AQS之CountDownLatch

CountDownLatch在Jdk1.5中引入，通过内部实现继承了AbstractQueuedSynchronizer的Sync类，并实现了tryAcquireShared(arg)、tryReleaseShared方法，使用方式如下

public class CountDownLatchDemo {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch=new CountDownLatch(3);
        for (int i=0;i<3;i++){
            new Thread(()->{
                System.out.println("thread "+Thread.currentThread().getName()+" entry");
                latch.countDown();
                System.out.println("thread "+Thread.currentThread().getName()+" exit");
            },"T"+i).start();
        }

        latch.await();
        System.out.println("main thread end");
    }
}

创建了一个初始值为3的CountDownLatch对象latch，然后创建了3个线程，每个线程执行时都会执行latch.countDown()使计数器的值减1，而主线程在执行到latch.await()时会等待直到计数器的值为0。输出的结果如下：

thread T0 entry
thread T2 entry
thread T1 entry
thread T2 exit
thread T0 exit
thread T1 exit
main thread end

await方法：

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

这是countDownLatch实现的方法，通过调用sync中的acquireSharedInterruptibly方法，实际调用了AQS框架中的acquireSharedInterruptibly，而coundoownlatch本身只需要实现tryAcquireShared方法即可。
acquireSharedInterruptibly方法：

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

coundownlatch自己实现的tryAcquireShared方法

  protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }
根据状态来判断，如果state等于0说明计数器为0了，返回1表示成功，否则返回-1表示失败，需要放入队列中继续等待state变为0

当state！=0时，执行doAcquireSharedInterruptibly方法：
1.创建共享的node节点，并加入等待队列
2.获取前置节点p，如果等于head节点，则尝试获取锁，如果state==0（r>=0），则将node节点设置为head，并向后面节点传播
3.如果不为head,则自旋，找到安全点后park自己，并在唤醒后返回是否被中断

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);
        }
    }

private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
        /*
         * Try to signal next queued node if:
         *   Propagation was indicated by caller,
         *     or was recorded (as h.waitStatus either before
         *     or after setHead) by a previous operation
         *     (note: this uses sign-check of waitStatus because
         *      PROPAGATE status may transition to SIGNAL.)
         * and
         *   The next node is waiting in shared mode,
         *     or we don't know, because it appears null
         *
         * The conservatism in both of these checks may cause
         * unnecessary wake-ups, but only when there are multiple
         * racing acquires/releases, so most need signals now or soon
         * anyway.
         */
CDL如果获取锁成功，则propagate=1
h.waitStatus>=0表示线程取消或刚初始化
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

    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) {
如果当前节点ws为-1,表明需要唤醒后继节点
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
ws==0,表明节点初始化状态，需要设置为Node.PRoPagate状态
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
如果节点发生变化，自旋
            if (h == head)                   // loop if head changed
                break;
        }
    }

CDL的countDown()方法

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

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

//CDL中重写了tryReleaseShared
1.自旋的进行CAS操作
2.当state==0时，去唤醒后继节点
      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;
            }
        }
    }

await(long time,TimeUtil unit)

    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
//如果获取到锁返回true（state==0?1:-1）
//否则进行入队，自旋获取锁，park等过程
        return tryAcquireShared(arg) >= 0 ||
            doAcquireSharedNanos(arg, nanosTimeout);
    }
//添加了自旋阈值的控制spinForTimeoutThreshold
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        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 true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

调用await时
共享锁获取失败（计数器还不为0），则将该线程封装为一个Node对象放入队列中，并阻塞该线程；
共享锁获取成功（计数器为0），则从第一个节点开始依次唤醒后继节点，实现共享状态的传播。
调用countDown时
如果计数器不为0，则释放，继续阻塞，并把state的值减1；
如果计数器为0，则唤醒节点，解除线程的阻塞状态。

共享和独占的区别：
共享锁在节点成为头节点获取到锁之后，立马唤醒后继节点，实现节点传播
独占锁在获取锁且未释放锁之前，其他节点一致阻塞
【参考博客】
http://www.ideabuffer.cn/2017/03/19/深入理解AbstractQueuedSynchronizer（二）