理解AQS原理一之阅读注释

自己学习使用，大神勿喷

为了深入了解AQS运行原理，先读了AbstractQueuedSynchronizer类中大量的英文注释，并对部分进行翻译、阅读，简单理解该类的功能作用。该篇为AQS原理第一篇，主要通过阅读注释简单了解其功能。具体的源码解析会另开文章分析。此篇注释翻译，也会不断更新。

AQS类的UML图示

生成的此UML图示，带有field字段和内部类；由于方法个数较多，篇幅有限，未带有method方法，方便简洁查看。

AQS类中部分注释的翻译

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic {@code int} value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of（依据） this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all（执行） queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated {@code int}
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 * 为实现那些依赖于先进先出等待队列的阻塞式锁和相关同步器（信号量、事件等），提供了框架。
 * 设计这个类的目的是：为大多数依赖于单个原子int变量来表示状态的同步器 提供有用的基础。
 * 子类必须定义那些改变state的protected方法，以及这些方法定义了该state在获取或释放该对象方面的含义。
 * 鉴于此，该类中的其余方法用来执行队列和阻塞机制。
 * 子类可以维持其他状态,但是只有使用getState、setState、compareAndSetState方法原子地更新int值（指代state），才能同步跟踪状态值。
 * 
 *
 * <p>Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their public methods.
 * 子类必须定义为非public内部辅助类，用于实现其封闭类的同步属性。
 * AQS类未实现任何同步接口。
 * 取而代之的是，它定义了诸如acquireInterruptibly的方法，具体锁和相关同步器可以根据需要调用它来实现它们的公共方法
 *
 * <p>This class supports either or both a default <em>exclusive</em>
 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not &quot;understand&quot; these differences except in the
 * mechanical sense that when a shared mode acquire succeeds, the next
 * waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 * 该类既支持默认的独占模式，又支持共享模式。
 * 在独占模式中获取时，其他线程的试图获取不会成功。
 * 在共享模式时，多个线程获取时可能会（但不需要）成功。
 * 这个类不“理解”这些差异，除非从机械意义上说，当共享模式获取成功时，下一个等待的线程（如果存在的话）必须确定得知其是否可获取成功。
 * 不同模式下的等待线程共享一个FIFO队列。
 * 通常，实现子类仅支持一种模式，但这两种模式都可以发挥作用，比如：ReadWriteLock类
 * 仅支持独占或共享模式一种的子类，不需要定义那些支持未使用模式的方法。
 *
 * <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
 * {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an {@code AbstractQueuedSynchronizer} for their
 * synchronization mechanics.
 * 此类为内部队列提供了检查、检测和监控的方法，也为condition对象提供了类似的方法。
 * 可以使用AbstractQueuedSynchronizer将这些同步机制按照需要导出到类中。
 *
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a {@code readObject} method that restores this to a known
 * initial state upon deserialization.
 * 该类的序列化仅存储底层维持状态的原子整数，所以反序列化对象有空线程对象。
 * 需要序列化的子类将定义一个readObject方法，在反序列化时来恢复它到一个已知的初始化状态。
 *
 * <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}:
 * 想要使用这个类作为同步器的基础，通过使用（getState、setState）方法和/或compareAndSetState方法，
 * 检查和/或修改同步状态，来按照需求重新定义以下方法
 *
 *
 * <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}.  Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the <em>only</em> supported
 * means of using this class. All other methods are declared
 * {@code final} because they cannot be independently varied.
 * 默认情况下，上述的每一个方法抛出UnsupportedOperationException异常。
 * 这些方法的实现必须是内部线程安全的，并且处理同流程通常应该较短且非阻塞。
 * 定义这些方法是使用该类唯一被支持的方式。
 * 所有这些方法应该被声明为final，因为他们不能被独立更改。
 *
 * <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.
 * 你可能也发现继承自AbstractOwnableSynchronizer类的方法，对于跟踪拥有独占同步器的线程非常有用。
 * 我们被鼓励使用它们--这使得监视和诊断工具能帮助用户确定哪些线程持有锁。
 *
 * <p>Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies.  The core
 * of exclusive synchronization takes the form:
 * 尽管这个类基于一个内部FIFO队列，但它不会自动执行FIFO获取策略。
 * 独占同步的核心遵循以下形式
 *
 * <pre>
 * Acquire:// 获取锁
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 * 		  // 如果线程未排队，则入队
 *		  // 可能阻塞当前线程
 *     }
 *
 * Release:// 释放锁
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 *			//  解除第一个排队线程的阻塞
 * </pre>
 *
 * (Shared mode is similar but may involve cascading signals.)
 * 共享模式可能类似，当时可能包含级联signals
 *
 * <p id="barging">Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 * others that are blocked and queued.  However, you can, if desired,
 * define {@code tryAcquire} and/or {@code tryAcquireShared} to
 * disable barging by internally invoking one or more of the inspection
 * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 * In particular, most fair synchronizers can define {@code tryAcquire}
 * to return {@code false} if {@link #hasQueuedPredecessors} (a method
 * specifically designed to be used by fair synchronizers) returns
 * {@code true}.  Other variations are possible.
 * 因为acquire过程中的检查在排队之前被调用，一个最新acquire的线程可能会在其他被阻塞和排队的线程之前闯入（barge）。
 * 无论如何，在必要时，你可以定义tryAcquire和/或tryAcquireShared来禁用barging，通过内部调用一个或多个检查方法的方式，从而提供一个公平的FIFO获取顺序。
 * 特别地，大部分公平同步锁可以定义tryAcquire方法来返回false，如果hasQueuedPredecessors返回true。
 * hasQueuedPredecessors是一个专门为公平同步器设计的方法。
 * 其他变化是可能的。
 *
 * <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.
 * While this is not guaranteed to be fair or starvation-free, earlier
 * queued threads are allowed to recontend before later queued
 * threads, and each recontention has an unbiased chance to succeed
 * against incoming threads.  Also, while acquires do not
 * &quot;spin&quot; in the usual sense, they may perform multiple
