AQS源码解读----AbstractQueuedSynchronizer

package cn.com.pep;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractOwnableSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.LockSupport;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;

import sun.misc.Unsafe;

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

    private static final long serialVersionUID = 7373984972572414691L;

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

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

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

    /**
     * The synchronization state.
     */
    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.
     *
     * @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.
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     */
    private Node enq(final Node node) {
        /*"自旋"，将给定的节点插入到同步队列的尾部*/
        for (;;) {
            Node t = tail;
            /*同步队列为空，则dummy哑节点作为同步队列的头结点head，并且将尾节点tail也指向头结点head*/
            if (t == null) {
                /*CAS操作，设置同步队列的头结点*/
                if (compareAndSetHead(new Node())) {
                    /*将尾节点设置为头结点，进入下次"自旋"*/
                    tail = head;
                }
            }else {
                /*尾部节点不为空，则进行正常添加动作*/
                node.prev = t;
                /*CAS操作，设置同步队列的头结点*/
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     * 以给定的模式包装当前线程节点，将当前节点加入到阻塞队列的队尾
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        /*以给定的模式将当前线程包装成node节点*/
        Node node = new Node(Thread.currentThread(), mode);
        
        /*快速采用尾插法，将当前节点插入到同步队列的队尾*/
        Node predNode = tail;
        if (predNode != null) {
            /*preNode <-- node*/
            node.prev = predNode;
            /*采用CAS将node设置阻塞队列的尾节点，设置成功，说明没有并发*/
            if (compareAndSetTail(tail, node)) {
                predNode.next = node;
                /*尾插法插入成功，则直接返回当前节点*/
                return node;
            }
        }
        /*"自旋"将节点加入到队列的尾部，直到成功为止*/
        enq(node);
        return node;
    }

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

    /**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        //在这里，这个节点其实是同步队列的头结点，头结点唤醒后继节点之后，使命就完成了，所以应该将其状态置为0
        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;
        //因为s.next相当于从同步队列的头部遍历所以可能会出现s == null的情况，上面分析过原因，不再赘述了。
        if (s == null || s.waitStatus > 0) {
            s = null;
            //从同步队列的尾部向前遍历，找到当前node节点(头结点)的最近的有效后继节点
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        
        /**
         * 找到最近的有效后继节点，则唤醒后继节点中的线程在parkAndCheckInterrupt()方法上的阻塞，去尝试竞争共享资源，
         * 这就体现了线程之间的协作，而在这个竞争的过程中也会忽略这个Node.CANCELLED状态的节点，这当前node节点也就放弃了竞争共享资源的机会，相当于出队了。
         */
        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.)
     */
    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) {
        //后继节点成功获取了共享锁，队列的"旧head"还没有改变，将其保存下来，锁定到方法的局部变量做后序的判断使用；
        Node h = head; // Record old head for check below
        /**
         * 将这个获取共享锁成功的后继节点设置为同步队列的“新head”,此时同步队列的head发生变化， 此线程还未唤起任何线程。
         */
        setHead(node);
        /**
         * 1、h == null这个条件什么时候成立呢？仔细翻了下AQS中的源码发现：
         * 这个setHeadAndPropagate()方法只在共享锁模式下，同步队列head的后继节点成功获取了共享锁才会调用。
         * 获取到共享锁的当前线程是同步队列的头结点的后继节点，"旧head"有后继节点，说明同步队列不为空,那么"旧head"也必定不为空，
         * 此方法中第一行通过h == head,在执行setHead(node)方法之前将"旧head"保存了下来，所以h == null必定不会成立,
         * 至于为什么这么写呢? 查阅了下资料网上说"发现这个是防止空指针异常发生的标准写法（既如果要取一个对象的某个属性进行判断的时候，首先对这个对象进行null判断）。"
         * 这说的过去吧?
         * 
         * 2、(h = head) == null这个条件什么时候成立呢？
         * 这个条件也是不可能成立的，下面这种情况应该是最常见的：
         *     （1）、例如有个Semaphore实例s初始化了2个许可，线程A首先调用s.acquire(2)申请了两个许可，成功申请到了许可；
         *     （2）、线程B调用了s.acquire()方法申请一个许可，申请失败，加入到同步队列；
         *     （3）、线程C调用了s.acquire()方法申请一个许可，申请失败，加入到同步队列；
         *  （4）、线程A调用了s.releaseShared(2)方法释放了两个许可，再调用doReleaseShared()方法，进行同步队列唤醒；
         *  （5）、首先唤醒了同步队列中的线程B，B线程获取到共享锁：
         *      a)、如果此时线程B还未setHead(Node)方法，还未改变同步队列的head头结点，那么线程A的唤醒工作就结束，也仅仅只是唤醒了同步队列中的线程B，
         *              则必定有（h = head） == Node(C) ！= null成立，线程C的唤醒工作仍然需要线程B去执行；
         *      b)、如果此时线程B执行了SetHead(Node)方法，改变了同步队列的head头结点，那么线程A同时也会唤醒线程C，相当于线程A同时唤醒了线程B和线程C：
         *         1）、如果线程C中的setHeadAndPropagate()在线程B前调用完毕（即线程C执行了setHead()方法改变了同步队列的head），那么 (h = head) == Node(C);
         *         2）、如果线程C中的setHeadAndPropagate()在线程B之后才调用(即线程C此时还未执行setHead()方法，未改变同步队列的head)，那么 (h = head) == Node(B)
         *  所以综上所述，只要执行过addWaiter()方法，向同步队列中添加过线程，那么（h = head）== null必定不成立。只能理解为“防止空指针的标准写法”。 
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            /**
             * s == null这种情况是可能存在的，如果当前唤醒的这个node节点是同步队列的尾节点就可能出现node.next == null;
             * s.isShared()指定是共享锁模式，当前线程获取共享锁之后，是需要尝试唤醒同步队列中的其它线程的。
             */
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

