AQS详解-CLH队列，线程等待状态

AtomicInteger解析：http://donald-draper.iteye.com/blog/2359555
锁持有者管理器AbstractOwnableSynchronizer：http://donald-draper.iteye.com/blog/2360109
AQS线程挂起辅助类LockSupport：http://donald-draper.iteye.com/blog/2360206
AQS作为高性能锁的基础，想要理解锁机制，我们需要深入地去剖析AbstractQueuedSynchronizer。今天我们不打算，将独占锁和共享锁的获取与释放，条件等待和唤醒，我们从源码帮助文档，简单看一下AQS是个什么东西，后面的文章我们详细说独占锁和共享锁的获取与释放，条件等待和唤醒。

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import sun.misc.Unsafe;

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic <tt>int</tt> value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated <tt>int</tt>
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 *
AbstractQueuedSynchronizer主要是依赖于FIFO等待队列，提供阻塞锁，和相关同步(信号量，事件等)
框架。AQS用一个原子的int值表示状态state，是多种同步器或锁的基础。
子类必须定义一个protected方法，用于改变状态state，此状态state以为着，
一个对象（锁）是否获取或释放。其他的方法，用于队列操作和阻塞机制。
子类可以通过setState和getState，#compareAndSetState方法，以原子，获取，改变state值。
 * <p>Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * <tt>AbstractQueuedSynchronizer</tt> does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their public methods.
 *
 子类必须提供非公开的内部类，用于实现同步器相关功能，操作相关属性。
 AQS不实现任何同步接口。AQS提供了具体的锁和相关同步器可以在其public方法，
invoked的方法，比如acquireInterruptibly

 * <p>This class supports either or both a default [i]exclusive[/i]
 * mode and a [i]shared[/i] mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not "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. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 *
 AQS支持独占锁和共享锁模式。当一个线程持有独占锁，在其没释放之前，其他线程
 尝试获取锁，则不能成功获取。共享锁可以被多个线程所持有。最大的不同是，
 共享锁模式下，当一个线程成功获取锁，下一个等待线程必须确定其是否可以获取
 锁。不同模式锁下等待的线程，在同一个FIFO队列中。子类可以实现两种模式中的
 一种，也可以都实现，比如读写锁ReadWriteLock。子类如果只提供一种模式的锁，
 不必实现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.  No
 * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
AQS定义一个内部类ConditionObject，用于子类提供Condition的实现。实现独占锁模式的子类，
isHeldExclusively方法可以用于判断当前线程是否持有锁，
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.
 上面这一段，暂时先放在这里，不能很好的翻译这段
 release方法用于完全释放锁。
  No
 * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it. 
ConditionObject的行为依赖于同步器具体的实现语义的过程。

 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an <tt>AbstractQueuedSynchronizer</tt> for their
 * synchronization mechanics.
 *
 此类提供一些方法用于监视，操作内部队列和条件对象。如果锁或同步器，使用AQS作为
 它的同步机制，这些方法需要暴露出去，或者说作为一个接口。
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a <tt>readObject</tt> method that restores this to a known
 * initial state upon deserialization.
 *
 序列化AQS，只序列胡State的原子整数值，反序列化，只有线程等待队列为空。
 需要序列化的子类必须定义一个readObject方法，用于恢复锁状态到一个先前的状态。
 * <h3>Usage</h3>
 *
 * <p>To use this class as the basis of a synchronizer, redefine the
 * following methods, as applicable, by inspecting and/or modifying
 * the synchronization state using {@link #getState}, {@link
 * #setState} and/or {@link #compareAndSetState}:
 *
 用AQS作为基本的同步器，需要从新定义一下方法。用#setState，#getState，#compareAndSetState
 方法修改，监视，获取同步状态
 * [list]
 * <li> {@link #tryAcquire}//尝试获取独占锁
 * <li> {@link #tryRelease}//尝试释放独占锁
 * <li> {@link #tryAcquireShared}//尝试获取共享锁
 * <li> {@link #tryReleaseShared}//尝试释放共享锁
 * <li> {@link #isHeldExclusively}//是否持有独占锁
 *[/list]
 *
 * Each of these methods by default throws {@link
 * UnsupportedOperationException}.  Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the [i]only[/i] supported
 * means of using this class. All other methods are declared
 * <tt>final</tt> because they cannot be independently varied.
 上述一些方法，默认抛出UnsupportedOperationException异常。
 实现这些方法必须是内部线程安全的，同时这个过程，在时间上，应尽量的短，同时无阻塞。
 定义这些方法意味着，锁或同步器的实现，是基于AQS。其他方法被定义为Final，以防被子类修改。
 *
 * <p>You may also find the inherited methods from {@link
 * AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer.  You are encouraged to use them
 * -- this enables monitoring and diagnostic tools to assist users in
 * determining which threads hold locks.
 *
 你可以用从AbstractOwnableSynchronizer继承的方法，获取独占锁的持有线程。我们鼓励
 用AbstractOwnableSynchronizer继承的方法，去确定那个线程持有锁。
 * <p>Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies.  The core
 * of exclusive synchronization takes the form:
 *尽管AQS的内部的队列是FIFO，但AQS不能保证FIFO的准确性，独占锁的核心同步如下
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        [i]enqueue thread if it is not already queued[/i];
 *        [i]possibly block current thread[/i];
 *     }
 *
 自旋尝试获取锁，获取失败，则查看线程是否在等待队列中，如果没有，
 则入队列，同时可能阻塞当前线程

