AQS 源码阅读

AQS 概念和数据结构介绍

AbstractQueuedSynchronizer 代码较多，对于不熟悉的同学来说，阅读可能有点困难。
此处先整理介绍几个重要的概念和数据，方便后面一行一行的死磕代码。

• AbstractQueuedSynchronizer 翻译： 抽象队列同步器，AQS是一种提供了原子式管理同步状态、阻塞和唤醒线程功能以及队列模型的简单框架。
• AQS定义两种资源共享方式：独占exclusive模式 和 共享shared模式

• 数据结构：共享资源（volatile int state） 和 静态内部类Node实现的CLH 队列（双向链表，使用时FIFO）

  
  
  image.png

代码阅读

一行一行的死磕。

类结构

类 AbstractQueuedSynchronizer 继承了 AbstractOwnableSynchronizer。AbstractOwnableSynchronizer 是抽象类。

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() { }
}

AbstractOwnableSynchronizer 抽象可独占同步器。

/**
 * A synchronizer that may be exclusively owned by a thread.  This
 * class provides a basis for creating locks and related synchronizers
 * that may entail a notion of ownership.  The
 * {@code AbstractOwnableSynchronizer} class itself does not manage or
 * use this information. However, subclasses and tools may use
 * appropriately maintained values to help control and monitor access
 * and provide diagnostics.
 * 独占模式的同步器。此类为创建可能需要所有权概念的锁和相关的同步器提供了基础。
 * 类 AbstractOwnableSynchronizer 自己不管理或使用这些信息。
 * 但是，子类和工具可以使用适当维护的值来帮助控制和监视访问并提供诊断。
 *
 * @since 1.6
 * @author Doug Lea
 */
public abstract class AbstractOwnableSynchronizer
    implements java.io.Serializable {

    protected AbstractOwnableSynchronizer() { }

    /**
     * The current owner of exclusive mode synchronization.
     * 独占模式下的拥有锁的线程
     */
    private transient Thread exclusiveOwnerThread;

    /**
     * Sets the thread that currently owns exclusive access.
     * A {@code null} argument indicates that no thread owns access.
     * This method does not otherwise impose any synchronization or
     * {@code volatile} field accesses.
     * 设置当前拥有独占访问权的线程。
     * null 表明表示没有线程拥有访问权限
     * 
     * @param thread the owner thread
     */
    protected final void setExclusiveOwnerThread(Thread thread) {
        exclusiveOwnerThread = thread;
    }

    /**
     * Returns the thread last set by {@code setExclusiveOwnerThread},
     * or {@code null} if never set.  This method does not otherwise
     * impose any synchronization or {@code volatile} field accesses.
     * 返回最近一次使用 setExclusiveOwnerThread 方法设置的线程
     * @return the owner thread
     */
    protected final Thread getExclusiveOwnerThread() {
        return exclusiveOwnerThread;
    }
}

数据结构

静态内部类 Node，用于构建CLH 队列（双向列表），用作AQS的阻塞队列，Condition的等待队列也是通过 Node 实现的。

/**
 * Wait queue node class.
 * 等待队列节点类
 *
 * 
The wait queue is a variant of a "CLH" (Craig, Landin, and
 * Hagersten) lock queue. CLH locks are normally used for
 * spinlocks.  We instead use them for blocking synchronizers, but
 * use the same basic tactic of holding some of the control
 * information about a thread in the predecessor of its node.  A
 * "status" field in each node keeps track of whether a thread
 * should block.  A node is signalled when its predecessor
 * releases.  Each node of the queue otherwise serves as a
 * specific-notification-style monitor holding a single waiting
 * thread. The status field does NOT control whether threads are
 * granted locks etc though.  A thread may try to acquire if it is
 * first in the queue. But being first does not guarantee success;
 * it only gives the right to contend.  So the currently released
 * contender thread may need to rewait.
 * 这个等待队列是 CLH 锁队列的变体。CLH 通常用于自旋锁。
 * 相反，我们将它们用于阻塞同步器，但是使用相同的基本策略，
 * 将有关线程的某些控制信息保存在其节点的前身中。
 * 每个节点中的“status”字段将跟踪线程是否应阻塞。
 * 节点的前任释放时会发出信号。
 * 否则，队列的每个节点都充当一个特定通知样式的监视器，其中包含一个等待线程。
 * 虽然状态字段不控制线程是否被授予锁等。
 * 如果线程在队列的第一位置，线程可能会尝试获取它。
 * 但是第一个并不能保证成功。它只赋予了竞争的权利。
 * 因此，当前发布的竞争者线程可能需要重新等待。
 *
 * 
To enqueue into a CLH lock, you atomically splice it in as new
 * tail. To dequeue, you just set the head field.
 * 
 *      +------+  prev +-----+       +-----+
 * head |      | <---- |     | <---- |     |  tail
 *      +------+       +-----+       +-----+
 * 
 *
 * 
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.
 *
 * 
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/
 *
 * 
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.)
 *
 * 
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.
 *
 * 
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.
 *
 * 
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.
 *
 * 
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 */
    // 标志：表明一个节点是在共享模式下等待
    static final Node SHARED = new Node();
    /** Marker to indicate a node is waiting in exclusive mode */
    // 标志：表明一个节点是在独占模式下等待
    static final Node EXCLUSIVE = null;

