/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on first-in-first-out
 * (FIFO) wait queues. 提供了一个实现阻塞锁和依赖FIFO的等待队列的相关的同步器（信号灯、事件等）框架

 * 
 * 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.
 * 这个类对于大多数使用一个单独原子类的int值来表示状态的同步器很有用
 * 
 * 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. 子类必须定义protected方法来改变这个状态值，并且定义状态值是获取还是释放对象
 * 
 * Given these, the other methods in this class carry out all queuing and
 * blocking mechanics. 鉴于此，这个类中的其他方法实现了所有排队和阻塞的机制

 * 
 * 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. 子类可以维护其他的状态值字段，但只有getState、setState和compareAndSetState
 * 方法是通过原子更新来实现同步的
 * 
 * <p>
 * Subclasses should be defined as non-public internal helper classes that are
 * used to implement the synchronization properties of their enclosing class.
 * 子类应该定义成非public的内部helper工具类，用于实现其封闭类的同步属性

 * 
 * Class {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface. AbstractQueuedSynchronizer类没有实现任何同步接口

 * 
 * 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.
 * 取而代之的是，它定义了像acquireInterruptibly这样的方法，通过调用恰当的具体 锁和相关同步器方法，以便实现他们自己的公共方法

 * 
 * <p>
 * This class supports either or both a default <em>exclusive</em> mode and a
 * <em>shared</em> mode. 这个类既支持默认的独占模式，也支持共享模式，也支持两种模式一起实现

 * 
 * When acquired in exclusive mode, attempted acquires by other threads cannot
 * succeed. 当在独占模式获取到锁时，其他线程再尝试获取锁会失败

 * 
 * Shared mode acquires by multiple threads may (but need not) succeed.
 * 共享模式，多个线程都能成功获取到锁

 * 
 * This class does not understand 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.
 * 这个类不会理解机制的不同，共享模式中的一个线程获取锁成功了，下一个线程 （如果存在）仍然会去确定它自己是否也可以获取
 * 
 * Threads waiting in the different modes share the same FIFO queue.
 * 线程虽在不同的模式中，却都在等待共享相同的FIFO队列
 * 
 * Usually, implementation subclasses support only one of these modes, but both
 * can come into play for example in a {@link ReadWriteLock}.
 * 通常，子类只需要实现这两种模式中的一种，但也能两种都实现，例如ReadWriteLock
 * 
 * Subclasses that support only exclusive or only shared modes need not define
 * the methods supporting the unused mode. 仅支持一种模式的子类，不必定义另一种模式下的方法

 * 
 * <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.
 * 这个类定义了一个嵌套的ConditionObject类，该类可以被支持独占模式的子类用作
 * Condition实现，为此，isHeldExclusively()报告当前线程是否持续保持同步，

 * release方法通过调用getState来完全释放当前对象，并且将当前的资源状态 再保存到state中，最后会将此对象恢复为先前的获取状态

 * 
 * No {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.
 * 没有AbstractQueuedSynchronizer方法去创建condition，因此如果不能满足 这个约束，就不要使用它

 * 
 * The behavior of {@link ConditionObject} depends of course on the semantics of
 * its synchronizer implementation. ConditionObject的行为依赖于其同步器实现的语义

 * 
 * <p>
 * This class provides inspection, instrumentation, and monitoring methods for
 * the internal queue, as well as similar methods for condition objects.
 * 这个类提供检查、追踪和监控内部队列的方法，类似于condition对象的方法

 * 
 * These can be exported as desired into classes using an
 * {@code AbstractQueuedSynchronizer} for their synchronization mechanics.
 * 可以根据需要使用AbstractQueuedSynchronizer，将它们导入到类中以实现其同步机制

 * 
 * <p>
 * Serialization of this class stores only the underlying atomic integer
 * maintaining state, so deserialized objects have empty thread queues.
 * 这个类仅序列化state的原子值，因此反序列化出来的对象中的线程队列是空的
 * 
 * Typical subclasses requiring serializability will define a {@code readObject}
 * method that restores this to a known initial state upon deserialization.
 * 需要序列化的子类可以在反序列化的时候定义一个readObject方法来恢复已知的初始状态

