AbstractQueuedSynchronizer.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import sun.misc.Unsafe;

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic {@code int} value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated {@code int}
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 *
 * <p>Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their public methods.
 *
 * <p>This class supports either or both a default <em>exclusive</em>
 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not &quot;understand&quot; these differences except in the
 * mechanical sense that when a shared mode acquire succeeds, the next
 * waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 *
 * <p>This class defines a nested {@link ConditionObject} class that
 * can be used as a {@link Condition} implementation by subclasses
 * supporting exclusive mode for which method {@link
 * #isHeldExclusively} reports whether synchronization is exclusively
 * held with respect to the current thread, method {@link #release}
 * invoked with the current {@link #getState} value fully releases
 * this object, and {@link #acquire}, given this saved state value,
 * eventually restores this object to its previous acquired state.  No
 * {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an {@code AbstractQueuedSynchronizer} for their
 * synchronization mechanics.
 *
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a {@code readObject} method that restores this to a known
 * initial state upon deserialization.
 *
 * <h3>Usage</h3>
 *
 * <p>To use this class as the basis of a synchronizer, redefine the
 * following methods, as applicable, by inspecting and/or modifying
 * the synchronization state using {@link #getState}, {@link
 * #setState} and/or {@link #compareAndSetState}:
 *
 * <ul>
 * <li> {@link #tryAcquire}
 * <li> {@link #tryRelease}
 * <li> {@link #tryAcquireShared}
 * <li> {@link #tryReleaseShared}
 * <li> {@link #isHeldExclusively}
 * </ul>
 *
 * Each of these methods by default throws {@link
 * UnsupportedOperationException}.  Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the <em>only</em> supported
 * means of using this class. All other methods are declared
 * {@code final} because they cannot be independently varied.
 *
 * <p>You may also find the inherited methods from {@link
 * AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer.  You are encouraged to use them
 * -- this enables monitoring and diagnostic tools to assist users in
 * determining which threads hold locks.
 *
 * <p>Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies.  The core
 * of exclusive synchronization takes the form:
 *
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 *     }
 *
 * Release:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 * </pre>
 *
 * (Shared mode is similar but may involve cascading signals.)
 *
 * <p id="barging">Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 * others that are blocked and queued.  However, you can, if desired,
 * define {@code tryAcquire} and/or {@code tryAcquireShared} to
 * disable barging by internally invoking one or more of the inspection
 * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 * In particular, most fair synchronizers can define {@code tryAcquire}
 * to return {@code false} if {@link #hasQueuedPredecessors} (a method
 * specifically designed to be used by fair synchronizers) returns
 * {@code true}.  Other variations are possible.
 *
 * <p>Throughput and scalability are generally highest for the
 * default barging (also known as <em>greedy</em>,
 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
 * While this is not guaranteed to be fair or starvation-free, earlier
 * queued threads are allowed to recontend before later queued
 * threads, and each recontention has an unbiased chance to succeed
 * against incoming threads.  Also, while acquires do not
 * &quot;spin&quot; in the usual sense, they may perform multiple
 * invocations of {@code tryAcquire} interspersed with other
 * computations before blocking.  This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. If so desired, you can
 * augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended}
 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 * is likely not to be contended.
 *
 * <p>This class provides an efficient and scalable basis for
 * synchronization in part by specializing its range of use to
 * synchronizers that can rely on {@code int} state, acquire, and
 * release parameters, and an internal FIFO wait queue. When this does
 * not suffice, you can build synchronizers from a lower level using
 * {@link java.util.concurrent.atomic atomic} classes, your own custom
 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 *
 * <h3>Usage Examples</h3>
 *
 * <p>Here is a non-reentrant mutual exclusion lock class that uses
 * the value zero to represent the unlocked state, and one to
 * represent the locked state. While a non-reentrant lock
 * does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor.
 * It also supports conditions and exposes
 * one of the instrumentation methods:
 *
 *  <pre> {@code
 * class Mutex implements Lock, java.io.Serializable {
 *
 *   // Our internal helper class
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Reports whether in locked state
 *     protected boolean isHeldExclusively() {
 *       return getState() == 1;
 *     }
 *
 *     // Acquires the lock if state is zero
 *     public boolean tryAcquire(int acquires) {
 *       assert acquires == 1; // Otherwise unused
 *       if (compareAndSetState(0, 1)) {
 *         setExclusiveOwnerThread(Thread.currentThread());
 *         return true;
 *       }
 *       return false;
 *     }
 *
 *     // Releases the lock by setting state to zero
 *     protected boolean tryRelease(int releases) {
 *       assert releases == 1; // Otherwise unused
 *       if (getState() == 0) throw new IllegalMonitorStateException();
 *       setExclusiveOwnerThread(null);
 *       setState(0);
 *       return true;
 *     }
 *
 *     // Provides a Condition
 *     Condition newCondition() { return new ConditionObject(); }
 *
 *     // Deserializes properly
 *     private void readObject(ObjectInputStream s)
 *         throws IOException, ClassNotFoundException {
 *       s.defaultReadObject();
 *       setState(0); // reset to unlocked state
 *     }
 *   }
 *
 *   // The sync object does all the hard work. We just forward to it.
 *   private final Sync sync = new Sync();
 *
 *   public void lock()                { sync.acquire(1); }
 *   public boolean tryLock()          { return sync.tryAcquire(1); }
 *   public void unlock()              { sync.release(1); }
 *   public Condition newCondition()   { return sync.newCondition(); }
 *   public boolean isLocked()         { return sync.isHeldExclusively(); }
 *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 *   public void lockInterruptibly() throws InterruptedException {
 *     sync.acquireInterruptibly(1);
 *   }
 *   public boolean tryLock(long timeout, TimeUnit unit)
 *       throws InterruptedException {
 *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 *   }
 * }}</pre>
 *
 * <p>Here is a latch class that is like a
 * {@link java.util.concurrent.CountDownLatch CountDownLatch}
 * except that it only requires a single {@code signal} to
 * fire. Because a latch is non-exclusive, it uses the {@code shared}
 * acquire and release methods.
 *
 *  <pre> {@code
 * class BooleanLatch {
 *
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     boolean isSignalled() { return getState() != 0; }
 *
 *     protected int tryAcquireShared(int ignore) {
 *       return isSignalled() ? 1 : -1;
 *     }
 *
 *     protected boolean tryReleaseShared(int ignore) {
 *       setState(1);
 *       return true;
 *     }
 *   }
 *
 *   private final Sync sync = new Sync();
 *   public boolean isSignalled() { return sync.isSignalled(); }
 *   public void signal()         { sync.releaseShared(1); }
 *   public void await() throws InterruptedException {
 *     sync.acquireSharedInterruptibly(1);
 *   }
 * }}</pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