    // Utilities for various versions of acquire

    /**
     * Cancels an ongoing attempt to acquire.
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        //当前节点为空，则说明当前线程永远不会被调度到了，所以直接返回
        if (node == null) {
            return;
        }
        
        /**
         * 接下来将点前Node节点从同步队列出队，主要做以下几件事：
         * 1、将当前节点不与任何线程绑定，设置当前节点为Node.CANCELLED状态；
         * 2、将当前取消节点的前置非取消节点和后置非取消节点"链接"起来；
         * 3、如果前置节点释放了锁，那么当前取消节点承担起后续节点的唤醒职责。
         */
        
        //1、取消当前节点与线程的绑定
        node.thread = null;
        
        //2、找到当前节点的有效前继节点pred
        Node pred = node.prev;
        while (pred.waitStatus > 0) {
            //为什么双向链表从后往前遍历呢？而不是从前往后遍历呢？
            node.prev = pred = pred.prev;
        }
        
        //用作CAS操作时候的条件判断需要使用的值
        Node predNext = pred.next;
        
        //3、将当前节点设置为取消状态
        node.waitStatus = Node.CANCELLED;
        
        /**
         * 接下来就需要将当前取消节点的前后两个有效节点"链接"起来了，"达成让当前node节点出队的目的"。
         * 这里按照node节点在同步队列中的不同位置分了三种情况：
         * 1、node节点是同步队列的尾节点tail;
         * 2、node节点既不是同步队列头结点head的后继节点，也不是尾节点tail;
         * 3、node节点是同步队列头结点head的后继节点；
         */
        