 * 
 * 
 * <h3>Usage</h3> 使用
 * 
 * <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}: 使用这个类作为同步器锁，需要重新定义以下方法：
 * 
 * <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}.
 * 这些方法默认抛出UnsupportedOperationException异常
 * 
 * Implementations of these methods must be internally thread-safe, and should
 * in general be short and not block. 这些方法的实现必须在内部是线程安全的，而且通常都很简短，没有阻塞
 * 
 * Defining these methods is the <em>only</em> supported means of using this
 * class. 定义这些方法是使用这个类唯一可行的方式

 * 
 * All other methods are declared {@code final} because they cannot be
 * independently varied. 所有其他的方法都被声明为final，因为他们无法独自变化

 * 
 * <p>
 * You may also find the inherited methods from
 * {@link AbstractOwnableSynchronizer} useful to keep track of the thread owning
 * an exclusive synchronizer.
 * 你可能也发现了继承自AbstractOwnableSynchronizer的方法对于跟踪拥有独占同步器的线程很有用
 * 
 * You are encouraged to use them -- this enables monitoring and diagnostic
 * tools to assist users in determining which threads hold locks.
 * 鼓励你使用它们——这使得监控和诊断工具能够帮助用户确定那些线程持有锁

 * 
 * <p>
 * Even though this class is based on an internal FIFO queue, it does not
 * automatically enforce FIFO acquisition policies.
 * 即使这个类是基于一个内部的FIFO队列，它也不会自动执行FIFO获得策略
 * 
 * The core of exclusive synchronization takes the form: 独占锁的核心采用以下形式：

 * 
 * <pre>
 * Acquire方法:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        使线程入队，如果它还没有在队列中的话
 *        <em>possibly block current thread</em>;
 *        可能会阻塞当前线程

 *     }
 * 
 * Release方法:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 *        解锁队列中的第一个线程

 * </pre>
 * 
 * (Shared mode is similar but may involve cascading signals.) 共享模式类似，但可能涉及级联信号
 *
 * <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.
 * 因为进入队列之前检查锁的获取，因此一个新的线程可能会插入其他阻塞或排队的线程之前
 * 
 * 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. 但如果你愿意的话，可以定义tryAcquire和/或tryAcquireShared方法禁止插队，从而提供

 * 一个公平的获取顺序
 * 
 * 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}.
 * 尤其是，如果hasQueuedPredecessors（专用于公平锁的方法）返回true，大多数公平锁 可以定义tryAcquire方法返回false
 * 
 * Other variations are possible. 其他变化也是可能的

 * 
 * <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.
 * 对于默认插入（也称为greedy，renouncement和convoy-avoidance）策略， 吞吐量和可扩展性通常是最高的
 * 
 * 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.
 * 尽管这不能保证公平，也不能保证没有饥饿，但是可以让较早排队的线程在较 晚排队的线程之前进行重新竞争
 * 
 * Also, while acquires do not spin in the usual sense, they may perform
 * multiple invocations of {@code tryAcquire} interspersed with other
 * computations before blocking.
 * 同样，尽管获得锁通常不会自旋，但它们在阻塞之前，可以执行多个对tryAcquire的调用与其他阻塞前的计算
 * 
 * This gives most of the benefits of spins when exclusive synchronization is
 * only briefly held, without most of the liabilities when it isn't.
 * 这提供了自旋的大部分好处，而在不进行排他同步时，也不会带来很多负担
 * 
 * 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. 如果需要，你可以通过在调用之前对获取方法进行“快速路径”检查来增强此功能，
 * 可能会预先检查hasContended和/或hasQueuedThreads
 * 
 * <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. 此类为同步提供了有效且可扩展的基础，部分原因是依赖于使用state，获取和释放参数 以及内部FIFO等待队列的同步器
 * 
 * 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.
 * 如果这不够，你可以使用原子类、实现Queue接口和LockSupport提供低级别的阻塞支持
 * 
 * <h3>Usage Examples</h3> 使用示例
 * 
 * <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.
 * 这是一个非重入互斥独占锁类，使用0表示非锁定状态，1表示锁定状态