    private static final long serialVersionUID = 7373984972572414691L;

    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance
     * with initial synchronization state of zero.
     */
    protected AbstractQueuedSynchronizer() { }

    /**
     * Wait queue node class.
     *
     * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
     * Hagersten) lock queue. CLH locks are normally used for
     * spinlocks.  We instead use them for blocking synchronizers, but
     * use the same basic tactic of holding some of the control
     * information about a thread in the predecessor of its node.  A
     * "status" field in each node keeps track of whether a thread
     * should block.  A node is signalled when its predecessor
     * releases.  Each node of the queue otherwise serves as a
     * specific-notification-style monitor holding a single waiting
     * thread. The status field does NOT control whether threads are
     * granted locks etc though.  A thread may try to acquire if it is
     * first in the queue. But being first does not guarantee success;
     * it only gives the right to contend.  So the currently released
     * contender thread may need to rewait.
     *
     * <p>To enqueue into a CLH lock, you atomically splice it in as new
     * tail. To dequeue, you just set the head field.
     * <pre>
     *      +------+  prev +-----+       +-----+
     * head |      | <---- |     | <---- |     |  tail
     *      +------+       +-----+       +-----+
     * </pre>
     *
     * <p>Insertion into a CLH queue requires only a single atomic
     * operation on "tail", so there is a simple atomic point of
     * demarcation from unqueued to queued. Similarly, dequeuing
     * involves only updating the "head". However, it takes a bit
     * more work for nodes to determine who their successors are,
     * in part to deal with possible cancellation due to timeouts
     * and interrupts.
     *
     * <p>The "prev" links (not used in original CLH locks), are mainly
     * needed to handle cancellation. If a node is cancelled, its
     * successor is (normally) relinked to a non-cancelled
     * predecessor. For explanation of similar mechanics in the case
     * of spin locks, see the papers by Scott and Scherer at
     * http://www.cs.rochester.edu/u/scott/synchronization/
     *
     * <p>We also use "next" links to implement blocking mechanics.
     * The thread id for each node is kept in its own node, so a
     * predecessor signals the next node to wake up by traversing
     * next link to determine which thread it is.  Determination of
     * successor must avoid races with newly queued nodes to set
     * the "next" fields of their predecessors.  This is solved
     * when necessary by checking backwards from the atomically
     * updated "tail" when a node's successor appears to be null.
     * (Or, said differently, the next-links are an optimization
     * so that we don't usually need a backward scan.)
     *
     * <p>Cancellation introduces some conservatism to the basic
     * algorithms.  Since we must poll for cancellation of other
     * nodes, we can miss noticing whether a cancelled node is
     * ahead or behind us. This is dealt with by always unparking
     * successors upon cancellation, allowing them to stabilize on
     * a new predecessor, unless we can identify an uncancelled
     * predecessor who will carry this responsibility.
     *
     * <p>CLH queues need a dummy header node to get started. But
     * we don't create them on construction, because it would be wasted
     * effort if there is never contention. Instead, the node
     * is constructed and head and tail pointers are set upon first
     * contention.
     *
     * <p>Threads waiting on Conditions use the same nodes, but
     * use an additional link. Conditions only need to link nodes
     * in simple (non-concurrent) linked queues because they are
     * only accessed when exclusively held.  Upon await, a node is
     * inserted into a condition queue.  Upon signal, the node is
     * transferred to the main queue.  A special value of status
     * field is used to mark which queue a node is on.
     *
     * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
     * Scherer and Michael Scott, along with members of JSR-166
     * expert group, for helpful ideas, discussions, and critiques
     * on the design of this class.
     */
    static final class Node {

        /*********************锁的模式：共享或排他*******************/
        /** 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;

        /*********************节点的状态***************************/

        //表示该节点维护的线程已经不在等待的状态
        static final int CANCELLED =  1;
        //表示该节点需要负责后继节点的唤醒
        static final int SIGNAL    = -1;
        //表示该节点在Condition的队列中
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3;

        /**
         * 节点的状态
         */
        volatile int waitStatus;