 * Release:
 *     if (tryRelease(arg))
 *        [i]unblock the first queued thread[/i];
 * </pre>
 *
 如果释放锁成功，则唤醒队列头部的线程，持有锁
 * (Shared mode is similar but may involve cascading signals.)
 *
 共享模式下的锁，获取锁和释放锁类型，但是为引起级联效应
 * <p><a name="barging">Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may [i]barge[/i] ahead of
 * others that are blocked and queued.  However, you can, if desired,
 * define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to
 * disable barging by internally invoking one or more of the inspection
 * methods, thereby providing a [i]fair[/i] FIFO acquisition order.
 * In particular, most fair synchronizers can define <tt>tryAcquire</tt>
 * to return <tt>false</tt> if {@link #hasQueuedPredecessors} (a method
 * specifically designed to be used by fair synchronizers) returns
 * <tt>true</tt>.  Other variations are possible.
 *
 在进入队列，进行获取锁检查时，一个新的获取锁线程也许会优先于，阻塞在队列中的
 线程获取锁。如果不想出现这种情况，可以选择通过tryAcquire和tryAcquireShared方法，
 通过一个或多个诊断方法，屏蔽这种情况，鉴于这种情况，提供一个精度较高的
 FIFO队列。在一些特殊情况下，大部分同步器的#hasQueuedPredecessors方法（专为公平锁设计的方法）
 返回true时，可以定义tryAcquire方法返回false，来保证公平性。

 * <p>Throughput and scalability are generally highest for the
 * default barging (also known as [i]greedy[/i],
 * [i]renouncement[/i], and [i]convoy-avoidance[/i]) strategy.
 * While this is not guaranteed to be fair or starvation-free, earlier
 * queued threads are allowed to recontend before later queued
 * threads, and each recontention has an unbiased chance to succeed
 * against incoming threads.  Also, while acquires do not
 * "spin" in the usual sense, they may perform multiple
 * invocations of <tt>tryAcquire</tt> interspersed with other
 * computations before blocking.  This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. If so desired, you can
 * augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended}
 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 * is likely not to be contended.
 *
 非公平锁（贪婪模式或闯入者模式）拥有一个相对较高的稳定性和吞吐量，强烈建议
 使用这种模式的锁，这种模式不能保证公平性或饥渴度平衡树，先入队列的线程，允许在
 后进入队列的线程之前竞争锁，每个竞争者与刚进来的线程拥有公平的机会，成功竞争锁。
 一般情况下，在线程阻塞前，竞争者会自旋，多次执行tryAcquire方法，尽最大可能获取锁。
自旋对于锁的持有者有利，而对于获取者没有任务坏处。如果竞争者不可能获取锁，但同时
有强烈的愿望持有锁，则可以在acquire方法前，通过#hasContended或hasQueuedThreads方法，
检查或预检查锁。
 * <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 <tt>int</tt> state, acquire, and
 * release parameters, and an internal FIFO wait queue. When this does
 * not suffice, you can build synchronizers from a lower level using
 * {@link java.util.concurrent.atomic atomic} classes, your own custom
 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 *
 AQS利用获取和释放原子的int state，内部FIFO等待队列为同步器提供有效和稳定的基础。
 当AQS性能不好时，可以利用Atomic和自己实现的Queue，和LockSupport，实现自己的锁机制。