 * 
 * 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.
 * 而非重入锁并不严格要求记录当前所有者线程，无论如何，这样做是为了更易于使用
 * 
 * It also supports conditions and exposes one of the instrumentation methods:
 * 它也支持conditions并公开了一种检测方法：
 * 
 * <pre>
 *  {@code
 * class Mutex implements Lock, java.io.Serializable {
 * 
 *   // Our internal helper class
 *   // 内部helper类

 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Reports whether in locked state
 *     // 是否持有锁

 *     protected boolean isHeldExclusively() {
 *       return getState() == 1;
 *     }
 * 
 *     // Acquires the lock if state is zero
 *     // 如果state是0就获得锁
 *     public boolean tryAcquire(int acquires) {
 *       assert acquires == 1; // Otherwise unused 断言acquires=1，否则退出

 *       if (compareAndSetState(0, 1)) {
 *         setExclusiveOwnerThread(Thread.currentThread());
 *         return true;
 *       }
 *       return false;
 *     }
 * 
 *     // Releases the lock by setting state to zero
 *     // 通过设置state=0来释放锁
 *     protected boolean tryRelease(int releases) {
 *       assert releases == 1; // Otherwise unused 断言acquires=1，否则退出

 *       if (getState() == 0) throw new IllegalMonitorStateException();
 *       setExclusiveOwnerThread(null);
 *       setState(0);
 *       return true;
 *     }
 * 
 *     // Provides a Condition
 *     Condition newCondition() {
 *         return new ConditionObject();
 *     }
 * 
 *     // Deserializes properly
 *     // 反序列化
 *     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.
 *   // 同步对象完成了所有困难的工作，我们只需要利用它实现下面的方法

 * 
 *   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.
 * 这是一个和CountDownLatch类很像的latch类，除了它仅仅需要一个获取信号启动外
 * 
 * Because a latch is non-exclusive, it uses the {@code shared} acquire and
 * release methods. 因为latch类是一个非独占锁，它使用共享的获取和释放方法

 * 
 * <pre>
 * {
 *     &#64;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.
     */
    /**
     * 用0初始化state同步状态，创建一个新的AbstractQueuedSynchronizer实例
     */
    protected AbstractQueuedSynchronizer() {
    }

    /**
     * Wait queue node class. 等待队列的Node类

     * 
     * <p>
     * The wait queue is a variant of a "CLH" (Craig, Landin, and Hagersten) lock
     * queue. 等待队列是CLH锁队列的变体
     * 
     * CLH locks are normally used for spinlocks. CLH锁通常用于自旋锁

     * 
     * 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. 我们将用他们用于阻塞同步器，但使用相同的基本策略， 将有关线程的某些控制信息保存在其节点的前继节点中
     * 
     * A "status" field in each node keeps track of whether a thread should block.
     * 每个节点中的“status”字段都保持线程是否应该阻塞的状态

     * 
     * A node is signalled when its predecessor releases. 当节点的前继释放时，会给当前节点发信号

     * 
     * 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.
     * 插入到CLH队列中只需要对tail执行一次原子操作，因此存在一个简单的原子分界点，即从未排队到排队
     * 同样，出队仅涉及更新head。但是，节点需要花费更多的精力来确定其后继者是谁，
     * 部分原因是要处理由于超时和中断而可能导致的取消
     * 
     * <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 {
        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;
        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED = 1;
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should unconditionally
         * propagate
         */
        static final int PROPAGATE = -3;

        /**
         * Status field, taking on only the values: 
         * 
         * SIGNAL: The successor of this node is (or will soon be) blocked (via park), 
         * so the current node must unpark its successor when it releases or cancels.
         * To avoid races, acquire methods must first indicate they need a signal, 
         * then retry the atomic acquire, and then, on failure, block.
         * 值为-1，表示当前节点的的后继节点将要或者已经被阻塞，在当前节点释放的时候需要unpark（唤醒）后继节点
         * 
         * CANCELLED: This node is cancelled due to timeout or interrupt. Nodes never 
         * leave this state. In particular, a thread with cancelled node never again blocks.
         * 值为1，表示当前节点被取消
         * 
         * CONDITION: This node is currently on a condition queue. It will not be used 
         * as a sync queue node until transferred, at which time the status will be set to 0.
         * (Use of this value here has nothing to do with the other uses of the field, 
         * but simplifies mechanics.)
         * 值为-2，表示当前节点在等待condition，即在condition队列中