    /** waitStatus value to indicate thread has cancelled */
    // 表明线程被取消
    // static final int CANCELLED =  1;
    /** waitStatus value to indicate successor's thread needs unparking */
    // 表示后继线程，需要通知取消挂起
    static final int SIGNAL    = -1;
    /** waitStatus value to indicate thread is waiting on condition */
    // 表示线程正在等待条件 condition
    static final int CONDITION = -2;
    /**
     * waitStatus value to indicate the next acquireShared should
     * unconditionally propagate
     * 下一个共享获取应该无条件传播
     */
    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.
     * SIGNAL：后继节点被阻塞，所以当前节点 release 或者 cancel 时， 必须取消挂起后继节点
     *   CANCELLED:  This node is cancelled due to timeout or interrupt.
     *               Nodes never leave this state. In particular,
     *               a thread with cancelled node never again blocks.
     * CANCELLED： 当前节点因为 timeout 或者 interrupt 中断被 cancel。节点不会离开这个状态。尤其 cancelled 线程不会再被阻塞
     *   CONDITION:  This node is currently on a condition queue.
     *               It will not be used as a sync queue node
     *               until transferred, at which time the status
     *               will be set to 0. (Use of this value here has
     *               nothing to do with the other uses of the
     *               field, but simplifies mechanics. 
     * CONDITION： 这个节点正在一个 condition 队列中。他不会在阻塞队列中使用，直到被转移。此时 status 被置0
     *   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.
     * PROPAGATE： 共享释放应该传输到其他节点。
     *   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.
     * 链接到 当前节点线程用来检查waitStatus的 先前节点。
     * 入队时分配，出队时置null.
     * 同样，在取消前任后，我们会短路，同时找到一个未取消的前任，这将一直存在，因为根节点从未取消:
     * 仅当成功获取节点后，节点才会成为头部.
     * 被取消的线程永远不会成功获取，并且一个线程只会取消自身，而不会取消任何其他节点。
     */
    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.
     * 链接到 当前节点线程在释放时将取消挂起的 后继节点。
     * 入队时分配，在取消的前任对象时进行调整，出队时置null。
     * 入队操作，并不分配 next 字段直到附属之后，所以当看到next字段为null时，不一定时队列的结束。
     * 但是，next 字段为 null 的情况出现，可以从结尾扫描前置节点做double check。cancceld 节点的 next 字段会被置为自己而不是null, 使isOnSyncQueue更加简单。
     */
    volatile Node next;

    /**
     * The thread that enqueued this node.  Initialized on
     * construction and nulled out after use.
     * 使该节点排队的线程，在构造中初始化，在用完之后置 null
     */
    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
     * 链接到 限一个等待condition的节点或者特定值SHARED
     * 因为条件队列仅在以独占模式保存时才被访问，所以我们只需要一个简单的链接队列即可在节点等待条件时保存节点。
     * 然后将它们转移到队列中以重新获取。
     * 并且由于conditions只能是互斥的，因此我们通过使用特殊值来表示共享模式来保存字段
     * mode.
     */
    Node nextWaiter;

    /**
     * Returns true if node is waiting in shared mode.
     * 节点时共享模式时，返回true
     */
    final boolean isShared() {
        return nextWaiter == SHARED;
    }

    /**
     * Returns previous node, or throws NullPointerException if null.
     * Use when predecessor cannot be null.  The null check could
     * be elided, but is present to help the VM.
     * 返回前置节点
     *
     * @return the predecessor of this node
     */
    final Node predecessor() throws NullPointerException {
        Node p = prev;
        if (p == null)
            throw new NullPointerException();
        else
            return p;
    }
    
    // 下面是构造

    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;
    }
}

成员变量介绍

head 加 tail + 上文提及的 Node 数据结构，构成双向列表。
state 是同步状态。

/**
 * Head of the wait queue, lazily initialized.  Except for
 * initialization, it is modified only via method setHead.  Note:
 * If head exists, its waitStatus is guaranteed not to be
 * CANCELLED.
 * 等待队列的头节点，懒加载。
 * 除初始化外，只能通过setHead方法进行修改。注意：如果head存在，则保证其waitStatus不被取消。
 */
private transient volatile Node head;

/**
 * Tail of the wait queue, lazily initialized.  Modified only via
 * method enq to add new wait node.
 * 队列的尾节点，懒加载。
 * 仅通过方法enq进行修改以添加新的等待节点
 */
private transient volatile Node tail;

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

/**
 * 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.
 * 相对于使用定时park而言，spin更快的纳秒数。粗略估计足以在非常短的超时时间内提高响应能力
 */
static final long spinForTimeoutThreshold = 1000L;

方法介绍

compareAndSetState 原子方式更新同步状态 state。

/**
 * 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.
 * 如果当前状态值等于期望值，则以原子方式将同步状态设置为给定的更新值。
 *
 * @param expect the expected value
 * @param update the new value
 * @return {@code true} if successful. False return indicates that the actual
 *         value was not equal to the expected value.
 */
protected final boolean compareAndSetState(int expect, int update) {
    // See below for intrinsics setup to support this
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

入队方法 enq

/**
 * Inserts node into queue, initializing if necessary. See picture above.
 * 入队操作
 * @param node the node to insert
 * @return node's predecessor 返回前置节点
 */
private Node enq(final Node node) {
    // 死循环， 直到入队操作成功
    for (;;) {
        Node t = tail;
        // 未初始化，需要初始化 head 和 tail 为同一节点, 进入下一次循环
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            // 已初始化， 设置当前节点的前置节点置为tail
            // head 节点的next = node
            // tail = node
            // node.prev = head
            node.prev = t;
            if (compareAndSetTail(t, node)) {
                t.next = node;
                return t;
            }
        }
    }
}

/**
 * CAS head field. Used only by enq.
 * cas 设置head节点,仅用于入队方法
 */
private final boolean compareAndSetHead(Node update) {
    return unsafe.compareAndSwapObject(this, headOffset, null, update);
}

/**
 * CAS tail field. Used only by enq.
 * cas 设置tail节点,仅用于入队方法
 */
private final boolean compareAndSetTail(Node expect, Node update) {
    return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}