        /**
         * 节点的前继节点，只有在AQS中才存在
         */
        volatile Node prev;

        /**
         * 节点的后继节点，只有在AQS中才存在
         */
        volatile Node next;

        /**
         * 节点对应的线程
         */
        volatile Thread thread;

        /**
         * 可以用来表示锁的模式 或是 下一等待者
         * 当Node在AQS的阻塞队列中，nextWaiter表示锁为排他或是共享
         * 当Node在Condition的等待队列中，nextWaiter表示等待队列的后继节点
         */
        Node nextWaiter;

        /**
         * 节点是否为共享模式
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * 返回该节点的前继节点，只有在AQS中才存在prev
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

        Node() {    // Used to establish initial head or SHARED marker
        }

        Node(Thread thread, Node mode) {     // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }

        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

    /**
     * 队列的头节点
     */
    private transient volatile Node head;

    /**
     * 队列的尾节点
     */
    private transient volatile Node tail;

    /**
     * state表示锁的状态，0表示没线程持有锁，>0表示有线程持有锁（锁被持有的次数）
     */
    private volatile int state;

    /**
     * 获取锁的state
     */
    protected final int getState() {
        return state;
    }

    /**
     * 获取锁的state
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * CAS更新state值
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

    // Queuing utilities

    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * 往队列中插入node
     */
    private Node enq(final Node node) {
        //循环尝试，直至CAS成功
        for (;;) {
            Node t = tail;
            //未初始化时，先初始化
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else { //添加node至队尾
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
     * 添加等待者
     */
    private Node addWaiter(Node mode) {
        //创建队列的节点（等待的线程和模式）
        Node node = new Node(Thread.currentThread(), mode);

        //CAS插入，先快速尝试一次
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        //失败，则进入完全CAS操作
        enq(node);
        return node;
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     * 唤醒node的后一个节点的线程
     */
    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);

        //找出node的未取消的下一节点
        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;
        }
        //如果successor存在，唤醒该线程
        if (s != null)
            LockSupport.unpark(s.thread);
    }

    /**
     * Release action for shared mode -- signals successor and ensures
     * propagation. (Note: For exclusive mode, release just amounts
     * to calling unparkSuccessor of head if it needs signal.)
     */
    //共享模式的释放锁
    //和独占模式最大的区别就是共享模式下的唤醒过程会开始传播，而独占模式下只会唤醒头节点的后继节点
    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;
            //如果头节点不为空 或者 头节点不等于尾节点 说明阻塞队列中有正在等待的线程
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                //如果节点的 waitStatus 为SIGNAL，表示有后继节点需要等待唤醒
                if (ws == Node.SIGNAL) {
                    //CAS操作更新waitStatus值
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) //CAS失败说明已经有其他线程修改了head的waitStatus
                        continue;            // loop to recheck cases
                    //唤醒等待的节点
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) //ws等于0说明没有后继节点， 通过CAS将waitStatus设置为 PROPAGATE
                    continue;                // loop on failed CAS
            }

            //如果头节点发生了改变，说明唤醒了后继线程，继续循环尝试唤醒剩下的等待节点
            //如果头节点未发生改变，说明唤醒失败（可能是没有等待的节点，也可能是其他线程已经在处理唤醒），线程完成唤醒任务
            if (h == head)                   // loop if head changed
                break;
        }
    }

    /**
     * Sets head of queue, and checks if successor may be waiting
     * in shared mode, if so propagating if either propagate > 0 or
     * PROPAGATE status was set.
     *
     * @param node the node
     * @param propagate the return value from a tryAcquireShared
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        //更新队列的头节点
        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.
         */
        //如果满足以下一个条件，则需要唤醒后继节点（如果存在的话）
        //1）propagate > 0 说明需要唤醒后继节点
        //2) h == null 说明没有阻塞的情况
        //3）waitStatus < 0
        //4）或者新的head满足以上2，3条件
        //TODO 后面几个判断对应什么情形还待研究
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

    // Utilities for various versions of acquire

    /**
     * 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;

        // Skip cancelled predecessors
        Node pred = node.prev;

        //处理已经被取消的前继节点
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        Node predNext = pred.next;

        node.waitStatus = Node.CANCELLED;


        if (node == tail && compareAndSetTail(node, pred)) { //如果是tail节点，更新tail，并将新tail的next设置为null
            //因为if中有CAS操作，其他线程会快速失败，进入这里的应该认为线程安全
            compareAndSetNext(pred, predNext, null);
        } else {
            int ws;
            // successor不需要唤醒，则只更新next
            if (pred != head &&
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else { //如果successor需要唤醒
                unparkSuccessor(node);
            }

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

    /**
     * 该方法会确认是否需要Park线程（当前继节点为SIGNAL时，需要Park）;
     * 并在过程中，更新节点的status
     */
    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) { //说明前节点已经被取消了，node需要延链表向前遍历，找到未被取消的节点
            //向前遍历
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * 将前节点的状态设置为SIGNAL，表示需要给后继节点信号
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * Convenience method to interrupt current thread.
     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /**
     * 阻塞线程，直到线程被再次唤醒
     * 并返回线程是否是由于interrupt被唤醒的
     */
    private final boolean parkAndCheckInterrupt() {
        //线程在这里阻塞 变成WAITING状态，LockSupport.unpark和Thread.interrupt()都可以让线程从WAITING会带RUNNABLE状态
        LockSupport.park(this);
        //返回是否是由于Thread.interrupt()造成的返回
        return Thread.interrupted();
    }