         * 
         * PROPAGATE: A releaseShared should be propagated to other nodes. This is set
         * (for head node only) in doReleaseShared to ensure propagation continues, even
         * if other operations have since intervened. 0: None of the above
         * 值为-3，表示releaseShared需要被传播给后续节点（仅在共享模式下使用）
         * 
         * The values are arranged numerically to simplify use. Non-negative values mean
         * that a node doesn't need to signal. So, most code doesn't need to check for
         * particular values, just for sign.
         * 
         * The field is initialized to 0 for normal sync nodes, and CONDITION for
         * condition nodes. It is modified using CAS (or when possible, unconditional
         * volatile writes).
         * 无状态，表示当前节点在队列中等待获取锁

         * 
         */
        volatile int waitStatus;

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

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

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

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

        /**
         * Returns true if node is waiting in shared mode.
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * Returns previous node, or throws NullPointerException if null. Use when
         * predecessor cannot be null. The null check could be elided, but is present to
         * help the VM.
         * 返回前继节点，如果为空则抛出异常
         * 
         * @return the predecessor of this node
         */
        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;
        }
    }

    /**
     * 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. 等待队列头部节点，懒加载，它仅仅通过setHead方法修改
     * 注意：如果头部节点存在，它的等待状态不保证会是CANCELLED
     */
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized. Modified only via method enq to
     * add new wait node. 等待队列的队尾节点，懒加载，只能通过enq方法加载新节点到队尾
     */
    private transient volatile Node tail;

    /**
     * The synchronization state. 同步状态

     * 该变量对不同的子类实现具有不同的意义
     * 对ReentrantLock来说，它表示加锁的状态：
     * 无锁时state=0，有锁时state>0
     * 第一次加锁时，将state+1
     * 而对于CountDownLatch来说，它是初始化时子线程的数量

     * 
     */
    private volatile int state;

    /**
     * Returns the current value of synchronization state. This operation has memory
     * semantics of a {@code volatile} read.
     * 
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state. This operation has memory semantics
     * of a {@code volatile} write.
     * 
     * @param newState the new state value
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Atomically sets synchronization state to the given updated value if the
     * current state value equals the expected value. This operation has memory
     * semantics of a {@code volatile} read and write. 以原子方式设置同步状态为指定的值

     * 
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
    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. 自旋超时时间，使用比park更快的纳秒，足以在非常短的时间内提高响应能力，默认值1000纳秒
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * 插入节点到队尾，如果有必要的话初始化
     * 
     * @param node the node to insert
     * @return node's predecessor
     */
    private Node enq(final Node node) {
        // 自旋
        for (;;) {
            // 将队尾指针给当前节点
            Node t = tail;
            if (t == null) { // Must initialize 必须初始化

                // 如果尾节点为null，说明队列还没有任何节点，那么头节点也就是尾节点
                if (compareAndSetHead(new Node())) {
                    tail = head;
                }
            } else {
                // 否则尾节点成为当前待加入节点的前继节点
                node.prev = t;
                // 将当前节点设置为尾节点
                if (compareAndSetTail(t, node)) {
                    // 尾节点的后续节点为当前节点
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     * 按给定模式将当前线程包装成一个入队的节点
     * 
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        // 将当前线程包装成节点
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        // 尝试快速入队
        Node pred = tail;
        // 尾节点是否为null
        if (pred != null) {
            // 将尾节点设置为当前节点的前继节点
            node.prev = pred;
            // 将当前节点设置为尾节点
            if (compareAndSetTail(pred, node)) {
                // 尾节点的后续节点为当前节点
                pred.next = node;
                return node;
            }
        }
        // 尾节点为null，则执行enq
        enq(node);
        // 返回当前节点
        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.
     * 将节点设置为队列头，从而让持有锁的节点出列，仅由acquire调用
     * 为了GC和抑制不必要的信号和遍历，也会清空未使用的字段

