;) {
        Node h = head;
        if (h != null && h != tail) {//有阻塞队列，而且头节点不是尾节点
            int ws = h.waitStatus;
            if (ws == Node.SIGNAL) {
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))//经过cas把头节点的waitStatus设置为0
                    continue;            // loop to recheck cases不成功从新设置
                unparkSuccessor(h);//唤醒下一个节点，以前的内容
            }
            else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed
            break;//
    }
}

刚刚wait的时候，doAcquireSharedInterruptibly中若是没获取到锁，就挂起。如今status为0，就唤醒他，继续自旋，次数看下面的代码

private void setHeadAndPropagate(Node node, int propagate) {
    Node h = head; // Record old head for check below获取头结点
    setHead(node);//当前节点设置为头节点
    
    if (propagate > 0 || h == null || h.waitStatus < 0 ||
        (h = head) == null || h.waitStatus < 0) {
        Node s = node.next;//获取下一个节点
        if (s == null || s.isShared())
            doReleaseShared();//唤醒下一个节点
    }
}

其余

CountDownLatch，实际上就是经过先设置state，再递减，等于0的时候，唤醒其余线程。
CyclicBarrier，也是先经过设置state，每一个await递减，state不等于0的时候放入Condition，等于0的时候唤醒。
Semaphore，也是先经过设置state，每次被获取state-1，释放+1,等于0就等待，大于0就唤醒