    /*
     * Various flavors of acquire, varying in exclusive/shared and
     * control modes.  Each is mostly the same, but annoyingly
     * different.  Only a little bit of factoring is possible due to
     * interactions of exception mechanics (including ensuring that we
     * cancel if tryAcquire throws exception) and other control, at
     * least not without hurting performance too much.
     */

    /**
     * 将当前线程添加进锁的队列，并park线程，直到前继节点unpark该线程后再返回
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {//表示获取成功
                    //判断内可以认为是线程安全的，因此没有CAS的操作
                    setHead(node);
                    //释放p结点
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //当前线程还不能获得锁
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt()) //if中第一步先检查是否需要park，第二步通过LockSuppot.park进行park
                    interrupted = true;
            }
        } finally {
            if (failed)//异常结束
                cancelAcquire(node);
        }
    }

    /**
     * 可中断的获取锁
     */
    private void doAcquireInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();

                //思考??? 非公平锁也要考虑前继节点是否为head?

                if (p == head && tryAcquire(arg)) { //是否有资格获取锁 且 成功得到锁

                    //更新头节点，并释放旧的头节点
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }

                //获取锁失败，检查是否需要被Park，如果需要则Park，并确认是否是因为interrupt而非unpark唤醒的线程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    // doAcquireInterruptibly和acquireQueued其实很像，只是如果线程是因为Interrupt被唤醒的会抛出一个异常
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * doAcquireNanos和doAcquire方法也类似
     */
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold) //增加了一步判断是否超时
                    LockSupport.parkNanos(this, nanosTimeout); //同时用LockSupport.parkNanos代替LockSupport.park，线程进入TIMED_WAITING
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared uninterruptible mode.
     * @param arg the acquire argument
     */
    //以共享方式获取锁
    private void doAcquireShared(int arg) {
        //往队列中添加节点
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            //自旋等待
            for (;;) {
                final Node p = node.predecessor();
                //如果前继节点已经是头节点，说明轮到唤醒本节点
                if (p == head) {
                    //先已共享方式获取锁，主要是看AQS的state是否已经等于0
                    int r = tryAcquireShared(arg);
                    if (r >= 0) { //获取锁成功
                        //唤醒后继节点
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                //如果前继节点不为头节点 或者获取锁失败
                //检查线程是否需要被Park，有则Park线程，线程会执行到parkAndCheckInterrupt中被挂起
                //并在唤醒后，检查是否由于中断而唤醒的
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    //以共享方式获取锁
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        //添加进阻塞队列，此时的模式是共享模式
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        //自循环等待
        try {
            for (;;) {
                final Node p = node.predecessor();
                //如果前继节点已经是头节点
                if (p == head) {
                    //再尝试以共享模式获取
                    int r = tryAcquireShared(arg);
                    //r >= 0 则说明获取成功
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                //获取失败，则先被Park，被唤醒后检查是否由中断唤醒，是则抛出InterruptedException
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    /**
     * 在指定时间内以共享方式获取锁
     * @param arg
     * @param nanosTimeout
     * @return
     * @throws InterruptedException
     */
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        //调用该方法时，说明先尝试过获取了锁，并且尝试获取锁失败
        //因此此时需要被添加进节点
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) { //如果前继节点已经是队列的头节点
                    //则说明该节点需要被唤醒
                    //此时再去尝试获取锁
                    int r = tryAcquireShared(arg);
                    if (r >= 0) { //如果成功（也可能因为非公平锁的性质导致头节点唤醒失败）
                        //则更新队列的头节点，并唤醒之后的节点
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                //唤醒后线程仍然获取失败
                nanosTimeout = deadline - System.nanoTime();
                //先计算是否超时
                if (nanosTimeout <= 0L)
                    return false;
                //是否需要继续挂起线程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                //线程是否被中断醒来
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    // Main exported methods

    /**
     * Attempts to acquire in exclusive mode. This method should query
     * if the state of the object permits it to be acquired in the
     * exclusive mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread. This can be used
     * to implement method {@link Lock#tryLock()}.
     *
     * <p>The default
     * implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return {@code true} if successful. Upon success, this object has
     *         been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in exclusive
     * mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *        passed to a release method, or the current state value upon
     *        entry to a condition wait.  The value is otherwise
     *        uninterpreted and can represent anything you like.
     * @return {@code true} if this object is now in a fully released
     *         state, so that any waiting threads may attempt to acquire;
     *         and {@code false} otherwise.
     * @throws IllegalMonitorStateException if releasing would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to acquire in shared mode. This method should query if
     * the state of the object permits it to be acquired in the shared
     * mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return a negative value on failure; zero if acquisition in shared
     *         mode succeeded but no subsequent shared-mode acquire can
     *         succeed; and a positive value if acquisition in shared
     *         mode succeeded and subsequent shared-mode acquires might
     *         also succeed, in which case a subsequent waiting thread
     *         must check availability. (Support for three different
     *         return values enables this method to be used in contexts
     *         where acquires only sometimes act exclusively.)  Upon
     *         success, this object has been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */

    /**
     * 尝试以共享的方式获取锁，负值表示获取失败，0表示获取成功，但后续无法在以共享模式继续获取锁，正值表示获取成功，且后续还能以共享模式获取锁
     * @param arg
     * @return
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in shared mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *        passed to a release method, or the current state value upon
     *        entry to a condition wait.  The value is otherwise
     *        uninterpreted and can represent anything you like.
     * @return {@code true} if this release of shared mode may permit a
     *         waiting acquire (shared or exclusive) to succeed; and
     *         {@code false} otherwise
     * @throws IllegalMonitorStateException if releasing would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Returns {@code true} if synchronization is held exclusively with
     * respect to the current (calling) thread.  This method is invoked
     * upon each call to a non-waiting {@link ConditionObject} method.
     * (Waiting methods instead invoke {@link #release}.)
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}. This method is invoked
     * internally only within {@link ConditionObject} methods, so need
     * not be defined if conditions are not used.
     *
     * @return {@code true} if synchronization is held exclusively;
     *         {@code false} otherwise
     * @throws UnsupportedOperationException if conditions are not supported
     */
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    /**
     * acquire方法会阻塞直到成功获取锁
     */
    public final void acquire(int arg) {
        //先通过tryAcquire尝试获取锁，如果获取成功，则上锁成功
        //否则将在AQS维护的队列（链表）中，添加一个新的节点Node，AQS会在需要时从链表中选择一个结点唤醒
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    /**
     * Acquires in exclusive mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking
     * at least once {@link #tryAcquire}, returning on
     * success.  Otherwise the thread is queued, possibly repeatedly
     * blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted.  This method can be
     * used to implement method {@link Lock#lockInterruptibly}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    /**
     * 可响应中断的获取锁
     * @param arg
     * @throws InterruptedException 如果等待过程中线程被中断，则抛出异常
     */
    public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

    /**
     * Attempts to acquire in exclusive mode, aborting if interrupted,
     * and failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquire}, returning on success.  Otherwise, the thread is
     * queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success or the thread is interrupted
     * or the timeout elapses.  This method can be used to implement
     * method {@link Lock#tryLock(long, TimeUnit)}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */

    /**
     * 在指定实现内尝试获取锁
     * @param arg
     * @param nanosTimeout
     * @return 是否获取成功
     * @throws InterruptedException
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }

    /**
     * 释放锁
     */
    public final boolean release(int arg) {
        //尝试释放
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                //唤醒后继节点
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    /**
     * Acquires in shared mode, ignoring interrupts.  Implemented by
     * first invoking at least once {@link #tryAcquireShared},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquireShared} until success.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquireShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     */
    /**
     * 以共享方式获取锁
     * @param arg
     */
    public final void acquireShared(int arg) {
        //尝试获取锁
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

    //可中断的以共享方式获取锁
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        //尝试以共享方式获取锁，如果返回值小于0，说明获取失败
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

    /**
     * Attempts to acquire in shared mode, aborting if interrupted, and
     * failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquireShared}, returning on success.  Otherwise, the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted or the timeout elapses.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquireShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 ||
            doAcquireSharedNanos(arg, nanosTimeout);
    }

    /**
     * Releases in shared mode.  Implemented by unblocking one or more
     * threads if {@link #tryReleaseShared} returns true.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryReleaseShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    /**
     * 共享模式释放锁
     * @param arg
     * @return
     */
    public final boolean releaseShared(int arg) {
        //先尝试以共享模式释放锁
        if (tryReleaseShared(arg)) {
            //如果成功，则唤醒等待节点
            doReleaseShared();
            return true;
        }
        return false;
    }

    // Queue inspection methods

    /**
     * Queries whether any threads are waiting to acquire. Note that
     * because cancellations due to interrupts and timeouts may occur
     * at any time, a {@code true} return does not guarantee that any
     * other thread will ever acquire.
     *
     * <p>In this implementation, this operation returns in
     * constant time.
     *
     * @return {@code true} if there may be other threads waiting to acquire
     */
    public final boolean hasQueuedThreads() {
        return head != tail;
    }

    /**
     * Queries whether any threads have ever contended to acquire this
     * synchronizer; that is if an acquire method has ever blocked.
     *
     * <p>In this implementation, this operation returns in
     * constant time.
     *
     * @return {@code true} if there has ever been contention
     */
    public final boolean hasContended() {
        return head != null;
    }

    /**
     * Returns the first (longest-waiting) thread in the queue, or
     * {@code null} if no threads are currently queued.
     *
     * <p>In this implementation, this operation normally returns in
     * constant time, but may iterate upon contention if other threads are
     * concurrently modifying the queue.
     *
     * @return the first (longest-waiting) thread in the queue, or
     *         {@code null} if no threads are currently queued
     */
    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }

    /**
     * Version of getFirstQueuedThread called when fastpath fails
     */
    private Thread fullGetFirstQueuedThread() {
        /*
         * The first node is normally head.next. Try to get its
         * thread field, ensuring consistent reads: If thread
         * field is nulled out or s.prev is no longer head, then
         * some other thread(s) concurrently performed setHead in
         * between some of our reads. We try this twice before
         * resorting to traversal.
         */
        Node h, s;
        Thread st;
        if (((h = head) != null && (s = h.next) != null &&
             s.prev == head && (st = s.thread) != null) ||
            ((h = head) != null && (s = h.next) != null &&
             s.prev == head && (st = s.thread) != null))
            return st;

        /*
         * Head's next field might not have been set yet, or may have
         * been unset after setHead. So we must check to see if tail
         * is actually first node. If not, we continue on, safely
         * traversing from tail back to head to find first,
         * guaranteeing termination.
         */