        //1、node是尾节点，并且执行过程中没有并发，直接将pred设置为同步队列的tail
        if (node == tail && compareAndSetTail(node, pred)) {
            /*
             * 此时pred已经设置为同步队列的tail，需要通过CAS操作，将pred的next指向null,没有节点再引用node，就完成了node节点的出队
             * 可以看出出队操作会破坏这个同步队列的next指针，这应该“向链表从后往前遍历呢？而不是从前往后遍历呢”的原因吧？
             */
            compareAndSetNext(pred, predNext, null);
        }else {
            /*
             * 2、node不是尾节点，也不是头结点head的后继节点,那么当前节点node出队以后，node的有效前继结点pred，
             *  就有义务在它自身释放资源的时候，唤醒node的有效后继节点successor,即将pred的状态设置为Node.SIGNAL;
             */
            int ws;
            //能执行到这里，说明当前node节点不是head的后继节点，也不是同步队列tail节点
            if (pred != head &&
                    ((ws = pred.waitStatus) == Node.SIGNAL ||
                    //前继节点状态虽然有效但不是SIGNAL,采用CAS操作设置为SIGNAL确保后继有效节点可以被唤醒
                    (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) && 
                    pred.thread != null) {
                Node next = node.next;
                //只负责唤醒有效后继节点
                if (next != null && next.waitStatus <= 0) {
                    /**
                     * 下面这段代码相当于将pred-->next，我们提到这个同步队列是个双向队列，那么pred<--next这是谁执行的呢？
                     * 答案是其他线程：其它线程在获取共享资源在同步队列中阻塞的时候，调用shouldParkAfterFailedAcquire()方法，
                     * 从后向前遍历队列，寻找能唤醒它的有效前继节点，当找到node的时候，因为它的状态已经是Node.CANCELLED,所以会忽略node节点，
                     * 直到遍历到有效前继节点pred,将next.prev执行pred,即next--->pred，没有节点再引用node节点，所以node节点至此才完成出队。
                     */
                    compareAndSetNext(pred, predNext, next);
                }
            }else {
                //3、说明node节点是同步队列head的后继节点，调用unparkSuccessor(Node)"出队"。
                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.
     *
     * @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;
        /*判断前驱结点的状态，只有前驱结点的状态为SIGNAL,后继节点才能被唤醒，所以其可以安心地挂起来了*/
        if (ws == node.waitStatus) {
            return true;
        }
        
        /*ws>0表示前驱结点中的线程已经被取消调度了，则认为其是无效节点，继续向前查找，直至找到有效状态的节点*/
        if (ws > 0) {
            do {
                //前驱结点已经被取消，则将前驱结点设置为pred = pred.prev
                node.prev = pred = pred.prev;
                //不断遍历，直到找到第一个不是取消状态的节点
            } while (pred.waitStatus > 0);
            //
            pred.next = node;
        }else {
            /*前驱结点状态正常，将前驱结点状态设置为SIGNAL,则前驱结点释放资源的时候，就可以尝试唤醒它的后继节点了*/
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

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

    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        //将当前线程挂起
        LockSupport.park(this);
        //检测当前线程的中断状态，并且会清除线程的中断标记
        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.
     */

    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     * 使线程阻塞在同步队列中等待获取资源，直到获取资源成功才返回，此过程中线程发生了中断就返回true,否则就返回false
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        /*标记等待过程中是否发生了异常*/
        boolean failed = true;
        
        try {
            /*标记线程阻塞的过程中是否发生了中断*/
            boolean interrupted = false;
            /*线程自旋阻塞*/
            for(;;){
                /*获取当前节点的前驱结点*/
                final Node p = node.predecessor();
                /*前驱结点是头结点，则说明当前线程有资格获取共享资源，尝试获取，获取成功，将当前节点设置为头结点*/
                if (p == head && tryAcquire(arg)) {
                    /*将当前节点设置为头结点*/
                    setHead(node);
                    p.next = null; //help GC
                    failed = false;
                    return interrupted;
                }
                
