AbstractQueuedSynchronizer

1 简介

AbstractQueuedSynchronizer简称AQS是一个抽象同步框架，可以用来实现一个依赖状态的同步器。
JDK1.5中提供的java.util.concurrent包中的大多数的同步器(Synchronizer)如Lock, Semaphore, Latch, Barrier等，这些类之间大多可以互相实现，如使用Lock实现一个Semaphore或者反过来，但是它们都是基于java.util.concurrent.locks.AbstractQueuedSynchronizer这个类的框架实现的，理解了这个稍微复杂抽象的类再去理解其他的同步器就很轻松了。

2 原理介绍

AQS的核心是一个线程等待队列，采用的是一个先进先出FIFO队列。用来实现一个非阻塞的同步器队列有主要有两个选择Mellor-Crummey and Scott (MCS) locks和Craig, Landin, and Hagersten (CLH) locks的变种。CLH锁更适合处理取消和超时，所以AQS基于CLH进行修改作为线程等待队列。
CLH队列使用pred引用前节点形成一个队列，入队enqueue和出队dequeue操作都可以通过原子操作完成。

在 AQS 内部，通过维护一个FIFO 队列来管理多线程的排队工作。在公平竞争的情况下，无法获取同步状态的线程将会被封装成一个节点，置于队列尾部。入队的线程将会通过自旋的方式获取同步状态，若在有限次的尝试后，仍未获取成功，线程则会被阻塞住。大致示意图如下：

当头结点释放同步状态后，且后继节点对应的线程被阻塞，此时头结点线程将会去唤醒后继节点线程。后继节点线程恢复运行并获取同步状态后，会将旧的头结点从队列中移除，并将自己设为头结点。大致示意图如下：

其中每个节点包含如下状态：

/** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3;

/**
 * Status field, taking on only the values:
 *   SIGNAL:     The successor of this node is (or will soon be)
 *               blocked (via park), so the current node must
 *               unpark its successor when it releases or
 *               cancels. To avoid races, acquire methods must
 *               first indicate they need a signal,
 *               then retry the atomic acquire, and then,
 *               on failure, block.
 *   CANCELLED:  This node is cancelled due to timeout or interrupt.
 *               Nodes never leave this state. In particular,
 *               a thread with cancelled node never again blocks.
 *   CONDITION:  This node is currently on a condition queue.
 *               It will not be used as a sync queue node
 *               until transferred, at which time the status
 *               will be set to 0. (Use of this value here has
 *               nothing to do with the other uses of the
 *               field, but simplifies mechanics.)
 *   PROPAGATE:  A releaseShared should be propagated to other
 *               nodes. This is set (for head node only) in
 *               doReleaseShared to ensure propagation
 *               continues, even if other operations have
 *               since intervened.
 *   0:          None of the above
 *
 * The values are arranged numerically to simplify use.
 * Non-negative values mean that a node doesn't need to
 * signal. So, most code doesn't need to check for particular
 * values, just for sign.
 *
 * The field is initialized to 0 for normal sync nodes, and
 * CONDITION for condition nodes.  It is modified using CAS
 * (or when possible, unconditional volatile writes).
 */
CANCELED表示线程等待已经取消，是唯一一个大于0的状态。
SINALG表示需要唤醒next节点
CONDITION表明线程正在等待一个条件
PROPAGATE用于acquireShared中向后传播

3 acquire

/**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

1、如果尝试获取锁成功整个获取操作就结束，否则转到2. 尝试获取锁是通过方法tryAcquire来实现的，AQS中并没有该方法的具体实现，只是简单地抛出一个不支持操作异常，在AQS简介中谈到tryAcquire有很多实现方法，这里不再细化，只需要知道如果获取锁成功该方法返回true即可；

2、如果获取锁失败，那么就创建一个代表当前线程的结点加入到等待队列的尾部，是通过addWaiter方法实现的，来看该方法的具体实现：

/**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

该方法创建了一个独占式结点，然后判断队列中是否有元素，如果有（pred!=null）就设置当前结点为队尾结点，即将当前节点插入到尾节点的后面，然后返回；

如果没有元素（pred==null），表示队列为空，走的是入队操作

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     */
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

enq方法采用的是变种CLH算法，先看头结点是否为空，如果为空就创建一个傀儡结点，头尾指针都指向这个傀儡结点，这一步只会在队列初始化时会执行；

如果头结点非空，就采用CAS操作将当前结点插入到头结点后面，如果在插入的时候尾结点有变化，就将尾结点向后移动直到移动到最后一个结点为止，然后再把当前结点插入到尾结点后面，尾指针指向当前结点，入队成功。