 * invocations of {@code tryAcquire} interspersed with other
 * computations before blocking.  This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. If so desired, you can
 * augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended}
 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 * is likely not to be contended.
 * 吞吐量和可伸缩性通常是默认barging（也称为贪心、放弃和避免护送）策略的*别，
 * 虽然不能保证这是公平或无中断的，但允许更早排队的线程在比较晚排队线程之前重新竞争，而且每次重新竞争又一次公平地机会同传入的线程竞争成功。
 * 另外，虽然通常情况acquires不会“自旋”，但在阻塞之前，他们可能执行多次tryAcquire的调用，并附带着其他计算。
 * 当独占同步被短暂挂起时，就提供了自旋的大部分好处，没有了大部分不利因素。
 * 如果需要，你可以通过前面的调用来增强这一点，即使用“fast-path”检查来获取方法，可能会预检查hasContended()和/或hasQueuedThreads()，只有在同步器可能不存在竞争的情况下才这样做。
 *
 *
 * <p>This class provides an efficient and scalable basis for
 * synchronization in part by specializing its range of use to
 * synchronizers that can rely on {@code int} state, acquire, and
 * release parameters, and an internal FIFO wait queue. When this does
 * not suffice, you can build synchronizers from a lower level using
 * {@link java.util.concurrent.atomic atomic} classes, your own custom
 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 * 这个类为同步提供了有效的和可伸缩的基础，通过将其使用范围专业化到（可以依赖int状态、获取和释放参数以及内部FIFO等待队列）的同步器。
 * 当这还未足够，你可以从使用了atomic类、自定义队列类和LockSupport阻塞支持，从较低的级别，来构建同步器
 *
 * <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:
 * 这有一个使用0值来表示未锁状态、1值表示加锁状态的不可重入互斥独占锁类。
 * 虽然一个不可重入锁不严格要求计入当前owner的线程，但是这类还是这么做了，以便于容易见识使用状况。
 *
 *  <pre> {@code
 * class Mutex implements Lock, java.io.Serializable {
 *
 *   // Our internal helper class
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Reports whether in locked state
 *     protected boolean isHeldExclusively() {
 *       return getState() == 1;
 *     }
 *
 *     // Acquires the lock if state is zero
 *     public boolean tryAcquire(int acquires) {
 *       assert acquires == 1; // Otherwise unused
 *       if (compareAndSetState(0, 1)) {
 *         setExclusiveOwnerThread(Thread.currentThread());
 *         return true;
 *       }
 *       return false;
 *     }
 *
 *     // Releases the lock by setting state to zero
 *     protected boolean tryRelease(int releases) {
 *       assert releases == 1; // Otherwise unused
 *       if (getState() == 0) throw new IllegalMonitorStateException();
 *       setExclusiveOwnerThread(null);
 *       setState(0);
 *       return true;
 *     }
 *
 *     // Provides a Condition
 *     Condition newCondition() { return new ConditionObject(); }
 *
 *     // Deserializes properly
 *     private void readObject(ObjectInputStream s)
 *         throws IOException, ClassNotFoundException {
 *       s.defaultReadObject();
 *       setState(0); // reset to unlocked state
 *     }
 *   }
 *
 *   // The sync object does all the hard work. We just forward to it.
 *   private final Sync sync = new Sync();
 *
 *   public void lock()                { sync.acquire(1); }
 *   public boolean tryLock()          { return sync.tryAcquire(1); }
 *   public void unlock()              { sync.release(1); }
 *   public Condition newCondition()   { return sync.newCondition(); }
 *   public boolean isLocked()         { return sync.isHeldExclusively(); }
 *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 *   public void lockInterruptibly() throws InterruptedException {
 *     sync.acquireInterruptibly(1);
 *   }
 *   public boolean tryLock(long timeout, TimeUnit unit)
 *       throws InterruptedException {
 *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 *   }
 * }}</pre>
 *
 * <p>Here is a latch class that is like a
 * {@link java.util.concurrent.CountDownLatch CountDownLatch}
 * except that it only requires a single {@code signal} to
 * fire. Because a latch is non-exclusive, it uses the {@code shared}
 * acquire and release methods.
 * 这是一个类似于CountDownLatch类的latch类，只不过它仅需一个signal来触发。
 * 因为latch是非独占的，所以它使用共享模式的acquire和release方法。
 *
 *  <pre> {@code
 * class BooleanLatch {
 *
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     boolean isSignalled() { return getState() != 0; }
 *
 *     protected int tryAcquireShared(int ignore) {
 *       return isSignalled() ? 1 : -1;
 *     }
 *
 *     protected boolean tryReleaseShared(int ignore) {
 *       setState(1);
 *       return true;
 *     }
 *   }
 *
 *   private final Sync sync = new Sync();
 *   public boolean isSignalled() { return sync.isSignalled(); }
 *   public void signal()         { sync.releaseShared(1); }
 *   public void await() throws InterruptedException {
 *     sync.acquireSharedInterruptibly(1);
 *   }
 * }}</pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
 public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance
     * with initial synchronization state of zero.
     */
    protected AbstractQueuedSynchronizer() { }