                /*判断当前线程是否可以挂起*/
                if (shouldParkAfterFailedAcquire(p, node)
                        /*当前线程可以挂起，则挂起线程，并且线程被unpark()唤醒，检查线程的状态*/
                        && parkAndCheckInterrupt()) {
                    interrupted = true;
                }
            }
        }finally{
            if (failed) {
                /*阻塞获取同步资源的时候，发生了异常，取消当前线程的在同步队列中的排队*/
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in exclusive interruptible mode.
     * @param arg the acquire argument
     */
    //尝试获取锁，并响应中断
    private void doAcquireInterruptibly(int arg)
        throws InterruptedException {
        //将当前线程包装成Node节点，加入到同步队列sync queue中
        final Node node = addWaiter(Node.EXCLUSIVE);
        //标记是否获取锁的过程中是否发生了异常
        boolean failed = true;
        try {
            //自旋
            for (;;) {
                //获取当前节点的前驱结点
                final Node p = node.predecessor();
                //如果当前节点的前驱结点是头结点head，尝试获取锁，当前线程才有资格获取排他锁（头结点head是个哑结点，不代表任何线程）
                if (p == head && tryAcquire(arg)) {
                    //当前节点获取锁成功，将当前节点设置为头结点head
                    setHead(node);
                    //将原来的头结点从同步队列sync queue中移除
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
                //尝试获取锁失败，判断是否可以将当前线程安全地挂起，只有当前线程有效的前驱结点状态为Node.SIGNAL，当前线程才可以安全地被挂起，后续也会被及时地唤醒
                if (shouldParkAfterFailedAcquire(p, node) &&
                        //将当前线程挂起，并等他唤醒的时候判断是否发生了线程中断，发生了线程中断，则进入finally块中，取消当前线程对锁的尝试获取
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in exclusive timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        //指定的时间<=0则直接返回false
        if (nanosTimeout <= 0L)
            return false;
        //计算当前线程阻塞的截至时间
        final long deadline = System.nanoTime() + nanosTimeout;
        //将当前线程包装成Node节点，加入到同步队列sync queue中
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            //自旋尝试获取锁
            for (;;) {
                //获取当前节点的前驱结点
                final Node p = node.predecessor();
                //前驱结点是头结点，则当前节点有资格尝试获取锁，则尝试获取锁
                if (p == head && tryAcquire(arg)) {
                    //获取锁成功，则将当前节点设置为头结点head
                    setHead(node);
                    //将同步队列sync queue中原来的头结点移除队列
                    p.next = null; // help GC
                    //表示获取锁的过程没有发生异常
                    failed = false;
                    return true;
                }
                //计算剩余的等待时间
                nanosTimeout = deadline - System.nanoTime();
                //剩余的等待时间<=0，直接返回false
                if (nanosTimeout <= 0L)
                    return false;
                //当前线程获取锁失败，判断是否可以将当前线程安全地挂起
                if (shouldParkAfterFailedAcquire(p, node) &&
                        //如果剩余的等待时间<= spinForTimeoutThreshold，则不用将当前线程挂起，进行自旋即可
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                //获取锁的过程中发生了中断，则直接抛出InterruptedException异常
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            //获取锁的过程中发生了异常，则将当前线程取消
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared uninterruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireShared(int arg) {
        //将竞争共享资源失败的线程加入到同步队列中，并标记为共享模式
        final Node node = addWaiter(Node.SHARED);
        //标记阻塞获取资源的过程中是否发生了异常
        boolean failed = true;
        
        try {
            //标记阻塞获取资源的过程中，是否发生了线程中断请求
            boolean interrupted = false;
            //线程阻塞等待获取资源，被有效前继节点唤醒后，尝试竞争共享资源
            for(;;){
                /**
                 * 当前线程被唤醒之后，什么时候有资格竞争共享资源呢？
                 * 之后当它的前继节点是头结点（头结点是当前持有共享资源的线程），在唤醒后继节点的过程中，可能释放了资源，所以后继节点尝试获取一次共享资源。
                 */
                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.
     * @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.
     *
     * @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

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

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

    /**
     * Attempts to acquire in shared mode. This method should query if
     * the state of the object permits it to be acquired in the shared
     * mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return a negative value on failure; zero if acquisition in shared
     *         mode succeeded but no subsequent shared-mode acquire can
     *         succeed; and a positive value if acquisition in shared
     *         mode succeeded and subsequent shared-mode acquires might
     *         also succeed, in which case a subsequent waiting thread
     *         must check availability. (Support for three different
     *         return values enables this method to be used in contexts
     *         where acquires only sometimes act exclusively.)  Upon
     *         success, this object has been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected int tryAcquireShared(int arg) {
        for(;;){
            //计算可用资源量
            int available = getState();
            //计算剩余资源量
            int remaining = available - arg;
            //计算资源数量，不够直接放回，加入CAS是为了保证state状态一定能够更新成功
            if (remaining < 0 ||
                    compareAndSetState(available, remaining)) {
                    return remaining;
            }
        }
        
        //throw new UnsupportedOperationException();
    }

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

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

    /**
     * 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}.
     *
     * @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) {
        /*加塞抢占共享资源（因为同步队列中可能还有其它节点等待），获取成功则直接返回*/
        if (!tryAcquire(arg)  
                /* 1、抢占共享资源失败，则将当前节点放入到同步队列的尾部，并标记为独占模式；
                 * 2、使线程阻塞在同步队列中获取资源，直到获取成功才返回；如果整个过程中被中断过就返回true，否则就返回false；*/
                && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) {
            /*阻塞获取资源的过程中是不响应线程中断的，内部进行了中断检测，所以这块进行线程中断*/
            selfInterrupt();
        }
    }

