AQS之CountDownLatch、Semaphore、CyclicBarrier

CountDownLatch

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

通常情况下，countDown如下调用

CountDownLatch countDownLatch = new CountDownLatch(1);
countDownLatch.countDown();
countDownLatch.await();

看一下countDown方法：

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

AQS中releaseShared方法如下：

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

CountDownLatch中tryReleaseShared方法如下：

// 方法判断许可如果减1之后是否为0，如果为0的话就执行doReleaseShared（）方法。
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;
            }
        }

来看doReleaseShared（）方法：

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;
        }
    }

不过尴尬的是，CountDownLatch这里未做任何事情。

再看一下await（）方法：

await方法会让当前线程进入wait状态，除非满足下面两个条件：

1. count到0
2. 线程中断

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);
    }

tryAcquireShared方法如下：

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

所以，当state不是0的时候进入doAcquireSharedInterruptibly方法。

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) {
                    // 只有当state为0时r为1
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                // 如果state不为0，该线程会进入wait状态
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

CountDownLatch文档中有一句非常重要的话：

Memory consistency effects: Until the count reaches zero, actions in a thread prior to calling countDown() happen-before actions following a successful return from a corresponding await() in another thread

大意是一个线程countdown()之前的操作happens-before另一个线程中await()之后的操作。

Semaphore

Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource.

Semaphore主要用来限制获取资源的线程数。

Actions in a thread prior to calling a "release" method such as release() happen-before actions following a successful "acquire" method such as acquire() in another thread

内存语义：release() happen-before acquire()之前

启一个springboot项目，写一个方法：

@RequestMapping("/test/semaphore")
    @ResponseBody
    public void test() throws InterruptedException {
        Semaphore semaphore = new Semaphore(5);
        for (int i = 0; i < 7; i++) {
            int finalI = i;
            new Thread(()->{
                try {

                    semaphore.acquire();
                    System.err.println(Thread.currentThread() + "获取了许可" + semaphore.availablePermits());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }, "线程" + i).start();

        }
        new Thread(()->{
            try {
                Thread.sleep(10000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.err.println(Thread.currentThread() + "要释放许可" + semaphore.availablePermits());
            semaphore.release();
        }, "线程7").start();
    }

一次输出如下：

Thread[线程1,5,main]获取了许可4

Thread[线程0,5,main]获取了许可3

Thread[线程3,5,main]获取了许可2

Thread[线程4,5,main]获取了许可0

Thread[线程2,5,main]获取了许可0

Thread[线程7,5,main]要释放许可0

Thread[线程5,5,main]获取了许可0

会发现，线程5获取许可之前是先等线程7释放许可。

至于线程6会因为由于许可为0，进入等待状态。直到有线程释放许可，来调用unparkSuccessor。

CyclicBarrier

A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.

Actions in a thread prior to calling await() happen-before actions that are part of the barrier action, which in turn happen-before actions following a successful return from the corresponding await() in other threads.

内部类Generation只有一个属性broken（默认false）

我们发现，await（）方法如下：

 public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

进入dowait方法：

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;

            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }

            // 来一个线程count减1，如果index为0，就会翻车
            int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // 没翻车（broken，interrupted，timed out）的话就执行下面的逻辑
            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    if (g == generation && ! g.broken) {
                        breakBarrier();
                        throw ie;
                    } else {
                        // We're about to finish waiting even if we had not
                        // been interrupted, so this interrupt is deemed to
                        // "belong" to subsequent execution.
                        Thread.currentThread().interrupt();
                    }
                }

                if (g.broken)
                    throw new BrokenBarrierException();

                if (g != generation)
                    return index;

                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

下面进入trip.await()方法

public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
    		// 往等待队列加入节点Node
            Node node = addConditionWaiter();
    		// 这里释放AQS中的state， 如果释放失败，会将node的waitstatus置为CANCELLED，这是传参node的唯一用处
            int savedState = fullyRelease(node);
            int interruptMode = 0;
    		// 如果node有next就肯定返回true
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
    		// 如果当前线程
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

进入addConditionWaiter()

private Node addConditionWaiter() {
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }

假如5个线程按顺序进入await（），则此时，trip这个ConditionObject上firstWaiterlastWaiternew Node("线程0对应的线程"， Node.CONDITION)

同时，因为dowait方法中的lock.lock()，AQS的同步队列如下：

head节点--》线程1--》线程2--》线程3--》线程4（tail）

等待队列: t0

当释放线程0的锁之后，唤醒线程1，将线程1加入等待队列，线程2/3也加入等待队列。此时同步队列还剩下线程4。此时队列情况是：

同步队列：head节点

等待队列：t0->t1->t2->t3

到了最后一个线程4执行的时候，index==0，执行nextGeneration，会signalAll trip这个Condition上的所有等待线程。所以经过signalAll之后，队列情况变成了：

同步队列：head->t0->t1->t2->t3

等待队列：空

此时线程4运行，释放锁之后唤醒同步队列上的第一个节点t0