    /**
     * Wait queue node class.
	 * 等待队列Node静态内部类声明
     *
     * <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.
     * 这个等待队列是“CLH”锁队列的变体。“CLH”锁通常用于自旋锁。
	 * 相反，我们使用其阻塞同步器，但使用相同的基本策略，即在该节点的前继续节点中持有线程的一些控制信息。
	 * 每个节点中的“status”字段跟踪线程是否应该阻塞。该节点的前继节点release时，该节点释放信号。
	 * 否则，队列中的每个节点都充当持有单个等待线程的“特定通知样式”的监视器。
	 * “status”字段不控制线程是否被授予锁。如果一个线程是队列中的第一个，它可能尝试acquire。
	 * 但是成为首个并不能保证成功；它仅仅被赋予了竞争的权利。
	 * 所以最近发布的竞争线程，可能需要重新等待。
	 *
     * <p>To enqueue into a CLH lock, you atomically splice it in as new
     * tail. To dequeue, you just set the head field.
	 * 要进入CLH锁队列，你需要原子地拼接新节点到队列，并作为新的tail。
	 * 要退出队列，你仅仅设置head字段即可。
	 *
     * <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CLH队列只需要在“tail”的一个原子操作，因此从无队列到队列只有一个简单的原子点划分。
	 * 相似地，退出队列仅包含更新“head”节点。
	 * 但是，节点需要更多的工作去驹诶定他们的继承者是谁，一定程度上是为了解决超时和中断可能引起的取消。
	 * 
     *
     * <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/
	 * 这个"prev"指针链接（不用于原始CLH锁）主要用于处理取消。
	 * 如果一个节点被取消，它的继承者（通常）会重新连接到一个未被取消的前继节点。
	 * 有关自旋锁的类似机制的解释见以下地址：
     *
     * <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.)
	 * 我们使用“next”指针链接来实现阻塞机制。
	 * 每个节点对应的线程Id都保存在自身节点中，故前继节点通过遍历next链接，来通知下一个节点苏醒，以确定他是哪个线程。
	 * 确定继承节点必须避免和新排队的节点竞争，以设置前继节点的“next”指针域。
	 * 当节点的继承节点可能为空时(或者换句话说，下一个链接是一个优化，因此我们通常不需要向后扫描)，从原子更新的“tail”向后检查，从而在必要时解决这个问题。
     *
     * <p>Cancellation introduces some conservatism（保守性） 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.
	 * 取消在基本算法中引入了一定的保守性。
	 * 由于我们必须轮询以取消其他节点，因此我们可能遗漏注意到一个已取消的节点是在前面还是后面。
     * 解决这个问题的方法是：总是在取消后继节点前unparking，允许后继节点稳定在一个新的前继节点上。
	 * 除非我们可以确定一个未被取消的，可以由其承担责任的前继节点
	 * 
     * <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队列需要一个虚拟的header节点来启动。
	 * 但是我们不用在结构上创建，因为假如从没有竞争，这将白白浪费。
	 * 反而，虚拟节点被构造，head和tail指针在第一次竞争时被设置。
     *
     * <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.
	 * 在Conditions条件等待的线程使用相同的节点，不过使用的另外一个链接。
	 * 条件只需要在简单（非并发）链接队列中链接节点，因为他们只要在独占持有时才会刚被访问。
	 * 等待时，将节点插入到条件队列中。
	 * 接到新号时，将节点传输到主队列。
	 * status字段的一个特殊值用于标记节点所在的队列
     *
     * <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 { 
        static final Node SHARED = new Node();
        static final Node EXCLUSIVE = null;