     * 
     * @param node the node
     */
    private void setHead(Node node) {
        // 将节点设置为队列头
        head = node;
        // 头节点没有线程
        node.thread = null;
        // 头节点没有前继节点
        node.prev = null;
    }

    /**
     * Wakes up node's successor, if one exists.
     * 唤醒节点的后续节点

     * 
     * @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.
         * 如果状态值为负，就尝试清除预期信号值

         * 如果失败或状态由等待线程更改，则OK
         */
        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);
        }
    }

    /**
     * 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.)
     * 共享模式下的释放行为——发出后续信号并确保传播

     * （注意：对于独占模式，释放只是在需要信号时调用head的unparkSuccessor方法）

     * 
     */
    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;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) {
                        continue; // loop to recheck cases
                    }
                    unparkSuccessor(h);
                } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) {
                    continue; // loop on failed CAS
                }
            }
            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
        setHead(node);
        /*
         * Try to signal next queued node if: Propagation was indicated by caller, or
         * was recorded (as h.waitStatus either before or after setHead) by a previous
         * operation (note: this uses sign-check of waitStatus because PROPAGATE status
         * may transition to SIGNAL.) and The next node is waiting in shared mode, or we
         * don't know, because it appears null
         * 
         * The conservatism in both of these checks may cause unnecessary wake-ups, but
         * only when there are multiple racing acquires/releases, so most need signals
         * now or soon anyway.
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared()) {
                doReleaseShared();
            }
        }
    }

    // Utilities for various versions of acquire

    /**
     * Cancels an ongoing attempt to acquire.
     * 取消一个不断尝试获取锁的线程节点

     * 
     * @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;
        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.
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            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 {
                unparkSuccessor(node);
            }

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

    /**
     * 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.
     * 节点获取锁失败时检查并且更新状态值，如果线程应该阻塞返回true
     * 在所有获取锁的循环中这是主要的信号控制

     *
     * @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) {
            /*
             * 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);
        }

        return false;
    }

    /**
     * Convenience method to interrupt current thread.
     * 中断当前线程的快捷方法

     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /**
     * Convenience method to park and then check if interrupted
     * 
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        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.
     * 在独占和共享模式中，获取锁有多种方式，大多数都相同

     * 由于异常机制（包括确保在tryAcquire抛出异常时取消）和其他控件的交互，

     * 性能可能会受一点影响，但至少不会造成太大的损害

     */

    /**
     * Acquires in exclusive uninterruptible mode for thread already in queue. Used
     * by condition wait methods as well as acquire.
     * 以独占不中断模式获取队列中已存在的线程。用于condition等待方法以及获取锁

     * 
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) {
                    interrupted = true;
                }
            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in exclusive interruptible mode.
     * 独占中断模式获取锁

     * 
     * @param arg the acquire argument
     */
    private void doAcquireInterruptibly(int arg) throws InterruptedException {
        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;
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in exclusive timed mode.
     * 独占超时模式获取锁

     * 
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    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) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }
                if (Thread.interrupted()) {
                    throw new InterruptedException();
                }
            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in shared uninterruptible mode.
     * 共享非中断模式获取锁
     * 
     * @param arg the acquire argument
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted) {
                            selfInterrupt();
                        }
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) {
                    interrupted = true;
                }
            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in shared interruptible mode.
     * 共享中断模式获取锁

     * 
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) {
                    throw new InterruptedException();
                }
            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in shared timed mode.
     * 共享超时模式获取锁

     * 
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
        if (nanosTimeout <= 0L) {
            return false;
        }
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L) {
                    return false;
                }
                if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }
                if (Thread.interrupted()) {
                    throw new InterruptedException();
                }
            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    // Main exported methods
    // 主要的自定义方法

    /**
     * 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.
     * 尝试以独占模式获取锁，此方法应该查询对象的状态state是否允许以独占模式获取锁，如果允许则获取锁

     * 
     * <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()}.
     * 此方法始终由执行获取锁的线程调用，如果获取失败，则会将线程放到CLH队列队尾（如果尚未排队），

     * 直到某个其他线程发出释放信号，这可用于实现接口方法tryLock
     * 
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     * 缺省实现是抛出UnsupportedOperationException异常
     * 
     * @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.
     *            获取锁的参数，表示需要获取锁的数量