将当前线程和指定模式（独占/共享）创建和入队节点 addWaiter 方法：

/**
 * Creates and enqueues node for current thread and given mode.
 * 将当前线程和指定模式（独占/共享）创建和入队节点
 *
 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 * @return the new node
 */
private Node addWaiter(Node mode) {
    // 创建入队的节点
    Node node = new Node(Thread.currentThread(), mode);
    // Try the fast path of enq; backup to full enq on failure
    // 如果tail 不为null
    // node.prev = tail
    // cas tail = node, 如果成功 tail.next = node, 返回node
    // 如果失败，执行enq操作
    
    Node pred = tail;
    if (pred != null) {
        node.prev = pred;
        if (compareAndSetTail(pred, node)) {
            pred.next = node;
            return node;
        }
    }
    enq(node);
    return node;
}

唤醒后置节点 unparkSuccessor：

/**
 * Wakes up node's successor, if one exists.
 * 唤醒后置节点
 *
 * @param node the node
 */
private void unparkSuccessor(Node node) {
    /*
     * If status is negative (i.e., possibly needing signal) try
     * to clear in anticipation of signalling.  It is OK if this
     * fails or if status is changed by waiting thread.
     * 如果状态是negative（即可能需要信号），尝试清除以预期发出信号。如果失败或等待线程更改状态，是没有关系的。
     */
    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.
     * 释放线程将保留在后续线程中，该线程通常只是下一个节点。
     * 但是，如果取消或出现null，从尾部向后移动以找到实际的未取消后继节点。
     */
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {
        s = null;
        for (Node t = tail; t != null && t != node; t = t.prev)
            if (t.waitStatus <= 0)
                s = t;
    }
    // 唤醒节点
    if (s != null)
        LockSupport.unpark(s.thread);
}

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

共享模式释放 doReleaseShared。

/**
 * Release action for shared mode -- signals successor and ensures
 * propagation. (Note: For exclusive mode, release just amounts
 * to calling unparkSuccessor of head if it needs signal.)
 * 共享模式的释放动作：
 * 通知后继节点并保证传播。
 * 对于独占模式，如果需要信号，释放仅相当于调用head的unparkSuccessor。
 */
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.
     * 即使有其他正在进行的acquires/releases，也要确保发行传播。
     * 如果需要信号，将以尝试取消headSuccessor的常规方式进行。
     * 但是，如果没有，则将状态设置为PROPAGATE，以确保释放后继续传播。
     * 此外，在执行此操作时，必须循环以防添加新节点。另外，与unparkSuccessor的其他用法不同，
     * 我们需要知道CAS重置状态是否失败，如果重新检查。
     *
     */
    // 死循环
    for (;;) {
        Node h = head;
        // 队列非空
        if (h != null && h != tail) {
            // 如果 waitStatus 为 SIGNAL，尝试 cas 变更 waitStatus 为0， 如果失败重试循环，否则执行unparkSuccessor唤醒后继节点
            // 如果 waitStatus 为 0 且 尝试 cas 变更 waitStatus 为 PROPAGATE， 如果失败重试循环
            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
        }
        // head 发生变化，需要继续循环，否则终止循环
        if (h == head)                   // loop if head changed
            break;
    }
}

设置 head 节点，并执行传播动作

/**
 * 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.
 * 设置队列的头部，并检查后继者是否可能在共享模式下等待，如果是，且 propagate > 0或PROPAGATE状态被设置，则传播。
 *
 * @param node the node
 * @param propagate the return value from a tryAcquireShared
 */
private void setHeadAndPropagate(Node node, int propagate) {
    Node h = head; // Record old head for check below
    setHead(node);
    /*
     * Try to signal next queued node if:
     *   Propagation was indicated by caller,
     *     or was recorded (as h.waitStatus either before
     *     or after setHead) by a previous operation
     *     (note: this uses sign-check of waitStatus because
     *      PROPAGATE status may transition to SIGNAL.)
     * and
     *   The next node is waiting in shared mode,
     *     or we don't know, because it appears null
     *
     * The conservatism in both of these checks may cause
     * unnecessary wake-ups, but only when there are multiple
     * racing acquires/releases, so most need signals now or soon
     * anyway.
     */
    if (propagate > 0 || h == null || h.waitStatus < 0 ||
        (h = head) == null || h.waitStatus < 0) {
        Node s = node.next;
        if (s == null || s.isShared())
            doReleaseShared();
    }
}

取消正在进行的 acquire 尝试 cancelAcquire。

/**
 * Cancels an ongoing attempt to acquire.
 * 取消正在进行的 acquire 尝试
 * @param node the node
 */
private void cancelAcquire(Node node) {
    // Ignore if node doesn't exist
    if (node == null)
        return;

    node.thread = null;

    // Skip cancelled predecessors
    // 跳过 cancelled 前置节点，CANCELLED=1
    Node pred = node.prev;
    while (pred.waitStatus > 0)
        node.prev = pred = pred.prev;

    // predNext is the apparent node to unsplice. CASes below will
    // fail if not, in which case, we lost race vs another cancel
    // or signal, so no further action is necessary.
    // predNext是要取消拼接的明显节点。如果没有，以下情况将失败，
    // 在这种情况下，我们输掉了比赛，而另一个取消或发出信号，因此不需要采取进一步的措施。
    Node predNext = pred.next;

    // Can use unconditional write instead of CAS here.
    // After this atomic step, other Nodes can skip past us.
    // Before, we are free of interference from other threads.
    // 可以在此处使用无条件写入而不是CAS。
    // 完成这一基本步骤后，其他节点可以跳过我们。在此以前，我们不受其他线程的干扰。
    node.waitStatus = Node.CANCELLED;