 * <h3>Usage Examples</h3>
 *
 * <p>Here is a non-reentrant mutual exclusion lock class that uses
 * the value zero to represent the unlocked state, and one to
 * represent the locked state. While a non-reentrant lock
 * does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor.
 * It also supports conditions and exposes
 * one of the instrumentation methods:
 *
 这里是一个非重入互斥锁的实现，用0表示锁打开状态，1表示锁住状态。
 非重入互斥锁不需要记录当前锁的持有者，所以用了简单的监视方法实现
 isHeldExclusively。Mutex支持Condition，暴露了一些使用方法。
 * <pre>
非重入互斥锁
 * class Mutex implements Lock, java.io.Serializable {
 *
 *   // Our internal helper class，内部锁helper
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Report whether in locked state，监控锁状态
 *     protected boolean isHeldExclusively() {
 *       return getState() == 1;
 *     }
 *
 *     // Acquire the lock if state is zero
 *     public boolean tryAcquire(int acquires) {
         //断言acquires为1，当开启断言检查时（VM -ea），acquires不为1，则中断程序
 *       assert acquires == 1; // Otherwise unused
 *       if (compareAndSetState(0, 1)) {
           //CAS操作获取锁，如果成功，则设置锁持有者为当前线程
 *         setExclusiveOwnerThread(Thread.currentThread());
           //返回true获取成功
 *         return true;
 *       }
 *       return false;
 *     }
 *
 *     // Release the lock by setting state to zero
 *     protected boolean tryRelease(int releases) {
       //断言releases为1，当开启断言检查时，releases不为1，则中断程序
 *       assert releases == 1; // Otherwise unused
	//如果锁为打开状态，抛出非法状态监控异常
 *       if (getState() == 0) throw new IllegalMonitorStateException();
         //设置锁持有者为null，即锁无持有者
 *       setExclusiveOwnerThread(null);
         //设置锁为打开状态
 *       setState(0);
         //释放成功
 *       return true;
 *     }
 *
 *     // Provide a Condition，创建条件
 *     Condition newCondition() { return new ConditionObject(); }
 *
 *     // Deserialize properly，反序列化方法
 *     private void readObject(ObjectInputStream s)
 *         throws IOException, ClassNotFoundException {
        //调用默认的反序列化
 *       s.defaultReadObject();
        //设置锁为打开状态
 *       setState(0); // reset to unlocked state
 *     }
 *   }
 *  //同步器sync，做了所有的关键工作，我们只需要利用它实现锁机制
 *   // The sync object does all the hard work. We just forward to it.
 *   private final Sync sync = new Sync();
 *  
 *   public void lock()                { sync.acquire(1); } //获取锁
 *   public boolean tryLock()          { return sync.tryAcquire(1); }//尝试获取锁
 *   public void unlock()              { sync.release(1); }//释放锁
 *   public Condition newCondition()   { return sync.newCondition(); }//创建条件
 *   public boolean isLocked()         { return sync.isHeldExclusively(); }//是否锁住
 *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }//锁是否有等待队列
    //以可中断的方式获取锁
 *   public void lockInterruptibly() throws InterruptedException {
 *     sync.acquireInterruptibly(1);//
 *   }
     //等待超时时间，再尝试获取锁
 *   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 CountDownLatch}
 * except that it only requires a single <tt>signal</tt> to
 * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt>
 * acquire and release methods.
 *BooleanLatch是单个signal的闭锁，就像CountDownLatch一样。因为闭锁是非独占锁
 ，所以它用acquire和release的共享版本，来获取与释放锁。
 * <pre>
 * class BooleanLatch {
 *   //内部同步器
 *   private static class Sync extends AbstractQueuedSynchronizer {
       //当锁状态不为零，代表锁处理打开状态，等待锁打开的线程，此时被唤醒
 *     boolean isSignalled() { return getState() != 0; }
 *     //获取共享信号锁，锁打开，则获取锁成功。
 *     protected int tryAcquireShared(int ignore) {
 *       return isSignalled() ? 1 : -1;
 *     }
 *     //释放共享锁，即打开锁
 *     protected boolean tryReleaseShared(int ignore) {
 *       setState(1);
 *       return true;
 *     }
 *   }
 *
 *   private final Sync sync = new Sync();
 *   public boolean isSignalled() { return sync.isSignalled(); }//锁是否打开
 *   public void signal()         { sync.releaseShared(1); }//已共享模式，打开锁
     //已共享可中断方式，等待锁打开信号
 *   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 <tt>AbstractQueuedSynchronizer</tt> instance
     * with initial synchronization state of zero.
     */
     //创建一个实例，初始化化状态为0，及闭锁状态
    protected AbstractQueuedSynchronizer() { }