     *            
     * @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.
     * 尝试将状态state设置为以独占模式释放锁

     * 
     * <p>
     * This method is always invoked by the thread performing release.
     * 此方法始终由执行释放的线程调用

     * 
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     * 缺省实现是抛出UnsupportedOperationException异常
     * 
     * @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.
     *            释放锁的参数，表示需要释放锁的数量，与tryAcquire中需要获取的数量一一对应
     *            
     * @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.
     * 共享模式尝试获取锁

     * 
     * <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.
     * 此方法始终由执行获取的线程调用，如果调用失败，则会将线程放到CLH队列队尾（如果尚未排队），

     * 直到某个其他线程发出释放信号
     * 
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     * 缺省实现是抛出UnsupportedOperationException异常
     * 
     * @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
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in shared mode.
     * 尝试将状态state设置为以共享模式释放锁

     * 
     * <p>
     * This method is always invoked by the thread performing release.
     * 此方法始终由执行获取的线程调用

     * 
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     * 缺省实现是抛出UnsupportedOperationException异常
     * 
     * @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}.)
     * 如果以独占方式保持与当前（调用）线程的同步，则返回true
     * 每次调用非等待的ConditionObject方法时都会调用此方法（等待方法改为调用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.
     * 缺省实现是抛出UnsupportedOperationException异常
     * 此方法仅在ConditionObject内部调用，因此如果不使用Condition，则无需定义
     * 
     * @return {@code true} if synchronization is held exclusively; {@code false}
     *         otherwise
     * @throws UnsupportedOperationException if conditions are not supported
     */
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    /**
     * Acquires in exclusive mode, ignoring interrupts. Implemented by invoking at
     * least once {@link #tryAcquire}, returning on success. Otherwise the thread is
     * queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success. This method can be used to implement
     * method {@link Lock#lock}.
     * 以独占模式获取锁，忽略中断，通过调用至少一次tryAcquire来实现，成功时返回，否则线程将排队

     * 可能会反复阻塞和解除阻塞，调用tryAcquire直到成功获取锁，此方法可用于实现接口方法lock
     * 
     * @param arg the acquire argument. This value is conveyed to
     *            {@link #tryAcquire} but is otherwise uninterpreted and can
     *            represent anything you like.
     */
    public final void acquire(int arg) {
        /**
         * 该方法主要做了如下工作：
         * 先看tryAcquire尝试获取独占锁是否成功，获取成功则返回

         * 否则用addWaiter方法将当前线程封装成Node对象，并添加到队列尾部

         * 自旋获取锁，并判断中断标志位
         * 如果中断标志位为true，则设置中断线程，否则返回

         */
        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}.
     * 独占模式获取锁，如果中断则取消

     * 首先检查中断状态，然后至少调用一次tryAcquire来实现方法，在成功时返回，否则线程将进入队尾
     * 可能会反复阻塞和解除阻塞，调用tryAcquire，直到成功或线程被中断

     * 此方法可用于实现接口方法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
     */
    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)}.
     * 尝试以独占模式获取锁，如果中断则中止，如果超时则失败
     * 通过首先检查中断状态，然后至少调用一次tryAcquire来实现，在成功时返回，否则线程将进入队尾
     * 可能会反复阻塞和解除阻塞，调用tryAcquire，直到成功或线程中断或超时结束

     * 此方法可用于实现接口方法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
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }

        return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout);
    }

    /**
     * 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}.
     * 独占模式时释放锁，通过解除一个或多个阻塞线程来实现，如果tryRelease返回true
     * 此方法可用于实现接口方法unlock
     * 
     * @param arg the release argument. This value is conveyed to
     *            {@link #tryRelease} but is otherwise uninterpreted and can
     *            represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0) {
                unparkSuccessor(h);
            }
            return true;
        }

        return false;
    }

    /**
     * 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.
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0) {
            doAcquireShared(arg);
        }
    }