        Node t = tail;
        Thread firstThread = null;
        while (t != null && t != head) {
            Thread tt = t.thread;
            if (tt != null)
                firstThread = tt;
            t = t.prev;
        }
        return firstThread;
    }

    /**
     * Returns true if the given thread is currently queued.
     *
     * <p>This implementation traverses the queue to determine
     * presence of the given thread.
     *
     * @param thread the thread
     * @return {@code true} if the given thread is on the queue
     * @throws NullPointerException if the thread is null
     */
    public final boolean isQueued(Thread thread) {
        if (thread == null)
            throw new NullPointerException();
        for (Node p = tail; p != null; p = p.prev)
            if (p.thread == thread)
                return true;
        return false;
    }

    /**
     * Returns {@code true} if the apparent first queued thread, if one
     * exists, is waiting in exclusive mode.  If this method returns
     * {@code true}, and the current thread is attempting to acquire in
     * shared mode (that is, this method is invoked from {@link
     * #tryAcquireShared}) then it is guaranteed that the current thread
     * is not the first queued thread.  Used only as a heuristic in
     * ReentrantReadWriteLock.
     */
    /**
     * 队列中的第一个节点是否为独占模式的节点
     * @return
     */
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null &&
            (s = h.next)  != null &&
            !s.isShared()         &&
            s.thread != null;
    }

    /**
     * 判断该线程之前是否有其它线程在等待
     */
    public final boolean hasQueuedPredecessors() {
        // 头尾节点不相等 且 头节点的下一个节点为空或下一节点并非当前线程 则说明该线程之前有其他线程在等待
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }


    // Instrumentation and monitoring methods

    /**
     * Returns an estimate of the number of threads waiting to
     * acquire.  The value is only an estimate because the number of
     * threads may change dynamically while this method traverses
     * internal data structures.  This method is designed for use in
     * monitoring system state, not for synchronization
     * control.
     *
     * @return the estimated number of threads waiting to acquire
     */
    public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null)
                ++n;
        }
        return n;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in exclusive mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to an exclusive acquire.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (!p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in shared mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to a shared acquire.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a string identifying this synchronizer, as well as its state.
     * The state, in brackets, includes the String {@code "State ="}
     * followed by the current value of {@link #getState}, and either
     * {@code "nonempty"} or {@code "empty"} depending on whether the
     * queue is empty.
     *
     * @return a string identifying this synchronizer, as well as its state
     */
    public String toString() {
        int s = getState();
        String q  = hasQueuedThreads() ? "non" : "";
        return super.toString() +
            "[State = " + s + ", " + q + "empty queue]";
    }


    // Internal support methods for Conditions

    /**
     * 返回节点是否在阻塞队列中
     */
    final boolean isOnSyncQueue(Node node) {
        //如果waitStuats == CONDITION 说明在等待队列
        //如果不存在前继节点，说明不可能在阻塞队列
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        //如果存在后继节点，说名字阻塞队列
        if (node.next != null) // If has successor, it must be on queue
            return true;
        //否则遍历AQS的阻塞队列查找
        return findNodeFromTail(node);
    }

    /**
     * 从尾部开始查找node是否在AQS的队列中
     */
    private boolean findNodeFromTail(Node node) {
        //从后AQS的阻塞队列尾部开始查找Node
        Node t = tail;
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }

    /**
     * 将节点从Condition的等待队列移动到AQS的阻塞队列（该方法只会在signal相关的方法中被调用）
     */
    final boolean transferForSignal(Node node) {
        //CAS操作更新节点的waitStatus，期待值为CONDITION
        //操作成功，说明没有其他线程在操作这个节点
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) //CAS操作失败，说明节点已经被中断操作过，waitStatus已经变成了0
            return false;

        //这里说明CAS操作成功
        //应该是线程安全的
        //将节点添加到阻塞队列的队尾
        Node p = enq(node);
        //判断此时阻塞队列是否有前继节点等待，有就Park线程，等待前继节点唤醒
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

    /**
     * Transfers node, if necessary, to sync queue after a cancelled wait.
     * Returns true if thread was cancelled before being signalled.
     *
     * @param node the node
     * @return true if cancelled before the node was signalled
     */
    /**
     * 判断转移是否发生在取消信号之后（此时线程一定是中断的，只是需要判断中断信号和SIGNAL先后顺序）
     * @param node
     * @return
     */
    final boolean transferAfterCancelledWait(Node node) {
        //CAS操作，期待值是CONDITION，说明此时唤醒是被取消（中断），因为如果是SIGNAL，那么waitingStatus不会CONDITION，而是0（可以见SIGNAL流程）
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            //即使是取消的，也需要移到AQS的阻塞队列
            enq(node);
            return true;
        }

        //说明线程先收到了SIGNAL信号
        //此时要等SIGNAL信号处理完成
        while (!isOnSyncQueue(node))
            Thread.yield();
        return false;
    }

    /**
     * 完全释放锁，不管重入了几次，释放干净
     * 注意传入的NODE是ConditionObject维护队列的节点，非AQS阻塞队列的节点
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            //获取锁被持有的此时
            int savedState = getState();
            //让锁直接释放被持有的次数
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            //如果释放失败了，则将节点标记为CANCELLED
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }

    // Instrumentation methods for conditions

    /**
     * Queries whether the given ConditionObject
     * uses this synchronizer as its lock.
     *
     * @param condition the condition
     * @return {@code true} if owned
     * @throws NullPointerException if the condition is null
     */
    public final boolean owns(ConditionObject condition) {
        return condition.isOwnedBy(this);
    }