    // If we are the tail, remove ourselves.
    // 如果是尾节点， 移除当前节点
    if (node == tail && compareAndSetTail(node, pred)) {
        compareAndSetNext(pred, predNext, null);
    } else {
        // If successor needs signal, try to set pred's next-link
        // so it will get one. Otherwise wake it up to propagate.
        // 如果后置节点需要通知，尝试赋值pred.next = node.next,即跳过当前节点
        int ws;
        if (pred != head &&
            ((ws = pred.waitStatus) == Node.SIGNAL ||
             (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
            pred.thread != null) {
            Node next = node.next;
            if (next != null && next.waitStatus <= 0)
                compareAndSetNext(pred, predNext, next);
        } else {
            // 唤醒后置节点
            unparkSuccessor(node);
        }

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

检查是否需要挂起并更新无法获取的节点的状态 shouldParkAfterFailedAcquire。

/**
 * 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.
 * 检查并更新无法获取的节点的状态。
 * 如果线程应阻塞，则返回true。
 * 这是所有采集循环中的主要信号控制。要求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;
    if (ws == Node.SIGNAL)
        /*
         * This node has already set status asking a release
         * to signal it, so it can safely park.
         */
        return true;
    if (ws > 0) {
        /*
         * Predecessor was cancelled. Skip over predecessors and
         * indicate retry.
         * 跳过 cancelled 节点
         */
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
        /*
         * waitStatus must be 0 or PROPAGATE.  Indicate that we
         * need a signal, but don't park yet.  Caller will need to
         * retry to make sure it cannot acquire before parking.
         * cas 设置 waitStatus 为 SIGNAL
         */
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}

exclusive uninterruptible 模式 acquere 方法：acquireQueued

/*
 * 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.
 * 各种获取方式，互斥共享和控制方式不同。
 * 每个都大体相同，但令人讨厌的不同。
 * 由于异常机制（包括确保在tryAcquire抛出异常时我们取消）和其他控件的相互作用，因此只有少量分解是可能的，至少在不影响性能的前提下。
 */

/**
 * Acquires in exclusive uninterruptible mode for thread already in
 * queue. Used by condition wait methods as well as acquire.
 * 以exclusive uninterruptible 模式获取已在队列中的线程。用于条件等待方法以及获取。
 *
 * @param node the node
 * @param arg the acquire argument
 * @return {@code true} if interrupted while waiting
 */
final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        // 死循环
        for (;;) {
            // 获取当前节点的前置节点
            final Node p = node.predecessor();
            // 如果当前节点的前置节点是head节点，执行独占模式获取方法，由字类实现
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            // 检查是否需要挂起并更新无法获取的节点的状态 且 挂起并检查是否中断
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        // 确保一定执行，如果 acquire 失败， 执行 cancelAcquire
        if (failed)
            cancelAcquire(node);
    }
}

/**
 * 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.
 * 独占模式acquire方法，这个方法需要先查询state是否允许acquire，如果可以再执行acquire方法
 * protected 方法由字类实现，否则抛出异常
 *
 * 
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()}.
 *
 * 
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();
}

    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

/**
 * Acquires in exclusive interruptible mode.
 * exclusive interruptible 模式下 acquire
 * @param arg the acquire argument
 */
private void doAcquireInterruptibly(int arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.EXCLUSIVE);
    boolean failed = true;
    try {
        // 死循环
        for (;;) {
            // 当前节点的前置节点
            final Node p = node.predecessor();
            // 如果当前节点前置节点是head, 执行独占模式获取方法
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return;
            }
            // 检查是否需要挂起并更新无法获取的节点的状态 ，如果interrupted返回true, 抛出异常
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

独享定时模式下 acquire 方法：doAcquireNanos

/**
 * Acquires in exclusive timed mode.
 * 独享定时模式下 acquire
 *
 * @param arg the acquire argument
 * @param nanosTimeout max wait time
 * @return {@code true} if acquired
 */
private boolean doAcquireNanos(int arg, long nanosTimeout)
        throws InterruptedException {
    if (nanosTimeout <= 0L)
        return false;
    final long deadline = System.nanoTime() + nanosTimeout;
    // 独占模式入队
    final Node node = addWaiter(Node.EXCLUSIVE);
    boolean failed = true;
    try {
        // 死循环
        for (;;) {
            // 前置节点p
            final Node p = node.predecessor();
            // 如果前置节点是head 尝试获取成功
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return true;
            }
            
            // 超时返回false
            nanosTimeout = deadline - System.nanoTime();
            if (nanosTimeout <= 0L)
                return false;
                
            // 获取失败需要挂起 且 超时时间 大于自旋阈值， 挂起线程
            if (shouldParkAfterFailedAcquire(p, node) &&
                nanosTimeout > spinForTimeoutThreshold)
                LockSupport.parkNanos(this, nanosTimeout);
            
            // 如果当前线程被中断，抛出异常           
            if (Thread.interrupted())
                throw new InterruptedException();
        }
    } finally {
        // 获取失败，执行cancelAcquire
        if (failed)
            cancelAcquire(node);
    }
}

shared uninterruptible 模式下获取doAcquireShared

/**
 * Acquires in shared uninterruptible mode.
 * shared uninterruptible 模式下获取
 * @param arg the acquire argument
 */
private void doAcquireShared(int arg) {
    // 入队
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        boolean interrupted = false;
        // 死循环
        for (;;) {
            // 前置节点
            final Node p = node.predecessor();
            // 如果前置节点是head, 尝试获取
            if (p == head) {
                int r = tryAcquireShared(arg);
                // 获取成功，设置 head 节点，并执行传播动作
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    if (interrupted)
                        selfInterrupt();
                    failed = false;
                    return;
                }
            }
            // 判断是否需要做获取失败挂起 和 挂起检查interrupt状态
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        // 如果失败，执行cancelAcquire
        if (failed)
            cancelAcquire(node);
    }
}