    /**
     * Acquires in shared mode, aborting if interrupted. 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.
     * 
     * @param arg the acquire argument. This value is conveyed to
     *            {@link #tryAcquireShared} but is otherwise uninterpreted and can
     *            represent anything you like.
     * @throws InterruptedException
     *             if the current thread is interrupted
     */
    public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }
        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}
     */
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            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.
     * 查询是否有线程正在等待获取锁
     * 请注意，由于中断和超时导致的取消可能随时发生，因此返回true不能保证任何其他线程将获得锁
     * 
     * <p>
     * In this implementation, this operation returns in constant time.
     * 在该实现中，操作以指定的时间返回
     * 
     * @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.
     * 查询是否有任何线程曾争用获取此同步器，也就是说，是否某个获取锁方法曾被阻塞

     * 
     * <p>
     * In this implementation, this operation returns in constant time.
     * 在该实现中，操作以指定的时间返回
     * 
     * @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.
     */
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null;
    }

    /**
     * Queries whether any threads have been waiting to acquire longer than the
     * current thread.
     * 查询是否有任何线程等待获取锁的时间超过当前线程

     * 
     * <p>
     * An invocation of this method is equivalent to (but may be more efficient
     * than):
     * 调用此方法相当于调用：getFirstQueuedThread() != Thread.currentThread() && hasQueuedThreads()
     * 
     * <pre>
     *  {@code getFirstQueuedThread() != Thread.currentThread() && hasQueuedThreads()}
     * </pre>
     * 
     * <p>
     * Note that because cancellations due to interrupts and timeouts may occur at
     * any time, a {@code true} return does not guarantee that some other thread
     * will acquire before the current thread. Likewise, it is possible for another
     * thread to win a race to enqueue after this method has returned {@code false},
     * due to the queue being empty.
     * 
     * <p>
     * This method is designed to be used by a fair synchronizer to avoid
     * <a href="AbstractQueuedSynchronizer#barging">barging</a>. Such a
     * synchronizer's {@link #tryAcquire} method should return {@code false}, and
     * its {@link #tryAcquireShared} method should return a negative value, if this
     * method returns {@code true} (unless this is a reentrant acquire). For
     * example, the {@codetryAcquire} method for a fair, reentrant, exclusive mode 
     * synchronizer might look like this:
     * 
     * <pre>
     *  {@code
     * protected boolean tryAcquire(int arg) {
     *   if (isHeldExclusively()) {
     *     // A reentrant acquire; increment hold count
     *     return true;
     *   } else if (hasQueuedPredecessors()) {
     *     return false;
     *   } else {
     *     // try to acquire normally
     *   }
     * }}
     * </pre>
     * 
     * @return {@code true} if there is a queued thread preceding the current
     *         thread, and {@code false} if the current thread is at the head of the
     *         queue or the queue is empty
     * @since 1.7
     */
    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        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

    /**
     * Returns true if a node, always one that was initially placed on a condition
     * queue, is now waiting to reacquire on sync queue.
     * 
     * @param node the node
     * @return true if is reacquiring
     */
    final boolean isOnSyncQueue(Node node) {
        if (node.waitStatus == Node.CONDITION || node.prev == null) {
            return false;
        }
        if (node.next != null) {// If has successor, it must be on queue
            return true;
        }

        /*
         * node.prev can be non-null, but not yet on queue because the CAS to place it
         * on queue can fail. So we have to traverse from tail to make sure it actually
         * made it. It will always be near the tail in calls to this method, and unless
         * the CAS failed (which is unlikely), it will be there, so we hardly ever
         * traverse much.
         */
        return findNodeFromTail(node);
    }

    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * 
     * @return true if present
     */
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (;;) {
            if (t == node) {
                return true;
            }
            if (t == null) {
                return false;
            }
            t = t.prev;
        }
    }

    /**
     * Transfers a node from a condition queue onto sync queue. Returns true if
     * successful.
     * 
     * @param node the node
     * @return true if successfully transferred (else the node was cancelled before
     *         signal)
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            return false;
        }