3、将新加入的结点放入队列之后，这个结点有两种状态，要么获取锁，要么就挂起，如果这个结点不是头结点的后继节点，就看看这个结点是否应该挂起，如果应该挂起，就挂起当前结点，是否应该挂起是通过shouldParkAfterFailedAcquire方法来判断的 　

    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     *
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                //判断前驱节点是否是头节点，如果是，则循环获取同步
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //如果获取同步状态失败或者前驱节点不是头节点，则判断是否将当前节点挂起
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

　　

    /**
     * Checks and updates status for a node that failed to acquire.
     * Returns true if thread should block. This is the main signal
     * control in all acquire loops.  Requires that pred == node.prev
     *
     * @param pred node's predecessor holding status
     * @param node the node
     * @return {@code true} if thread should block
     */
/**
 * 该方法主要用途是，当线程在获取同步状态失败时，根据前驱节点的等待状态，决定后续的动作。比如前驱
 * 节点等待状态为 SIGNAL，表明当前节点线程应该被阻塞住了。不能老是尝试，避免 CPU 忙等。
 *    —————————————————————————————————————————————————————————————————
 *    | 前驱节点等待状态 |                   相应动作                     |
 *    —————————————————————————————————————————————————————————————————
 *    | SIGNAL         | 阻塞                                          |
 *    | CANCELLED      | 向前遍历, 移除前面所有为该状态的节点               |
 *    | waitStatus < 0 | 将前驱节点状态设为 SIGNAL, 并再次尝试获取同步状态   |
 *    —————————————————————————————————————————————————————————————————
 */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        //首先检查前趋结点的waitStatus位，如果为SIGNAL,表示前趋结点会通知它，那么它可以放心大胆地挂起了
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            //如果前趋结点是一个被取消的结点怎么办呢？那么就向前遍历跳过被取消的结点，直到找到一个没有被取消的结点为止，将找到的这个结点作为它的前趋结点，
　　　　　　　//将找到的这个结点的waitStatus位设置为SIGNAL,返回false表示线程不应该被挂起，继续尝试获取锁
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
         /*
         * 等待状态为 0 或 PROPAGATE，设置前驱节点等待状态为 SIGNAL，
         * 并再次尝试获取同步状态。
         */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

private final boolean parkAndCheckInterrupt() {
    // 调用 LockSupport.park 阻塞自己
    LockSupport.park(this);
    return Thread.interrupted();
}

4 release

    /**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryRelease} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

　　

1、release过程比acquire要简单，首先调用tryRelease释放锁，如果释放失败，直接返回；

2、释放锁成功后需要唤醒继任结点，是通过方法unparkSuccessor实现的

    /**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

1、node参数传进来的是头结点，首先检查头结点的waitStatus位，如果为负，表示头结点还需要通知后继结点，这里不需要头结点去通知后继，因此将该该标志位清0.

2、然后查看头结点的下一个结点，如果下一个结点不为空且它的waitStatus<=0,表示后继结点没有被取消，是一个可以唤醒的结点，于是唤醒后继结点返回；如果后继结点为空或者被取消了怎么办？寻找下一个可唤醒的结点，然后唤醒它返回。

这里并没有从头向尾寻找，从队列尾部开始向前查找，找到队列最前面没有被取消的节点，然后，将其唤醒。

为什么需要从队列尾部开始向前查找呢？

因为在CLH队列中的结点随时有可能被中断，被中断的结点的waitStatus设置为CANCEL,而且它会被踢出CLH队列，如何个踢出法，就是它的前趋结点的next并不会指向它，而是指向下一个非CANCEL的结点,而它自己的next指针指向它自己。一旦这种情况发生，如何从头向尾方向寻找继任结点会出现问题，因为一个CANCEL结点的next为自己，那么就找不到正确的继任接点。

有的人又会问了，CANCEL结点的next指针为什么要指向它自己，为什么不指向真正的next结点？为什么不为NULL？

第一个问题的答案是这种被CANCEL的结点最终会被GC回收，如果指向next结点，GC无法回收。