shared interruptible 模式下获取方法doAcquireSharedInterruptibly

/**
 * Acquires in shared interruptible mode.
 * shared interruptible 模式下获取
 * @param arg the acquire argument
 */
private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    // 入队
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        // 死循环
        for (;;) {
            final Node p = node.predecessor();
            // 如果前置节点是head, 尝试获取 获取成功，设置 head 节点，并执行传播动作
            if (p == head) {
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            // 判断是否需要做获取失败挂起 和 挂起检查interrupt状态， 如果interrupted 抛出异常
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        // 如果失败，执行cancelAcquire
        if (failed)
            cancelAcquire(node);
    }
}

shared timed 模式下获取doAcquireSharedNanos

/**
 * Acquires in shared timed mode.
 * shared timed 模式下获取
 *
 * @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;
    // 入队
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        // 死循环
        for (;;) {
            // 如果前置节点是head, 尝试获取 获取成功，设置 head 节点，并执行传播动作
            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;
                }
            }
            // 超时 返回false
            nanosTimeout = deadline - System.nanoTime();
            if (nanosTimeout <= 0L)
                return false;
            // 获取失败需要挂起 且 超时时间 大于自旋阈值， 挂起线程
            if (shouldParkAfterFailedAcquire(p, node) &&
                nanosTimeout > spinForTimeoutThreshold)
                LockSupport.parkNanos(this, nanosTimeout);
            // 如果interrupted 抛出异常
            if (Thread.interrupted())
                throw new InterruptedException();
        }
    } finally {
        // 如果失败，执行cancelAcquire
        if (failed)
            cancelAcquire(node);
    }
}

需要字类实现的几个方法：

//  exclusive 模式下 尝试获取
protected boolean tryAcquire(int arg) {
    throw new UnsupportedOperationException();
}

//  尝试设置状态以反映exclusive模式下的release。
protected boolean tryRelease(int arg) {
    throw new UnsupportedOperationException();
}

//  shared 模式下 尝试获取
protected int tryAcquireShared(int arg) {
    throw new UnsupportedOperationException();
}

//  尝试设置状态以反映 shared 模式下的release。
protected boolean tryReleaseShared(int arg) {
    throw new UnsupportedOperationException();
}

//  exclusive 模式下， 状态是否被占用
protected boolean isHeldExclusively() {
    throw new UnsupportedOperationException();
}

主要的公开方法

exclusive 模式获取状态，忽略中断。 acquire

/**
 * 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}.
 * exclusive 模式获取状态，忽略中断。
 *
 *
 * @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) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

exclusive 模式获取状态，如果中断抛出异常acquireInterruptibly

/**
 * 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}.
 * exclusive 模式获取状态，如果中断抛出异常
 *
 * @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);
}

exclusive 模式获取状态，如果中断抛出异常，超时失败 tryAcquireNanos

/**
 * 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)}.
 * exclusive 模式获取状态，如果中断抛出异常，超时失败
 *
 * @param arg the acquire argument.  This value is conveyed to
 *        {@link #tryAcquire} but is otherwise uninterpreted and
 *        can represent anything you like.
 * @param nanosTimeout the maximum number of nanoseconds to wait
 * @return {@code true} if acquired; {@code false} if timed out
 * @throws InterruptedException if the current thread is interrupted
 */
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    return tryAcquire(arg) ||
        doAcquireNanos(arg, nanosTimeout);
}

exclusive 模式的释放状态:release

/**
 * 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}.
 * exclusive 模式的释放状态
 *
 * @param arg the release argument.  This value is conveyed to
 *        {@link #tryRelease} but is otherwise uninterpreted and
 *        can represent anything you like.
 * @return the value returned from {@link #tryRelease}
 */
public final boolean release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);
        return true;
    }
    return false;
}

shared 模式获取状态，忽略中断

/**
 * 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.
 * shared 模式获取状态，忽略中断
 * @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);
}

shared 模式获取状态，中断抛出异常acquireSharedInterruptibly

/**
 * 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.
 * shared 模式获取状态，中断抛出异常
 * @param arg the acquire argument.
 * This value is conveyed to {@link #tryAcquireShared} but is
 * otherwise uninterpreted and can represent anything
 * you like.
 * @throws InterruptedException if the current thread is interrupted
 */
public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}

shared 模式获取状态，中断抛出异常 ，超时抛出异常tryAcquireSharedNanos

/**
 * 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.
 * shared 模式获取状态，中断抛出异常 ，超时抛出异常
 * 
 * @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);
}

shared 模式释放状态releaseShared

/**
 * Releases in shared mode.  Implemented by unblocking one or more
 * threads if {@link #tryReleaseShared} returns true.
 * shared 模式释放状态
 * @param arg the release argument.  This value is conveyed to
 *        {@link #tryReleaseShared} but is otherwise uninterpreted
 *        and can represent anything you like.
 * @return the value returned from {@link #tryReleaseShared}
 */
public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        doReleaseShared();
        return true;
    }
    return false;
}

队列检查相关方法

/**
 * Queries whether any threads are waiting to acquire. Note that
 * because cancellations due to interrupts and timeouts may occur
 * at any time, a {@code true} return does not guarantee that any
 * other thread will ever acquire.
 * 查看是否有线程在等待获取资源。
 * 注意：由于中断和超时导致的cancel可能随时发生，因此 true 返回值不能保证任何其他线程都可以获取。
 *
 * 
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.
 * 查询是否有任何线程争夺该同步器
 *
 * 
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.
 * 返回队列中第一个线程
 *
 * 
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.
 * 如果给定线程当前正在排队，则返回true。
 *
 * 
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.
 * 如果明显的第一个排队线程（如果存在）正在exclusive模式下等待，则返回{@code true}。
 */
final boolean apparentlyFirstQueuedIsExclusive() {
    Node h, s;
    return (h = head) != null &&
        (s = h.next)  != null &&
        !s.isShared()         &&
        s.thread != null;
}