        /*
         * Splice onto queue and try to set waitStatus of predecessor to indicate that
         * thread is (probably) waiting. If cancelled or attempt to set waitStatus
         * fails, wake up to resync (in which case the waitStatus can be transiently and
         * harmlessly wrong).
         */
        Node p = enq(node);
        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
     */
    final boolean transferAfterCancelledWait(Node node) {
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed until it finishes its
         * enq(). Cancelling during an incomplete transfer is both rare and transient,
         * so just spin.
         */
        while (!isOnSyncQueue(node)) {
            Thread.yield();
        }

        return false;
    }

    /**
     * Invokes release with current state value; returns saved state. Cancels node
     * and throws exception on failure.
     * 
     * @param node the condition node for this wait
     * @return previous sync state
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            int savedState = getState();
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            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. */
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        private transient Node lastWaiter;

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

        // Internal methods

        /**
         * Adds a new waiter to wait queue.
         * 
         * @return its new wait node
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null) {
                firstWaiter = node;
            } else {
                t.nextWaiter = node;
            }
            lastWaiter = node;

            return node;
        }

        /**
         * Removes and transfers nodes until hit non-cancelled one or null. Split out
         * from signal in part to encourage compilers to inline the case of no waiters.
         * 
         * @param first (non-null) the first node on condition queue
         */
        private void doSignal(Node first) {
            do {
                if ((firstWaiter = first.nextWaiter) == null) {
                    lastWaiter = null;
                }
                first.nextWaiter = null;
            } while (!transferForSignal(first) && (first = firstWaiter) != null);
        }

        /**
         * Removes and transfers all nodes.
         * 
         * @param first (non-null) the first node on condition queue
         */
        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.
         */
        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

        /**
         * Moves the longest-waiting thread, if one exists, from the wait queue for this
         * condition to the wait queue for the owning lock.
         * 
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        public final void signal() {
            if (!isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            }
            Node first = firstWaiter;
            if (first != null) {
                doSignal(first);
            }
        }

        /**
         * Moves all threads from the wait queue for this condition to the wait queue
         * for the owning lock.
         * 
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        public final void signalAll() {
            if (!isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            }
            Node first = firstWaiter;
            if (first != null) {
                doSignalAll(first);
            }
        }

        /**
         * Implements uninterruptible condition wait.
         * 实现不可中断的condition等待
         * 
         * <ol>
         * <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.
         * <li>Reacquire by invoking specialized version of {@link #acquire} with saved
         * state as argument.
         * </ol>
         */
        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;
                }
            }
            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.
         */

        /** 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;

        /**
         * Checks for interrupt, returning THROW_IE if interrupted before signalled,
         * REINTERRUPT if after signalled, or 0 if not interrupted.
         */
        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();
            }
        }

        /**
         * Implements interruptible condition wait.
         * 实现可中断的condition等待
         * 
         * <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 or interrupted.
         * <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>
         */
        public final void await() throws InterruptedException {
            if (Thread.interrupted()) {
                throw new InterruptedException();
            }
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) {
                    break;
                }
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE) {
                interruptMode = REINTERRUPT;
            }
            if (node.nextWaiter != null) {// clean up if cancelled
                unlinkCancelledWaiters();
            }
            if (interruptMode != 0) {
                reportInterruptAfterWait(interruptMode);
            }
        }

        /**
         * Implements timed condition wait.
         * 实现超时condition等待
         * 
         * <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>
         */
        public final long awaitNanos(long nanosTimeout) throws InterruptedException {
            if (Thread.interrupted()) {
                throw new InterruptedException();
            }
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }
                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 deadline - System.nanoTime();
        }

        /**
         * Implements absolute timed condition wait.
         * 实现绝对的超时condition等待
         * 
         * <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;
        }

        /**
         * Implements timed condition wait.
         * 实现超时condition等待
         * 
         * <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 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) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }
                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

        /**
         * Returns true if this condition was created by the given synchronization
         * object.
         * 
         * @return {@code true} if owned
         */
        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }

        /**
         * Queries whether any threads are waiting on this condition. Implements
         * {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
         * 
         * @return {@code true} if there are any waiting threads
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        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;
        }

        /**
         * Returns an estimate of the number of threads waiting on this condition.
         * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
         * 
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively} returns {@code false}
         */
        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;
        }

        /**
         * Returns a collection containing those threads that may be waiting on this
         * Condition. Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
         * 
         * @return the collection of threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively} returns {@code false}
         */
        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;
        }
    }

    /**
     * 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);
    }
}