    /**
     * Wait queue node class.
     *等待队列节点
     * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
     * Hagersten) lock queue. CLH locks are normally used for
     * spinlocks.  We instead use them for blocking synchronizers, but
     * use the same basic tactic of holding some of the control
     * information about a thread in the predecessor of its node.  A
     * "status" field in each node keeps track of whether a thread
     * should block.  A node is signalled when its predecessor
     * releases.  Each node of the queue otherwise serves as a
     * specific-notification-style monitor holding a single waiting
     * thread. The status field does NOT control whether threads are
     * granted locks etc though.  A thread may try to acquire if it is
     * first in the queue. But being first does not guarantee success;
     * it only gives the right to contend.  So the currently released
     * contender thread may need to rewait.
     线程等待队列是CLH 锁队列的一个变种。CLH锁一般用于自旋锁场景。
     我们用它阻塞同步器，及一些基本的策略用于描述线程的前驱线程节点。
     每个几点的状态status属性，用于描述一个线程是否应该被阻塞。当线程
     节点的前驱节点释放锁时，将会唤醒其后继线程节点。队列中的每个线程
     节点，描述的是等待线程的状态。节点的status field不能控制节点线程
     ，是否可以持有锁。队列的头结点线程，会尝试着获取锁。头节点线程
     ，虽然是第一个尝试获取锁的，但是不能保证能够成功获取锁，而是合适的
     竞争者。所以当竞争线程释放锁时，想要重新获取锁，必须重新等待。




     *
     * <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>
     *
     对于CLH队列，当进入队列时，只需要，新建一个尾节点，挂入队列即可；
     当出队列时，只需要设置队列的头节点，即可。
     * <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, dequeing
     * 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.
     *
     每次进入CLH队列时，需要对尾节点进入队列过程，是一个原子性操作。
     在出队列时，我们只需要更新head节点即可。在节点确定它的后继节点时，
     需要花一些功夫，用于处理那些，由于等待超时时间结束或中断等原因，
     而取消等待锁的线程。

     * <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/
     *节点的前驱指针，主要用于处理，取消等待锁的线程。如果一个节点
     取消等待锁，则此节点的前驱节点的后继指针，要指向，此节点后继节点中，
     非取消等待锁的线程（有效等待锁的线程节点）。自旋锁的相同机制，
     可以看Scott and Scherer的论文。

     * <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.)
     *
     我们用next指针连接实现阻塞机制。每个节点线程，控制着它自己的节点，
     节点通过节点的后继连接唤醒其后继节点。为了避免节点的后继节点与
     刚要进队列的线程竞争，通常把刚进的线程节点作为它后继，把节点的后继，
     设为刚进来线程节点的后继。上面说的这一段，是非公平可重入锁的特性，为了
     提高性能和吞吐量，这个我们后面的文章会说。上述的处理手段，节点了更新
     尾节点时，尾节点的后继为null的问题。可以说时next连接的一种优化，
     不必要再往后检查节点。

     * <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.
     *
     线程的取消，引入了一些保守的基本算法。由于我们必须poll其他节点
     的cancellation，而忽略了节点是否是头结点或为节点后继。除非我们能确定
     一个非取消前驱节点能够负责这些工作，否则Cancellation机制，总是unpark
     后继节点，并需要他们有一个新的前驱。