/**
 * Queries whether any threads have been waiting to acquire longer
 * than the current thread.
 * 查询是否有任何线程在等待获取比当前线程更长的时间
 *
 * 
An invocation of this method is equivalent to (but may be
 * more efficient than):
 *  
 {@code
 * getFirstQueuedThread() != Thread.currentThread() &&
 * hasQueuedThreads()}
 *
 * 
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.
 *
 * 
This method is designed to be used by a fair synchronizer to
 * avoid barging.
 * 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:
 *
 *  
 {@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
 *   }
 * }}
 *
 * @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.
 * 返回队列估算的长度，因为队列一直在变化
 *
 * @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.
 *  返回一个包含可能正在等待获取的线程的集合
 *
 * @return the collection of threads
 */
public final Collection getQueuedThreads() {
    ArrayList list = new ArrayList();
    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.
 * exclusive模式下，返回一个包含可能正在等待获取的线程的集合。
 * @return the collection of threads
 */
public final Collection getExclusiveQueuedThreads() {
    ArrayList list = new ArrayList();
    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.
 * shared 模式下，返回一个包含可能正在等待获取的线程的集合。
 *
 * @return the collection of threads
 */
public final Collection getSharedQueuedThreads() {
    ArrayList list = new ArrayList();
    for (Node p = tail; p != null; p = p.prev) {
        if (p.isShared()) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
    }
    return list;
}

Conditions 相关的内部支持方法

/**
 * Returns true if a node, always one that was initially placed on
 * a condition queue, is now waiting to reacquire on sync queue.
 * 如果一个节点（总是一个最初放置在条件队列中的节点）现在正等待在同步队列上重新获取，则返回true。
 * @param node the node
 * @return true if is reacquiring
 */
final boolean isOnSyncQueue(Node node) {
    if (node.waitStatus == Node.CONDITION || node.prev == null)
        return false;
    if (node.next != null) // If has successor, it must be on queue
        return true;
    /*
     * node.prev can be non-null, but not yet on queue because
     * the CAS to place it on queue can fail. So we have to
     * traverse from tail to make sure it actually made it.  It
     * will always be near the tail in calls to this method, and
     * unless the CAS failed (which is unlikely), it will be
     * there, so we hardly ever traverse much.
     */
    return findNodeFromTail(node);
}

/**
 * Returns true if node is on sync queue by searching backwards from tail.
 * 从同步队列尾部开始查找节点， 找到则返回true
 * Called only when needed by isOnSyncQueue.
 * @return true if present
 */
private boolean findNodeFromTail(Node node) {
    Node t = tail;
    for (;;) {
        if (t == node)
            return true;
        if (t == null)
            return false;
        t = t.prev;
    }
}

/**
 * Transfers a node from a condition queue onto sync queue.
 * 将节点由条件队列转移到同步队列
 * 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.
     * 如果无法修改waitStatus，这个节点就是被cancelled
     */
    if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
        return false;

    /*
     * Splice onto queue and try to set waitStatus of predecessor to
     * indicate that thread is (probably) waiting. If cancelled or
     * attempt to set waitStatus fails, wake up to resync (in which
     * case the waitStatus can be transiently and harmlessly wrong).
     * 节点加入同步队列，并尝试设置前置节点状态为SIGNAL，表明这个线程在等待。
     * 如果前置节点被cancel 或者 设置waitStatus失败，唤醒重新同步
     */
     
    Node p = enq(node);
    int ws = p.waitStatus;
    if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        LockSupport.unpark(node.thread);
    return true;
}

/**
 * Transfers node, if necessary, to sync queue after a cancelled wait.
 * Returns true if thread was cancelled before being signalled.
 * 如果有必要，将节点转移到同步队列，在cancel 等待之后
 *
 * @param node the node
 * @return true if cancelled before the node was signalled
 */
final boolean transferAfterCancelledWait(Node node) {
    if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
        enq(node);
        return true;
    }
    /*
     * If we lost out to a signal(), then we can't proceed
     * until it finishes its enq().  Cancelling during an
     * incomplete transfer is both rare and transient, so just
     * spin.
     */
    while (!isOnSyncQueue(node))
        Thread.yield();
    return false;
}

/**
 * Invokes release with current state value; returns saved state.
 * Cancels node and throws exception on failure.
 * 执行 release 当前状态，返回已保存的状态。失败情况下 会cancel node 并抛出异常
 * @param node the condition node for this wait
 * @return previous sync state
 */
final int fullyRelease(Node node) {
    boolean failed = true;
    try {
        int savedState = getState();
        if (release(savedState)) {
            failed = false;
            return savedState;
        } else {
            throw new IllegalMonitorStateException();
        }
    } finally {
        if (failed)
            node.waitStatus = Node.CANCELLED;
    }
}
/**
 * Queries whether the given ConditionObject
 * uses this synchronizer as its lock.
 * 查询给定的ConditionObject是否使用此同步器作为锁
 *
 * @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.
 * 查询是否有任何线程正在等待与此同步器关联的给定条件。
 * 请注意，由于超时和中断可能随时发生，因此返回 true 并不能保证将来的signal 会唤醒任何线程。
 * 此方法主要设计用于监视系统状态。
 *
 * @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.
 * 返回等待与此同步器关联的给定条件的线程数的估计值。
 * 请注意，由于超时和中断可能随时发生，因此估算值仅用作实际侍者数的上限。
 * 此方法设计用于监视系统状态，而不用于同步控制。
 *
 * @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.
 * 返回一个集合，其中包含那些可能正在等待与此同步器相关的给定条件的线程。
 * 因为实际的线程集在构造此结果时可能会动态变化，
 * 所以返回的集合只是尽力而为的估计。返回的集合的元素没有特定的顺序。
 *
 * @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 getWaitingThreads(ConditionObject condition) {
    if (!owns(condition))
        throw new IllegalArgumentException("Not owner");
    return condition.getWaitingThreads();
}