        static final int CANCELLED =  1;
        static final int SIGNAL    = -1;
        static final int CONDITION = -2;
        static final int PROPAGATE = -3;
        
         /**
         * Status field, taking on only the values: // 状态字段，仅接受以下的值
         *   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.
		 *   这个节点的后继节点是（或即将是）阻塞的（通过park），所以当它释放或取消时，当前节点必须unpark它的后继节点。
		 * 为了避免竞争，acquire方法必须首先表明他们需要一个信号，然后重新尝试原子获取，然后在失败、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.
		 * 此节点由于超时或中断而被取消。节点永远不会离开这个状态。特别是，具有取消节点的线程永远不会阻塞。
		 * 
         *   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.)
		 * 此节点当前处于条件队列中。它将不会被用作同步队列节点，直到被传输到主队列，此时的状态将被设置为0。（此处使用此值与该字段的其他用途无关，只是简化了机制。）
		 *
         *   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.
		 * 已发布共享的节点应该被传播到其他节点。这是在doReleaseShared中设置（仅针对head节点）来确保传播继续，即使后来又其他操作干预
		 *
         *   0:          None of the above
         *
         * The values are arranged numerically to simplify use.
         * Non-negative values mean that a node doesn't need to
         * signal. So, most code doesn't need to check for particular
         * values, just for sign.
         * 该字段使用数值以简化使用。非负值意味着节点不需要发出信号。因此，大多数代码不需要检查特定的值，只需检查签名即可。
		 *
         * 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).
		 * 正常同步节点，字段初始化为0；条件节点，字段初始化为CONDITION（-2）。它使用CAS进行修改(或者在可能的情况下，无条件volatile写)。
		 *
         */
        volatile int waitStatus;
        volatile Node prev;
        volatile Node next;
        volatile Thread thread;
        Node nextWaiter;
        .....
     }
     
     private transient volatile Node head;
     private transient volatile Node tail;
     private volatile int state;

	....
}