    /**
     * Queries whether any threads are waiting on the given condition
     * associated with this synchronizer. Note that because timeouts
     * and interrupts may occur at any time, a {@code true} return
     * does not guarantee that a future {@code signal} will awaken
     * any threads.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @param condition the condition
     * @return {@code true} if there are any waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    public final boolean hasWaiters(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.hasWaiters();
    }

    /**
     * Returns an estimate of the number of threads waiting on the
     * given condition associated with this synchronizer. Note that
     * because timeouts and interrupts may occur at any time, the
     * estimate serves only as an upper bound on the actual number of
     * waiters.  This method is designed for use in monitoring of the
     * system state, not for synchronization control.
     *
     * @param condition the condition
     * @return the estimated number of waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    public final int getWaitQueueLength(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitQueueLength();
    }

    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this
     * synchronizer.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate. The elements of the
     * returned collection are in no particular order.
     *
     * @param condition the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitingThreads();
    }

    /**
     * Condition implementation for a {@link
     * AbstractQueuedSynchronizer} serving as the basis of a {@link
     * Lock} implementation.
     *
     * <p>Method documentation for this class describes mechanics,
     * not behavioral specifications from the point of view of Lock
     * and Condition users. Exported versions of this class will in
     * general need to be accompanied by documentation describing
     * condition semantics that rely on those of the associated
     * {@code AbstractQueuedSynchronizer}.
     *
     * <p>This class is Serializable, but all fields are transient,
     * so deserialized conditions have no waiters.
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
        //该ConditionObject维护的等待队列的头节点
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        //该ConditionObject维护的等待队列的尾节点
        private transient Node lastWaiter;

        /**
         * Creates a new {@code ConditionObject} instance.
         */
        public ConditionObject() { }

        // Internal methods

        /**
         * 将线程包装成节点，并添加到Condition的等待队列
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;

            // 找出该ConditionObject的等待队列中 真正未被取消的最后一个节点，并更新为lastWaiter
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }

            //将该线程包装成Node
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            //如果此时ConditionObject队列为空，初始化链表且头节点为node
            if (t == null)
                firstWaiter = node;
            else //否则将node添加队尾
                t.nextWaiter = node;
            //更新链表尾节点为node
            lastWaiter = node;
            return node;
        }

        /**
         * 唤醒操作（此时占有锁，为线程安全），
         * 找出等待队列中第一个真正要被唤醒的节点，移动到阻塞队列，
         */
        private void doSignal(Node first) {
            do {
                //找出第一个需要唤醒的节点
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                //断开和之后等待队列的联系，并更新等待队列的头节点
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null); //等待队列的头节点移动失败 且 等待队列中还有其他节点，则继续尝试其他节点
        }

        /**
         * Removes and transfers all nodes.
         * @param first (non-null) the first node on condition queue
         */
        //唤醒所有等待的节点
        //其实就是将此时等待队列中所有的节点都移动到阻塞队列
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }

        /**
         * Unlinks cancelled waiter nodes from condition queue.
         * Called only while holding lock. This is called when
         * cancellation occurred during condition wait, and upon
         * insertion of a new waiter when lastWaiter is seen to have
         * been cancelled. This method is needed to avoid garbage
         * retention in the absence of signals. So even though it may
         * require a full traversal, it comes into play only when
         * timeouts or cancellations occur in the absence of
         * signals. It traverses all nodes rather than stopping at a
         * particular target to unlink all pointers to garbage nodes
         * without requiring many re-traversals during cancellation
         * storms.
         */
        //此方法并不保证线程安全，因此调用此方法时，必须要在获取锁的情况下调用
        //此方法的目的是为了整理Condition的等待队列，将非CONDITION状态的节点从等待队列中移除
        private void unlinkCancelledWaiters() {
            Node t = firstWaiter;
            Node trail = null;
            while (t != null) {
                Node next = t.nextWaiter;
                if (t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if (trail == null)
                        firstWaiter = next;
                    else
                        trail.nextWaiter = next;
                    if (next == null)
                        lastWaiter = trail;
                }
                else
                    trail = t;
                t = next;
            }
        }

        // public methods

        /**
         * 唤醒等待节点
         * 主要的流程就是将节点从Condition的等待队列移到AQS的阻塞队列中，让其重新等待锁的获取
         */
        public final void signal() {
            //先验证唤醒者是否是锁的持有者
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            //唤醒等待队列的第一个节点
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);
        }

        /**
         * 唤醒所有等待的线程（重新移到阻塞队列）
         */
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }

        /**
         * 和await不同的一点是，即便等待过程中线程被中断，也不会抛出INTERRUPT EXCEPTION
         */
        public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            //如果在等待或获取锁是发生了中断，并不抛出异常，只是保留线程的中断状态，可以在调用出测试
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

        /*
         * For interruptible waits, we need to track whether to throw
         * InterruptedException, if interrupted while blocked on
         * condition, versus reinterrupt current thread, if
         * interrupted while blocked waiting to re-acquire.
         */

        //常量，用来标记节点的interrupt mode

        /** Mode meaning to reinterrupt on exit from wait */
        private static final int REINTERRUPT =  1;
        /** Mode meaning to throw InterruptedException on exit from wait */
        private static final int THROW_IE    = -1;