    /**
     * Acquires in exclusive mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking
     * at least once {@link #tryAcquire}, returning on
     * success.  Otherwise the thread is queued, possibly repeatedly
     * blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted.  This method can be
     * used to implement method {@link Lock#lockInterruptibly}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

    /**
     * Attempts to acquire in exclusive mode, aborting if interrupted,
     * and failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquire}, returning on success.  Otherwise, the thread is
     * queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success or the thread is interrupted
     * or the timeout elapses.  This method can be used to implement
     * method {@link Lock#tryLock(long, TimeUnit)}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        //当前线程被中断，直接抛出异常
        if (Thread.interrupted())
            throw new InterruptedException();
        //尝试直接获取锁，获取成功就直接返回
        return tryAcquire(arg) ||
                //否则就“阻塞”指定时间，再尝试获取锁
            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}.
     *
     * @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;
            //当前线程已经释放锁，但是sync queue同步队列不为空，并且头结点head未被取消
            if (h != null && h.waitStatus != 0)
                //说明还有其它线程等待竞争锁，则尝试唤醒sync queue队列中等待锁的线程
                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 {
        //当前线程已经被中断，则直接抛出InterruptedException异常
        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) {
        //尝试释放锁，释放成功，则返回true
        if (tryReleaseShared(arg)) {
            //唤醒同步队列中，等待获取锁的线程
            doReleaseShared();
            return true;
        }
        return false;
    }