     * <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.
     *
     CLH队列需要一个头结点作为开始节点，头结点非实际线程节点。
     我们不会再构造函数中，创建它，因为如果没有线程竞争锁，那么，
     努力就白费了。取而代之额方案是，当有第一个竞争者时，我们才
     构造头指针和尾指针。
     * <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.
     *
     线程以那个同一节点等待条件，但是用另外一个连接。条件只需要放在一个
     非并发的连接队列与节点关联，因为只有当线程独占持有锁的时候，才会去访问条件。
     当一个线程等待条件的时候，节点将会出入到条件队列中。当条件触发时，
     节点将会转移到主队列中。有一个状态值，用于描述节点在哪一个队列上。
     * <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.
     */感谢各位JSR-166规范的成员，对此类设计的批评与建议。
    static final class Node {
        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();//标记节点等待一个共享锁
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;//标记节点等待一个独占锁

        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1;//表示等待锁的线程，被取消
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL    = -1;//表示后继线程需要被唤醒
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;//表示在等待条件
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3;//表示下一个获取共享锁的线程，无条件传递获取

        /**
         * 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.
         SIGNAL：节点的后继由于park等原因被阻塞，当节点释放锁或取消时，要
	 unpark后继节点。为了避免竞争，acquire方法必须，首先检查他们是否
	 需要唤醒后继节点，再原子获取锁，获成功，失败，阻塞。


         *   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.
         CANCELLED:节点有等待锁超时或者中断等原因，被取消，节点不会停留在这个状态。
          如果一个线程被取消，线程就不会再被阻塞。


         *   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.)
         CONDITION: 处于这个状态的节点线程，放在条件队列中。它永远不会被
	 用作一个同步队列节点，知道等待的条件发生，节点将被转移到同步队列中。
	 （这个状态与其他状态，没有关联，只是一种简化的机制）。

         *   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
         *
	PROPAGATE: 处于此模式下，释放共享锁具有传递性。头节点调用
	doReleaseShared方法，保证传递释放共享锁，即使有其他的操作干涉。

         * 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.
         *
	 这些状态值使用数字，表示状态。当值为负值时，表示节点不需要唤醒，
	 所以当编码时，不用检查精确的值，比较即可。
         * 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).
         */
        field初始化为0，表示一个正常的同步节点。CONDITION属于条件节点。
	此field，用CAS的手段进行修改等操作。

	//等待状态
        volatile int waitStatus;

        /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueing, 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.
         */
        当前线程用，前驱节点检查等待状态。为了给GC提供便利，当节点入队列以后，
	如果出队列，前继为nulled。如果前驱节点，处于取消状态，我们应该进行一个短暂的
	循环，剔除取消的节点，寻到一个非取消节点作为后继，节点总会存在，
	因为队列的头结点是，成功获取锁的节点。取消线程节点，不会成功获取锁，
	且只能取消它自己。
        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.
         */
	 当前线程释放锁，根据后继连接，unpark线程。当出队列时，节点的后继为nulled，
	 以便gc回收。入队列操作不能保证next不为null，直到处理队列链接中，所以一个
	 节点的后继为null，不意味着，没有入队列。如果一个节点的后继为null，
	 我们可以从对尾，浏览他的前继，做双保险检查。为了是节点在同步队列中的
	 生命周期简单化，当一个取消线程节点，取消时，他的后继节点不为null，而是
	 指向自己。
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on
         * construction and nulled out after use.
         */
	 进入队列的节点线程
        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.
         */
	 节点下一个等待条件或共享锁的节点。当线程持有独占锁时，只需要
	 访问条件队列，所以我们只需要一个简单的连接队列，存储等待条件的线程。
	 当他们转移到主队列时，可以重新获取锁。由于条件可以是互斥的，
	 所以我们用，特殊的值，去表示共享模式。
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode
         */
	检点是否是共享模式
        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.
         *
         * @return the predecessor of this node
         */
	 返回节点的前继，如果为null，抛出空指针异常。前继不内为null，
	 空值检查可以剔除这种情况，帮助VM回收。
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }
        //创建初始化head，和共享模式
        Node() {    // Used to establish initial head or SHARED marker
        }
        //构建等待条件节点
        Node(Thread thread, Node mode) {     // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }
        //构建等待状态节点
        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.
     */
    //等待队列的头节点，懒加载，通过setHead方法，初始化及修改头节点。
    如果头节点已经存在，要保证他的状态不能为CANCELLED.
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
     //等待队列的尾节点，懒加载。通过添加一个新的等待节点来修改
    private transient volatile Node tail;