以上方法都涉及到一个内部类 ConditionObject。ConditionObject 实现了 Condition 接口, 结构如下：

image.png

Condition 接口方法：

public interface Condition {
    /**
    * 当前线程进入等待状态，
    * 如果在等待状态中被中断会抛出被中断异常；
    */
    void await() throws InterruptedException;
    /**
    * 当前线程进入等待状态，不会响应线程中断操作，只能通过唤醒的方式让线程继续
    */
    void awaitUninterruptibly();
    /**
    * 当前线程进入等待状态直到被通知，中断或者超时；
    */
    long awaitNanos(long nanosTimeout) throws InterruptedException;
    /**
    * 当前线程进入等待状态直到被通知，中断或者超时，支持自定义时间单位
    */  
    boolean await(long time, TimeUnit unit) throws InterruptedException;
    /**
    * 当前线程进入等待状态直到被通知，中断或者超时， 指定超时时间点
    */      
    boolean awaitUntil(Date deadline) throws InterruptedException;
    /**
    * 唤醒一个等待在condition上的线程，将该线程从等待队列中转移到同步队列中，如果在同步队列中能够竞争到Lock则可以从等待方法中返回。
    */      
    void signal();
    /**
    * 唤醒全部等待在condition上的线程，将该线程从等待队列中转移到同步队列中，如果在同步队列中能够竞争到Lock则可以从等待方法中返回。
    */          
    void signalAll();
}

ConditionObject 实现

/**
 * 维护一个等待队列，等待和通知唤醒动作，都是针对等待队列中的节点
 *
 */
public class ConditionObject implements Condition, java.io.Serializable {
    private static final long serialVersionUID = 1173984872572414699L;
    /** 第一个节点 */
    private transient Node firstWaiter;
    /** 最后一个节点 */
    private transient Node lastWaiter;

    /** 构造 */
    public ConditionObject() { }


    /**
     * Adds a new waiter to wait queue.
     * 添加一个新 waiter 进入等待队列
     * @return its new wait node
     */
    private Node addConditionWaiter() {
        Node t = lastWaiter;
        // If lastWaiter is cancelled, clean out.
        // 如果最后一个节点被cancel, 清除
        if (t != null && t.waitStatus != Node.CONDITION) {
            unlinkCancelledWaiters();
            t = lastWaiter;
        }
        Node node = new Node(Thread.currentThread(), Node.CONDITION);
        if (t == null)
            firstWaiter = node;
        else
            t.nextWaiter = node;
        lastWaiter = node;
        return node;
    }

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

    /**
     * Removes and transfers all nodes.
     * 删除和转移全部节点
     * @param first (non-null) the first node on condition queue
     */
    private void doSignalAll(Node first) {
        lastWaiter = firstWaiter = null;
        do {
            Node next = first.nextWaiter;
            first.nextWaiter = null;
            transferForSignal(first);
            first = next;
        } while (first != null);
    }

    /**
     * Unlinks cancelled waiter nodes from condition queue.
     * Called only while holding lock. This is called when
     * cancellation occurred during condition wait, and upon
     * insertion of a new waiter when lastWaiter is seen to have
     * been cancelled. This method is needed to avoid garbage
     * retention in the absence of signals. So even though it may
     * require a full traversal, it comes into play only when
     * timeouts or cancellations occur in the absence of
     * signals. It traverses all nodes rather than stopping at a
     * particular target to unlink all pointers to garbage nodes
     * without requiring many re-traversals during cancellation
     * storms.
     * 从条件队列中取消 取消的服务者节点的链接。
     * 仅在保持锁定状态下调用。
     * 当在条件等待期间发生取消操作时，以及在看到lastWaiter已被取消时插入新的服务程序时，将调用此方法。
     * 需要这种方法来避免在没有信号的情况下保留垃圾。
     * 因此，即使可能需要完全遍历，它也只有在没有信号的情况下发生超时或取消时才起作用。
     * 它遍历所有节点，而不是停在特定目标上以取消链接所有指向垃圾节点的指针，而无需在取消风暴期间进行多次遍历。
     */
    private void unlinkCancelledWaiters() {
        Node t = firstWaiter;
        Node trail = null;
        while (t != null) {
            Node next = t.nextWaiter;
            if (t.waitStatus != Node.CONDITION) {
                t.nextWaiter = null;
                if (trail == null)
                    firstWaiter = next;
                else
                    trail.nextWaiter = next;
                if (next == null)
                    lastWaiter = trail;
            }
            else
                trail = t;
            t = next;
        }
    }

    // public methods

    /**
     * Moves the longest-waiting thread, if one exists, from the
     * wait queue for this condition to the wait queue for the
     * owning lock.
     * 将等待时间最长的线程（如果存在）从该条件的等待队列移至拥有锁的等待队列。
     *
     * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
     *         returns {@code false}
     */
    public final void signal() {
        if (!isHeldExclusively())
            throw new IllegalMonitorStateException();
        Node first = firstWaiter;
        if (first != null)
            doSignal(first);
    }

