/**
     * 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; // 等待队列的 head

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

    /**
     * The synchronization state.
     */
    private volatile int state; // 同步状态, 这就是所谓的 lock


    /**
     * The current owner of exclusive mode synchronization.
     */
    private transient Thread exclusiveOwnerThread;//继承自 AbstractOwnableSynchronizer

    volatile int waitStatus; // 下一个 Node 等待状态
    volatile Node prev; // 前驱节点
    volatile Node next; // 后继节点
    volatile Thread thread; // 入队等待的线程

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

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread(); // 当前申请获取锁的线程
            int c = getState(); // lock 的状态
            if (c == 0) { // lock 是 unlock 状态
                if (!hasQueuedPredecessors() && // 看一下队列中是不是已有在等待获取锁的线程，这就是所谓公平的体现，FIFO
                    compareAndSetState(0, acquires)) { // 没有在等待获取锁的线程, 获取锁，这里 CAS 可能会失败
                    setExclusiveOwnerThread(current); // 本线程持有这个锁
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) { // 该线程已经持有了锁
                int nextc = c + acquires; // 直接改变状态，这就是所谓可重入的意思,已经获取锁的线程，可以再次获取该锁
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded"); // 这里可能会抛异常
                setState(nextc);
                return true;
            }
            return false;
        }

    /**
     * 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) { // 说明队列不空
            node.prev = pred; // 设置 node 前驱
            if (compareAndSetTail(pred, node)) { // AQS 设置新的 tail
                pred.next = node; // 设置 pred 后继
                return node; // 返回包装申请锁的线程的 Node 节点
            }
        }
        enq(node); // 说明队列为空
        return node; // 返回包装申请锁的线程的 Node 节点
    }

    /**
     * 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 (;;) { // 注意是循环
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) { // 是等待队列的第一个 && tryAcquire , true 表明获取了锁
                    setHead(node); // 设置成 head, 旧的 head 出队
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) && // 锁已被其他线程获取, 会走到这里
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Cancels an ongoing attempt to acquire.
     *
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;

        node.thread = null; // 取消线程

        // Skip cancelled predecessors 跳过取消的节点
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary.
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves. 队尾
        if (node == tail && compareAndSetTail(node, pred)) { // expect update
            compareAndSetNext(pred, predNext, null); // node expect null
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            // 不是队尾，也就是等待队列的其他节点
            int ws;
            if (pred != head && // 不是等待队列中第一个节点
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) { // pred.thread != null 表示前驱节点有线程在等待获取锁
                Node next = node.next; 
                if (next != null && next.waitStatus <= 0) // 取消节点有后继节点 next 且后继节点 waitStatus <= 0
                    compareAndSetNext(pred, predNext, next); // pred 设置新的 next 节点
            } else {
                // 取消等待的节点是等待队列中的第一个节点, 叫醒后继节点
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

if (pred != head && 
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) 

    /**
     * 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;
        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) { // 线程取消了获取锁, 找到一个没有取消的
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            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.
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL); // 设置前驱节点的 waitStatus 为 -1
        }
        return false;
    }

    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this); // 调用线程执行到这里就不再继续向下执行了, 这是理解线程 waiting 的关键
        return Thread.interrupted(); // 线程被唤醒后才会 return, 执行下一轮的 acquireQueued
    }

    /**
     * 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)) { // 释放锁
            Node h = head;
            if (h != null && h.waitStatus != 0) // waitStatus 为 -1, 表明下一个节点需要被唤醒
                unparkSuccessor(h); // 唤醒 head 节点后的节点，也就是等待队列中第一个节点
            return true;
        }
        return false;
    }

      protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread()) // 是不是持有该锁的线程在 release
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) { // 已完全释放，因为可以重入
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c); // lock 设置成 unlock 状态，也就是 0，这就是所谓的释放锁
            return free;
        }

    /**
     * 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); // 唤醒这个线程
    }