/** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;

        /** 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).
         */
        volatile int waitStatus;

        /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueuing, and nulled
         * out (for sake of GC) only upon dequeuing.  Also, upon
         * cancellation of a predecessor, we short-circuit while
         * finding a non-cancelled one, which will always exist
         * because the head node is never cancelled: A node becomes
         * head only as a result of successful acquire. A
         * cancelled thread never succeeds in acquiring, and a thread only
         * cancels itself, not any other node.
         */
        volatile Node prev;

        /**
         * Link to the successor node that the current node/thread
         * unparks upon release. Assigned during enqueuing, adjusted
         * when bypassing cancelled predecessors, and nulled out (for
         * sake of GC) when dequeued.  The enq operation does not
         * assign next field of a predecessor until after attachment,
         * so seeing a null next field does not necessarily mean that
         * node is at end of queue. However, if a next field appears
         * to be null, we can scan prev‘s from the tail to
         * double-check.  The next field of cancelled nodes is set to
         * point to the node itself instead of null, to make life
         * easier for isOnSyncQueue.
         */
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on
         * construction and nulled out after use.
         */
        volatile Thread thread;

        /**
         * Link to next node waiting on condition, or the special
         * value SHARED.  Because condition queues are accessed only
         * when holding in exclusive mode, we just need a simple
         * linked queue to hold nodes while they are waiting on
         * conditions. They are then transferred to the queue to
         * re-acquire. And because conditions can only be exclusive,
         * we save a field by using special value to indicate shared
         * mode.
         */
        Node nextWaiter;

    /**
     * Head of the wait queue, lazily initialized.  Except for
     * initialization, it is modified only via method setHead.  Note:
     * If head exists, its waitStatus is guaranteed not to be
     * CANCELLED.
     */
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
    private transient volatile Node tail;

    /**
     * The synchronization state.
     */
    private volatile int state;

    /**
     * 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) {
        //首先执行tryAcquire(arg)尝试获取资源，如果成功则直接返回
        //如果tryAcquire(arg)获取资源失败，则讲当前线程封装成Node节点加入到sync queue队列中，并通过acquireQueued进行获取资源直至成功(如果尚未有资源可获取，那么acquireQueued会阻塞当前线程)
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }    

    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 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) {
        //生成指定模式的Node节点
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        //以下几行进行入队操作，如果失败，交给enq进行入队处理。其实，我认为可以直接调用enq，不知道作者设置如下几行的意图
        if (pred != null) {
            node.prev = pred;
            //通过CAS操作串行化并发入队操作，仅有一个线程会成功，由于node节点的prev字段是在CAS操作之前进行的，一旦CAS操作成功，node节点的prev字段就是指向了其前继节点，因此说prev字段是安全的
            if (compareAndSetTail(pred, node)) {
                //这里直接通过赋值操作赋值next字段，注意，可能有别的线程会在next字段赋值之前访问到next字段，因此next字段是非可靠的(一个节点的next字段为null并不代表该节点没有后继)
                pred.next = node;
                //一旦next字段赋值成功，那么next字段又变为可靠的了
                return node;
            }
        }
        //通过enq入队
        enq(node);
        return node;
    }

    /**
     * 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) {
        //死循环进行入队操作，CAS操作常规模式
        for (;;) {
            Node t = tail;
            //此时队列为空，需要初始化
            if (t == null) { // Must initialize
                //此时可能多个线程都在执行该方法，因此只有一个线程才能初始化sync queue，此处添加的节点我称之为Dummy Node，该节点没有关联线程
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                //以下四行与addWaiter类似，不再赘述
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }                

    /**
     * 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) {
        //用于记录是否获取成功，我现在还不清楚何时会失败= =
        boolean failed = true;
        try {
            //记录是否被中断过，如果被中断过，则需要在acquire方法中恢复中断现场
            boolean interrupted = false;
            //同样的套路，CAS配合死循环
            for (;;) {
                //获取node节点的前继节点p
                final Node p = node.predecessor();
                //当p为sync queue头结点时，才有资格尝试获取资源，换言之，当且仅当一个节点是sync queue中第二个节点时，它才有资格获取资源
                if (p == head && tryAcquire(arg)) {
                    //一旦获取成功，以下语句都是线程安全的，所有字段直接赋值即可，不需要CAS或者加锁
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //否则，找到前继节点，并将其设置为SIGNAL状态后阻塞自己
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                //如果失败了
                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
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        //一旦发现前继节点是SIGNAL状态，就返回true，在acquireQueued方法中会阻塞当前线程
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            //这里给出两个问题：
                 //1.如果在当前线程阻塞之前，前继节点就唤醒了当前线程，那么当前线程不就永远阻塞下去了吗？
                 //2.万一有别的线程更改了前继节点的状态，导致前继节点不唤醒当前线程，那么当前线程不就永远阻塞下去了吗？
            return true;

        //如果前继节点处于CANCELL状态(仅有CANCELL状态大于0)
        if (ws > 0) {
            //那么跳过那些被CANCELL的节点，先前找到第一个有效节点
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
        //前继节点状态要么是0，要么是PROPAGATE，将其通过CAS操作设为SIGNAL，不用管是否成功，退回到上层函数acquireQueued进行再次判断
            /*
             * 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;
    }

    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        //返回是否被中断过
        return Thread.interrupted();
    }

    /**
     * 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) {
        //调用tryRelease尝试释放资源
        if (tryRelease(arg)) {
            Node h = head;
            //只要头节点不为空且状态不为0，就唤醒后继节点，对于独占模式也就只有SIGNAL状态一种，头结点在任何情况下都不可能为CANCELL状态
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 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;
        //若节点状态小于0，将其通过CAS操作改为0，表明本次SIGNAL的任务已经完成，至于CAS是否成功，或者是否再次被其他线程修改，都与本次无关unparkSuccessor无关，只是该节点被赋予了新的任务而已。
        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.
         */
        //这里通过非可靠的next字段直接获取后继，如果非空，那么说明该字段可靠，如果为空，那么利用可靠的prev字段从tail向前找到当前node节点的后继节点
        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;
        }
        //唤醒后继节点
        if (s != null)
            LockSupport.unpark(s.thread);
    }