    // Queue inspection methods

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

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

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

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

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

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

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

    /**
     * Returns {@code true} if the apparent first queued thread, if one
     * exists, is waiting in exclusive mode.  If this method returns
     * {@code true}, and the current thread is attempting to acquire in
     * shared mode (that is, this method is invoked from {@link
     * #tryAcquireShared}) then it is guaranteed that the current thread
     * is not the first queued thread.  Used only as a heuristic in
     * ReentrantReadWriteLock.
     */
    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.
     *
     * <p>An invocation of this method is equivalent to (but may be
     * more efficient than):
     *  <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 {@code
     * tryAcquire} 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
     */
    //判断当前线程是否在sync queue同步队列中。
    final boolean isOnSyncQueue(Node node) {
        /**
         * 1、当前节点node的状态waitStatus为Node.CONDITION，则其必定还在条件队列condition中；
         * 2、如果当前节点node的node.prev == null，说明当前节点还未被加入到sync queue队列中，因为node节点要加入到sync queue中（无论本次加入是否成功），则本次尝试必有node.prev != null；
         */
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        //如果当前节点node.next != null，说明其已经有后继节点了，则其必定在sync queue同步队列中
        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.
         */
        /**
         * 能走到这里来，说明当前节点node.waitStatus != Node.CONDITION && node.prev != null && node.next == null
         * ，说明已经开始尝试将当前节点node加入到sync queue同步队列中，已经完成了node.prev = tail,但是进行接下来的CAS操作设置sync queue的尾节点，有可能失败
         * 导致当前节点入队失败，所以从sync queue的尾节点开始遍历，进一步确认当前节点是否成功加入到了sync queue中。
         */
        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
     */
    //判断指定的node节点是否在sync queue同步队列中
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        //从链表的尾部开始遍历
        for (;;) {
            //遍历到的当前节点等于指定的节点，则返回true
            if (t == node)
                return true;
            //遍历到的当前节点已经为空，说明链表已经遍历结束，未找到指定的节点，则返回false
            if (t == null)
                return false;
            //移动指针，进入下一次自旋遍历
            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.
         */
        //调用signal方法的时候，当前node节点已经取消了等待，则忽略这个节点
        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).
         */
        //如果这个节点node在条件队列condition中正常等待，则将node加入到sync queue同步队列中，并返回这个新结点的前驱结点
        Node p = enq(node);
        int ws = p.waitStatus;
        /**
         * 同步队列sync queue中的节点是通过前驱结点来唤醒的，前驱结点已经被取消，或者前驱结点的waitStatus设置Node.SIGNAL失败，
         * 后继节点不能通过前驱结点正常唤醒，则直接唤醒这个刚刚加入到同步队列sync queue中的节点
         */
        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) {
        /**
         * 判断一个node是否被signal()过，最简单有效的方式就是是否离开了condition条件队列，进入到了sync queue同步队列中
         */
        //如果一个节点的状态还是Node.CONDITION，说明它还未被signal过，是因为中断导致了它被唤醒（从condition条件队列转移到sync queue同步队列中）
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            //将当前被唤醒的节点加入到同步队列sync queue中，准备竞争锁，但是还没有将这个节点从条件队列中移除，所以后面的处理一下
            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
     */
    //完全释放线程所占用的锁
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            //获取当前锁状态
            int savedState = getState();
            //完全释放锁，对于重入锁而言，无论重入了几次都一次释放；同时持有锁的线程不是当前线程时候，也会直接抛出IllegalMonitorStateException异常，直接进入finally块
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            /**
             * 如果释放锁失败，则将刚才新加入到条件队列尾节点的node节点的waitStatus设置为Node.CANCELLED，
             * 后序其它线程被包装成Node节点加入到条件队列中时候，会将这些已经被取消的节点移除
             */
            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
         */
        //将当前线程包装成Node节点，加入到条件队列中
        private Node addConditionWaiter() {
            //条件队列的尾节点
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            //条件尾节点不为空，说明条件队列不为空，但是它的尾节点的waitStatus已经不是Node.CONDITION了，说明其已经取消了，则清除掉条件队列中已经取消的节点
            if (t != null && t.waitStatus != Node.CONDITION) {
                //清除掉条件队列中已经取消的节点
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            //将当前线程包装成Node节点，并将新的node节点的状态设置为Node.CONDITION
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            //如果条件队列为空，则将条件队列的头结点firstWaiter指向刚创建出来的node节点
            if (t == null)
                firstWaiter = node;
            else
                //否则， 将条件队列旧的尾节点的nextWaiter指向新创建的node节点
                t.nextWaiter = node;
            //将新创建的node节点设置为条件队列的尾节点
            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
         */
        //唤醒条件队列中第一个未被取消的节点
        private void doSignal(Node first) {
            do {
                //如果条件队列中只有一个节点，则将条件队列清空
                if ( (firstWaiter = first.nextWaiter) == null)
                    //则将条件队列的尾节点lastWaiter也置为null
                    lastWaiter = null;
                //将当前要被唤醒的节点从条件队列中移除
                first.nextWaiter = null;
                //当前节点已经被取消了，并且条件队列中还有节点，则进行下一次循环
            } 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) {
            //清空条件队列
            lastWaiter = firstWaiter = null;
            do {
                //获取条件队列头结点的下一个结点
                Node next = first.nextWaiter;
                //将条件队列的头结点从链表上移除
                first.nextWaiter = null;
                //将这个刚从条件队列condition中移除的节点转移到sync queue同步队列中
                transferForSignal(first);
                //修改指针，准备继续下一次的遍历
                first = next;
                //first ！= null表示链表还未遍历完成，继续下次遍历
            } 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() {
            //当前节点,从表示条件队列链表的头结点firstWaiter开始遍历
            Node t = firstWaiter;
            //表示条件队列中最后一个waitStatus不为Node.CANCELLED的节点
            Node trail = null;
            //条件队列不为空
            while (t != null) {
                //当前节点的后继节点
                Node next = t.nextWaiter;
                //如果当前节点t已经取消了正常等待，则将节点t从条件队列中移除
                if (t.waitStatus != Node.CONDITION) {
                    //将当前节点从条件队列移除
                    t.nextWaiter = null;
                    //trail == null表示条件队列当前节点之前的都被取消了，所以将条件队列的头结点firstWaiter重置指向next
                    if (trail == null)
                        firstWaiter = next;
                    else
                        //当前节点已经被取消，trail != null,则需要将trail与当前节点断开，指向next;
                        trail.nextWaiter = next;
                    //如果next == null说明队列已经完成遍历，将原来的条件队列中最后一个正常等待的节点赋值给lastWaiter；
                    if (next == null)
                        lastWaiter = trail;
                }
                else{
                    //如果当前节点未被取消，就将当前节点记为队列中最后一个未被取消的节点，令trail = t;
                    trail = t;
                }
                //修改当前节点的指针，指向下一个节点next
                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() {
            //判断当前线程是否是持有锁的线程，如果不是则直接抛出IllegalMonitorStateException异常
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            //条件队列condition不为空
            if (first != null)
                //唤醒条件队列condition中的头结点
                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() {
            //判断当前线程是否是持有锁的线程，如果不是，则直接抛出IllegalMonitorStateException异常
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            //判断条件队列是否为空
            if (first != null)
                //唤醒条件队列中进行等待的节点
                doSignalAll(first);
        }

        /**
         * Implements uninterruptible condition wait.
         * <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>
         */
        /**
         * await()方法中，中断和signal()起到了相同的效果，将当前线程唤醒，但是中断一个正常等待的线程，线程被唤醒后，
         * 抢到了锁，但是发现等待的条件还未满足，线程会重新被挂起，所以我们希望有一个方法，在等待的时候，不要响应线程中断，
         * 所以下面awaitUnitertuptibly()方法就实现了这个功能。
         */
        public final void awaitUninterruptibly() {
            //将当前线程包装成Node节点，加入到条件队列condition中
            Node node = addConditionWaiter();
            //释放当前线程所持有的锁
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            //判断当前线程是否已经在sync queue同步队列
            while (!isOnSyncQueue(node)) {
                //当前线程未在sync queue同步队列中，说明其还没有资格竞争独占锁，将当前线程挂起
                LockSupport.park(this);
                /**
                 * 判断当前线程是否是由于被其他线程中断唤醒的，如果是则只是简单记录下其中断标记即可，
                 * 当前线程要跳出此while循环则满足的唯一条件是，被其他线程signal()，进入到同步队列
                 */
                if (Thread.interrupted())
                    interrupted = true;
            }
            
            /**
             * 这也是一个阻塞操作，当前线程已经加入到同步队列中，尝试竞争锁，竞争失败，找一个合适的地方，
             * 将当前线程挂起，等待其他线程signal()，如果在竞争锁的过程中，发生了异常，则需要记录一下
             */
            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) {
            //判断在等待的过程中，线程是否发生了中断
            return Thread.interrupted() ?
                    //判断线程中断时发生在signal()之前，还是signal()之后，THROW_IE表示中断发生在signal()之前，REINTERRUPT表示中断发生在signal()之后，中断来得太晚了
                (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                0;
        }

        /**
         * Throws InterruptedException, reinterrupts current thread, or
         * does nothing, depending on mode.
         */
        private void reportInterruptAfterWait(int interruptMode)
            throws InterruptedException {
            //表示还未被其他线程signal()就发生了中断，当前线程是由于中断被唤醒的，所以抛出InterruptedException异常
            if (interruptMode == THROW_IE)
                throw new InterruptedException();
            //表示被其它线程signal()过了之后才发生了线程中断，中断来得太迟了，所以补一下中断标记就行了
            else if (interruptMode == REINTERRUPT)
                selfInterrupt();
        }

        /**
         * Implements interruptible condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled 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 {
            //调用await()方法时，当前线程已经被中断，则直接抛出中断异常
            if (Thread.interrupted())
                throw new InterruptedException();
            //将当前线程包装成Node节点，加入到条件队列中
            Node node = addConditionWaiter();
            //释放当前线程所占用的锁
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            //判断代表当前线程节点是否在同步队列中，如果不在，说明还没有其它线程调用signal()方法，将当前线程从条件队列Conditon种转移到同步队列中
            while (!isOnSyncQueue(node)) {
                //当前线程还在条件队列中，没有资格竞争锁，则直接将当前线程挂起
                LockSupport.park(this);
                //能走到这里，说明当前线程被其他线程signal()或者被其它线程中断了
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            //interruptMode != THROW_IE表示当前线程不是由于线程中断而被唤醒的
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            //上面将从条件队列condition中唤醒的节点加入到sync queue条件队列中时候，并没有从条件队列中移除，这里需要清理一下
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            //当前节点在等待的过程中发生了中断
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

        /**
         * Implements timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        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.
         * <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.
         * <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);
    }
}