        /**
         * 检查线程在WAITING状态期间，是否有被中断
         * 如果没有返回0；如果是在SIGNAL之前被中断，返回 THROW_IE；如果在SIGNAL之后被中断，则返回REINTERRUPT
         */
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ?
                (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                0;
        }

        /**
         * Throws InterruptedException, reinterrupts current thread, or
         * does nothing, depending on mode.
         */
        private void reportInterruptAfterWait(int interruptMode)
            throws InterruptedException {
            if (interruptMode == THROW_IE)
                throw new InterruptedException();
            else if (interruptMode == REINTERRUPT)
                selfInterrupt();
        }

        /**
         * 让线程进入等待
         */
        public final void await() throws InterruptedException {
            //如果线程已经被标记为中断，则抛出异常
            if (Thread.interrupted())
                throw new InterruptedException();
            //将线程添加进等待队列
            //注意等待队列和AQS维护的阻塞队列是两个不同的队列
            //正常流程当线程能调用await()，说明线程此时拥有锁，此时AQS的阻塞队列中，线程应该在head节点
            Node node = addConditionWaiter();

            //释放掉锁（如果释放失败，NODE的waitStatus被更新为CANCELLED）
            //同时因为释放掉了锁，该线程在阻塞队列中的节点也已经被移除
            int savedState = fullyRelease(node);

            //这里会将线程挂起，除非线程节点被移到AQS的阻塞队列或是线程被外部中断
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                //检查是否是由于被中断而唤醒，如果是，则跳出循环
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            //在阻塞队列中尝试获取锁
            //savedState需要在线程重新获取锁的时候，重新回到之前持有锁的次数
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            //节点已经在阻塞队列中，与Condition的等待队列联系断开
            //对于SIGNAL唤醒的线程而言，SIGNAL时除了将节点移到阻塞队列，同时也清空了node.nextWaiter
            //而对于中断唤醒的线程而言，只是将节点移到阻塞队列，并没有清空node.nextWaiter（因为此时线程不持有，操作等待线程并非线程安全）
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            //根据interruptMode 决定是否需要抛出异常
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

        /**
         * Implements timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        //带超时时间的等待
        public final long awaitNanos(long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();

            /*************正常情况下，这里线程占有锁************/
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);

            /*************此时已经不再占有锁*******************/
            //确定重试截止时间
            final long deadline = System.nanoTime() + nanosTimeout;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                //如果已经超时
                if (nanosTimeout <= 0L) {
                    //尝试将节点移动至阻塞队列，或是有其他线程也在移动节点，等其他线程移动完成
                    //因为此时等待已经超时，因此不再关心超时后是否发生中断
                    transferAfterCancelledWait(node);
                    break;
                }
                //线程被Park，直到在超时前一点的时间点
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                //如果被唤醒，检查是由于SIGNAL还是INTERRUPT
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break; //如果是由于中断，则跳出
                //重新计算超时时间
                nanosTimeout = deadline - System.nanoTime();
            }

            /************线程会在这里等待，直到重新占有锁***********/
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            //对于中断而移至阻塞队列的节点，会在这里清理与等待队列的联系
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            //检查是否需要抛出中断异常
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return deadline - System.nanoTime();
        }

        /**
         * Implements absolute timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        public final boolean awaitUntil(Date deadline)
                throws InterruptedException {
            long abstime = deadline.getTime();
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (System.currentTimeMillis() > abstime) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkUntil(this, abstime);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        /**
         * 和awaitNanos流程类似，只是这个方法可以指定时间的单位，并且返回值为是否在规定时间内唤醒
         */
        public final boolean await(long time, TimeUnit unit)
                throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    //这里需要确认是先超时还是先因为超时而被CANCELLED，返回true说明先超时
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                //这里是处理中断的情况
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        //  support for instrumentation

        /**
         * 判断Condition是否为给定的AQS持有
         */
        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }

        /**
         * 判断Condtion是否有等待的线程
         */
        protected final boolean hasWaiters() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }

        /**
         * 获取Condition等待队列的长度
         */
        protected final int getWaitQueueLength() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }

        /**
         * 获取Condition等待队列的线程
         */
        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    }

    /******************CAS操作相关**********************/

    /**
     * Setup to support compareAndSet. We need to natively implement
     * this here: For the sake of permitting future enhancements, we
     * cannot explicitly subclass AtomicInteger, which would be
     * efficient and useful otherwise. So, as the lesser of evils, we
     * natively implement using hotspot intrinsics API. And while we
     * are at it, we do the same for other CASable fields (which could
     * otherwise be done with atomic field updaters).
     */
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long stateOffset;
    private static final long headOffset;
    private static final long tailOffset;
    private static final long waitStatusOffset;
    private static final long nextOffset;

    static {
        try {
            stateOffset = unsafe.objectFieldOffset
                (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
            headOffset = unsafe.objectFieldOffset
                (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset
                (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset
                (Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset
                (Node.class.getDeclaredField("next"));

        } catch (Exception ex) { throw new Error(ex); }
    }

    /**
     * CAS head field. Used only by enq.
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    /**
     * CAS tail field. Used only by enq.
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    /**
     * CAS waitStatus field of a node.
     */
    private static final boolean compareAndSetWaitStatus(Node node,
                                                         int expect,
                                                         int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset,
                                        expect, update);
    }

    /**
     * CAS next field of a node.
     */
    private static final boolean compareAndSetNext(Node node,
                                                   Node expect,
                                                   Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
    }
}