    /**
     * The synchronization state.
     */
     //同步状态
    private volatile int state;

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a <tt>volatile</tt> read.
     * @return current state value
     */
    获取同步状态，从内存中直接读取
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a <tt>volatile</tt> write.
     * @param newState the new state value
     */
     设置同步状态，直接写内存
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * 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).
     */
    支持CAS操作。为了增强permitting future，我们需要本地化的实现，我们
    不用使用实现AtomicInteger的子类，AtomicInteger在其他方面是高效有用的。
    为了得到最优的性能，我们使用VM本地化的API，在CAS性质的fields，操作中
    使用相同的机制。
    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); }
    }

    /**
     * Condition implementation for a {@link
     * AbstractQueuedSynchronizer} serving as the basis of a {@link
     * Lock} implementation.
     * 作为AQS实现锁的一个基础实现Condition。
     * <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
     * <tt>AbstractQueuedSynchronizer</tt>.
     *方法文档用于描述这个条件实现机制，不是锁和条件的使用者，可以使用的操作。
     此类的版本与AbstractQueuedSynchronizer相关联。
     * <p>This class is Serializable, but all fields are transient,
     * so deserialized conditions have no waiters.
     */
     //这个所有的all fields are transient，所以反序列化时，条件没有等待者。
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
	队列中第一个等待节点线程
        private transient Node firstWaiter;
        /** Last node of condition queue. */
	队列中最后一个等待条件的节点线程
        private transient Node lastWaiter;
        剩下的我们会在后面的文章单独将,敬请期待..........
	}
}

总结：
从阅读源码帮助文档可看出，AQS使用CAS原始，修改锁的状态state；
等待锁的线程被放入到等待队列（CLH队列）中，每个线程等待状态用NODE来描述。
NODE有共享模式和独占模式，独占模式为NULL。NODE有CANCELLED，SIGNAL，SIGNAL，PROPAGATE
4中状态值。

SIGNAL：节点的后继由于park等原因被阻塞，当节点释放锁或取消时，要
unpark后继节点。为了避免竞争，acquire方法必须，首先检查他们是否
需要唤醒后继节点，再原子获取锁，获成功，失败，阻塞。
简单说，节点释放锁，是否需要唤醒后继节点
CANCELLED:节点有等待锁超时或者中断等原因，被取消，节点不会停留在这个状态。
          如果一个线程被取消，线程就不会再被阻塞。
简单说，单节点处于这个状态，将被移除到等待队列

CONDITION: 处于这个状态的节点线程，放在条件队列中。它永远不会被
用作一个同步队列节点，直到等待的条件发生，节点将被转移到同步队列中。
（这个状态与其他状态，没有关联，只是一种简化的机制）。

PROPAGATE: 处于此模式下，释放共享锁具有传递性。头节点调用
doReleaseShared方法，保证传递释放共享锁，即使有其他的操作干涉。
这个时共享模式下的状态。

CLH队列由于虚头节点，队列中线程等待节点有一个前驱和一个后继节点，NODE有一个状态
waitStatus，描述线程的当前状态，有一个线程field用于表示当前等待线程，同时还有
nextWaiter节点，用于描述，节点时候有等待条件，或共享模式，获取锁时，需要通知其他线程。

Node nextWaiter：节点下一个等待条件或共享锁的节点。当线程持有独占锁时，只需要
访问条件队列，所以我们只需要一个简单的连接队列，存储等待条件的线程。
当他们转移到主队列时，可以重新获取锁。由于条件可以是互斥的，
所以我们用，特殊的值，去表示共享模式。
AQS有一个状态state表示锁的状态，一个CLH队列存放等待锁的线程节点。NODE还可以用于描述节点的等待条件节点线程，用nextWaiter去关联，组成的队列是条件队列。条件队列和等待队列并不冲突，当等待条件的线程被唤醒时，可以尝试获取锁，加入到等待对列。当一个等待队列节点线程获取独占锁时，可以访问条件队列，唤醒等待条件的线程。AQS还有一个ConditionObject我们，下一篇文章再讲。