    /**
     * Moves all threads from the wait queue for this condition to
     * the wait queue for the owning lock.
     * 将所有线程从这种情况的等待队列移到拥有锁的等待队列。
     *
     * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
     *         returns {@code false}
     */
    public final void signalAll() {
        if (!isHeldExclusively())
            throw new IllegalMonitorStateException();
        Node first = firstWaiter;
        if (first != null)
            doSignalAll(first);
    }

    /**
     * Implements uninterruptible condition wait.
     * 不被中断的wait实现
     * 

     * 
 Save lock state returned by {@link #getState}.
     * 
 Invoke {@link #release} with saved state as argument,
     *      throwing IllegalMonitorStateException if it fails.
     * 
 Block until signalled.
     * 
 Reacquire by invoking specialized version of
     *      {@link #acquire} with saved state as argument.
     * 
     */
    public final void awaitUninterruptibly() {
        Node node = addConditionWaiter();
        int savedState = fullyRelease(node);
        boolean interrupted = false;
        while (!isOnSyncQueue(node)) {
            LockSupport.park(this);
            if (Thread.interrupted())
                interrupted = true;
        }
        if (acquireQueued(node, savedState) || interrupted)
            selfInterrupt();
    }

    /** Mode meaning to reinterrupt on exit from wait */
    // 等待退出时，重新中断
    private static final int REINTERRUPT =  1;
    /** Mode meaning to throw InterruptedException on exit from wait */
    // 等待退出时，抛出异常
    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.
     * 检查是否有中断，如果在发出信号之前被中断，则返回THROW_IE；在发出信号之后，则返回REINTERRUPT；否则，则返回0。
     */
    private int checkInterruptWhileWaiting(Node node) {
        return Thread.interrupted() ?
            (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
            0;
    }

    /**
     * Throws InterruptedException, reinterrupts current thread, or
     * does nothing, depending on mode.
     * 根据模式，抛出InterruptedException，中断当前线程或不执行任何操作。
     */
    private void reportInterruptAfterWait(int interruptMode)
        throws InterruptedException {
        if (interruptMode == THROW_IE)
            throw new InterruptedException();
        else if (interruptMode == REINTERRUPT)
            selfInterrupt();
    }

    /**
     * Implements interruptible condition wait.
     * 实现可中断的条件等待。
     * 

     * 
 If current thread is interrupted, throw InterruptedException.
     * 
 Save lock state returned by {@link #getState}.
     * 
 Invoke {@link #release} with saved state as argument,
     *      throwing IllegalMonitorStateException if it fails.
     * 
 Block until signalled or interrupted.
     * 
 Reacquire by invoking specialized version of
     *      {@link #acquire} with saved state as argument.
     * 
 If interrupted while blocked in step 4, throw InterruptedException.
     * 
     */
    public final void await() throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        Node node = addConditionWaiter();
        int savedState = fullyRelease(node);
        int interruptMode = 0;
        while (!isOnSyncQueue(node)) {
            LockSupport.park(this);
            if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                break;
        }
        if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
            interruptMode = REINTERRUPT;
        if (node.nextWaiter != null) // clean up if cancelled
            unlinkCancelledWaiters();
        if (interruptMode != 0)
            reportInterruptAfterWait(interruptMode);
    }

    /**
     * Implements timed condition wait.
     * 实现定时的条件等待。
     * 

     * 
 If current thread is interrupted, throw InterruptedException.
     * 
 Save lock state returned by {@link #getState}.
     * 
 Invoke {@link #release} with saved state as argument,
     *      throwing IllegalMonitorStateException if it fails.
     * 
 Block until signalled, interrupted, or timed out.
     * 
 Reacquire by invoking specialized version of
     *      {@link #acquire} with saved state as argument.
     * 
 If interrupted while blocked in step 4, throw InterruptedException.
     * 
     */
    public final long awaitNanos(long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        Node node = addConditionWaiter();
        int savedState = fullyRelease(node);
        final long deadline = System.nanoTime() + nanosTimeout;
        int interruptMode = 0;
        while (!isOnSyncQueue(node)) {
            if (nanosTimeout <= 0L) {
                transferAfterCancelledWait(node);
                break;
            }
            if (nanosTimeout >= spinForTimeoutThreshold)
                LockSupport.parkNanos(this, nanosTimeout);
            if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                break;
            nanosTimeout = deadline - System.nanoTime();
        }
        if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
            interruptMode = REINTERRUPT;
        if (node.nextWaiter != null)
            unlinkCancelledWaiters();
        if (interruptMode != 0)
            reportInterruptAfterWait(interruptMode);
        return deadline - System.nanoTime();
    }

    /**
     * Implements absolute timed condition wait.
     * 实现绝对时间超时的条件等待。
     * 

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

    /**
     * Implements timed condition wait.
     * 实现可超时条件等待。
     * 

     * 
 If current thread is interrupted, throw InterruptedException.
     * 
 Save lock state returned by {@link #getState}.
     * 
 Invoke {@link #release} with saved state as argument,
     *      throwing IllegalMonitorStateException if it fails.
     * 
 Block until signalled, interrupted, or timed out.
     * 
 Reacquire by invoking specialized version of
     *      {@link #acquire} with saved state as argument.
     * 
 If interrupted while blocked in step 4, throw InterruptedException.
     * 
 If timed out while blocked in step 4, return false, else true.
     * 
     */
    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)}.
     * 查询是否有线程在条件队列中等待
     *
     * @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.
     * 返回当前条件中的等待的估算的线程数目
     * 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.
     * 返回当前条件中等待的线程集合
     * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
     *
     * @return the collection of threads
     * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
     *         returns {@code false}
     */
    protected final Collection getWaitingThreads() {
        if (!isHeldExclusively())
            throw new IllegalMonitorStateException();
        ArrayList list = new ArrayList();
        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;
    }
}

还有一些cas相关的实现，此处不在赘述。