5-1. AQS（AbstarctQueuedSynchronizer）

 1 //acquire操作
 2 while (当前同步器的状态不允许获取操作) { 
 3     如果当前线程不在队列中，则将其插入队列
 4     阻塞当前线程
 5 }
 6 如果线程位于队列中，则将其移出队列
 7 
 8 
 9 //release操作
10 更新同步器的状态
11 if (新的状态允许某个被阻塞的线程获取成功){
12     解除队列中一个或多个线程的阻塞状态
13 }

public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

private Node addWaiter(Node mode) {
        // 当前线程构造成Node节点
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        // 尝试快速在尾节点后新增节点 提升算法效率 先将尾节点指向pred
        Node pred = tail;
        if (pred != null) {
            //尾节点不为空  当前线程节点的前驱节点指向尾节点
            node.prev = pred;
            //并发处理 尾节点有可能已经不是之前的节点 所以需要CAS更新
            if (compareAndSetTail(pred, node)) {
                //CAS更新成功 当前线程为尾节点 原先尾节点的后续节点就是当前节点
                pred.next = node;
                return node;
            }
        }
        //第一个入队的节点或者是尾节点后续节点新增失败时进入enq
        enq(node);
        return node;
    }
private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                //尾节点为空  第一次入队  设置头尾节点一致 同步队列的初始化
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                //所有的线程节点在构造完成第一个节点后 依次加入到同步队列中
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                //获取当前线程节点的前驱节点
                final Node p = node.predecessor();
                //前驱节点为头节点且成功获取同步状态
                if (p == head && tryAcquire(arg)) {
                    //设置当前节点为头节点
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //是否阻塞
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        //前驱节点的状态决定后续节点的行为
　　　　 int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*前驱节点为-1 后续节点可以被阻塞
             * 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.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*前驱节点是初始或者共享状态就设置为-1 使后续节点阻塞
             * 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.
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }
private final boolean parkAndCheckInterrupt() {
        //阻塞线程
        LockSupport.park(this);
        return Thread.interrupted();
    }

public final boolean release(int arg) {
        if (tryRelease(arg)) {//同步状态释放成功
            Node h = head;
            if (h != null && h.waitStatus != 0)
                //直接释放头节点
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
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);

        /*寻找符合条件的后续节点
         * 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)
            //唤醒后续节点
            LockSupport.unpark(s.thread);
    }

public final void acquireShared(int arg) {
        //获取同步状态的返回值大于等于0时表示可以获取同步状态
        //小于0时表示可以获取不到同步状态  需要进入队列等待
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }
private void doAcquireShared(int arg) {
        //和独占式一样的入队操作
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            //自旋
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        //前驱结点为头节点且成功获取同步状态 可退出自旋
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        //退出自旋的节点变成首节点
        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.
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

public final boolean releaseShared(int arg) {
        //释放同步状态
        if (tryReleaseShared(arg)) {
            //唤醒后续等待的节点
            doReleaseShared();
            return true;
        }
        return false;
    }
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.
         */
        //自旋
　　　　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
                    //唤醒后续节点
　　　　　　　　　　　　unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }