JDK 8 AQS(AbstractQueuedSynchronizer) 源码详解（详细注释版）-优快云博客

JDK 8 AQS(AbstractQueuedSynchronizer) 源码详解（详细注释版）

1. AQS核心概念

/*
 * AbstractQueuedSynchronizer (AQS) 抽象队列同步器
 * 是Java并发包中最重要的基础类之一
 * 为实现依赖于先进先出 (FIFO) 等待队列的阻塞锁和相关同步器（信号量、事件等）提供框架
 * 
 * AQS的核心思想：
 * 1. 使用一个int类型的成员变量state表示同步状态
 * 2. 通过内置的FIFO等待队列来完成资源获取的排队工作
 * 3. 通过CAS操作保证状态修改的原子性
 * 
 * AQS支持两种资源获取方式：
 * - 独占式：同一时刻只有一个线程可以获取资源
 * - 共享式：同一时刻可以有多个线程获取资源
 * 
 * AQS的主要应用：
 * - ReentrantLock
 * - ReentrantReadWriteLock
 * - Semaphore
 * - CountDownLatch
 * - ThreadPoolExecutor.Worker
 */

2. AQS核心数据结构

/*
 * AQS核心类源码
 */
public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    
    private static final long serialVersionUID = 7373984972572414691L;
    
    /**
     * Node内部类 - 等待队列的节点
     * 等待队列是CLH (Craig, Landin, and Hagersten) 队列的变体
     */
    static final class Node {
        // 共享模式标记
        static final Node SHARED = new Node();
        // 独占模式标记
        static final Node EXCLUSIVE = null;
        
        // 线程等待状态常量
        static final int CANCELLED =  1;  // 线程已取消
        static final int SIGNAL    = -1;  // 线程需要被唤醒
        static final int CONDITION = -2;  // 线程在等待条件
        static final int PROPAGATE = -3;  // 传播状态（共享模式下使用）
        
        // 等待状态
        volatile int waitStatus;
        
        // 前驱节点
        volatile Node prev;
        
        // 后继节点
        volatile Node next;
        
        // 当前节点关联的线程
        volatile Thread thread;
        
        // 下一个等待节点（条件队列中使用）或共享模式标记
        Node nextWaiter;
        
        /**
         * 判断是否为共享模式
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }
        
        /**
         * 获取前驱节点
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }
        
        Node() {    // 用于建立初始头或SHARED标记
        }
        
        Node(Thread thread, Node mode) {     // 用于addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }
        
        Node(Thread thread, int waitStatus) { // 用于条件节点
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }
    
    /*
     * AQS核心字段
     */
    
    // 等待队列的头节点
    private transient volatile Node head;
    
    // 等待队列的尾节点
    private transient volatile Node tail;
    
    // 同步状态
    private volatile int state;
    
    /**
     * 获取同步状态
     */
    protected final int getState() {
        return state;
    }
    
    /**
     * 设置同步状态
     */
    protected final void setState(int newState) {
        state = newState;
    }
    
    /**
     * CAS设置同步状态
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }
    
    // 等待队列是否需要自旋（用于自检）
    private static final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null &&
            (s = h.next)  != null &&
            !s.isShared()         &&
            s.thread != null;
    }
    
    // Unsafe相关字段和静态初始化
    private static final sun.misc.Unsafe unsafe = sun.misc.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); }
    }
}

3. AQS核心方法实现

/*
 * AQS核心同步方法实现
 */
public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    
    /**
     * 创建节点并加入等待队列
     * 
     * @param mode Node.EXCLUSIVE 独占模式, Node.SHARED 共享模式
     * @return 新创建的节点
     */
    private Node addWaiter(Node mode) {
        // 创建新节点
        Node node = new Node(Thread.currentThread(), mode);
        // 尝试快速入队（假设队列尾部不会频繁变化）
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            // CAS设置尾节点
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        // 快速入队失败，使用完整入队方法
        enq(node);
        return node;
    }
    
    /**
     * 完整的入队操作
     * 
     * @param node 要入队的节点
     * @return 节点的前驱节点
     */
    private Node enq(final Node node) {
        for (;;) {  // 自旋直到入队成功
            Node t = tail;
            if (t == null) { // 队列为空，初始化
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                // 将节点添加到队列尾部
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }
    
    /**
     * 设置头节点
     * 
     * @param node 新的头节点
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }
    
    /**
     * 唤醒后继节点
     * 
     * @param node 要唤醒其后继的节点
     */
    private void unparkSuccessor(Node node) {
        int ws = node.waitStatus;
        // 如果节点状态为负数，尝试将其置为0
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);
        
        // 获取后继节点
        Node s = node.next;
        // 如果后继节点为空或已取消，从尾部向前查找有效节点
        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;
        }
        // 唤醒找到的节点
        if (s != null)
            LockSupport.unpark(s.thread);
    }
    
    /**
     * 共享模式下传播释放信号
     * 
     * @param propagate 要传播的资源数
     * @param node 当前节点
     * @param mode 传播模式
     */
    private void doReleaseShared() {
        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;
        }
    }
    
    /**
     * 设置头节点并传播（共享模式）
     * 
     * @param node 新的头节点
     * @param propagate 要传播的资源数
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }
    
    /**
     * 取消正在进行的获取尝试
     * 
     * @param node 要取消的节点
     */
    private void cancelAcquire(Node node) {
        // 忽略无效节点
        if (node == null)
            return;
        
        node.thread = null;
        
        // 跳过已取消的前驱节点
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;
        
        // predNext是有效的前驱节点的下一个节点
        Node predNext = pred.next;
        
        // 设置节点状态为取消
        node.waitStatus = Node.CANCELLED;
        
        // 如果是尾节点，移除并更新尾节点
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            // 如果不是尾节点，需要跳过取消的节点
            int ws;
            if (pred != head &&
                ((ws = pred.waitStatus) == 0 ||
                 ws == Node.SIGNAL) &&
                pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                unparkSuccessor(node);
            }
            
            node.next = node; // help GC
        }
    }
    
    /**
     * 判断是否应该阻塞当前节点
     * 
     * @param pred 节点的前驱
     * @param node 当前节点
     * @return 如果应该阻塞返回true
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            // 前驱节点状态为SIGNAL，表示当前节点应该被阻塞
            return true;
        if (ws > 0) {
            // 前驱节点已取消，跳过取消的节点
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            // 设置前驱节点状态为SIGNAL
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }
    
    /**
     * 中断当前线程
     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }
    
    /**
     * 阻塞当前线程并检查中断状态
     * 
     * @return 如果线程被中断返回true
     * @throws InterruptedException 如果线程被中断且中断被抛出
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }
    
    /**
     * 独占模式下获取资源的核心方法
     * 
     * @param arg 获取资源的参数
     * @throws InterruptedException 如果线程被中断
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                // 获取前驱节点
                final Node p = node.predecessor();
                // 如果前驱是头节点且获取资源成功
                if (p == head && tryAcquire(arg)) {
                    setHead(node);  // 设置为新头节点
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 检查是否应该阻塞，如果应该则阻塞
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    /**
     * 独占模式下可中断地获取资源
     * 
     * @param arg 获取资源的参数
     * @throws InterruptedException 如果线程被中断
     */
    private void doAcquireInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    /**
     * 独占模式下带超时地获取资源
     * 
     * @param arg 获取资源的参数
     * @param nanosTimeout 超时时间（纳秒）
     * @return 如果成功获取返回true，超时返回false
     * @throws InterruptedException 如果线程被中断
     */
    private boolean doAcquireNanos(int arg, long nanosTimeout)
        throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    /**
     * 共享模式下获取资源的核心方法
     * 
     * @param arg 获取资源的参数
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    /**
     * 共享模式下可中断地获取资源
     * 
     * @param arg 获取资源的参数
     * @throws InterruptedException 如果线程被中断
     */
    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);
        }
    }
    
    /**
     * 共享模式下带超时地获取资源
     * 
     * @param arg 获取资源的参数
     * @param nanosTimeout 超时时间（纳秒）
     * @return 如果成功获取返回true，超时返回false
     * @throws InterruptedException 如果线程被中断
     */
    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);
        }
    }
}

4. AQS对外提供的核心方法

/*
 * AQS对外提供的核心同步方法
 */
public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    
    /**
     * 独占模式下获取资源
     * 子类需要实现tryAcquire方法
     * 
     * @param arg 获取资源的参数
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
    
    /**
     * 独占模式下可中断地获取资源
     * 
     * @param arg 获取资源的参数
     * @throws InterruptedException 如果线程被中断
     */
    public final void acquireInterruptibly(int arg)
        throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }
    
    /**
     * 独占模式下带超时地获取资源
     * 
     * @param arg 获取资源的参数
     * @param nanosTimeout 超时时间（纳秒）
     * @return 如果成功获取返回true，超时返回false
     * @throws InterruptedException 如果线程被中断
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
        throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }
    
    /**
     * 独占模式下释放资源
     * 子类需要实现tryRelease方法
     * 
     * @param arg 释放资源的参数
     * @return 如果成功释放返回true
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
    
    /**
     * 共享模式下获取资源
     * 子类需要实现tryAcquireShared方法
     * 
     * @param arg 获取资源的参数
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }
    
    /**
     * 共享模式下可中断地获取资源
     * 
     * @param arg 获取资源的参数
     * @throws InterruptedException 如果线程被中断
     */
    public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
    
    /**
     * 共享模式下带超时地获取资源
     * 
     * @param arg 获取资源的参数
     * @param nanosTimeout 超时时间（纳秒）
     * @return 如果成功获取返回true，超时返回false
     * @throws InterruptedException 如果线程被中断
     */
    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
        throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 ||
            doAcquireSharedNanos(arg, nanosTimeout);
    }
    
    /**
     * 共享模式下释放资源
     * 子类需要实现tryReleaseShared方法
     * 
     * @param arg 释放资源的参数
     * @return 如果成功释放返回true
     */
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    
    /**
     * 查询是否有正在等待获取资源的线程
     * 
     * @return 如果有等待线程返回true
     */
    public final boolean hasQueuedThreads() {
        return head != tail;
    }
    
    /**
     * 查询是否有线程曾经等待过
     * 
     * @return 如果有线程曾经等待过返回true
     */
    public final boolean hasContended() {
        return head != null;
    }
    
    /**
     * 获取队列中的第一个线程（如果不是当前线程）
     * 
     * @return 队列中的第一个线程，如果队列为空返回null
     */
    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }
    
    /**
     * 判断给定线程是否在队列中等待
     * 
     * @param thread 要检查的线程
     * @return 如果线程在队列中等待返回true
     */
    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;
    }
    
    /**
     * 获取等待队列的长度
     * 
     * @return 等待队列的长度
     */
    public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null)
                ++n;
        }
        return n;
    }
    
    /**
     * 获取等待队列中的所有线程集合
     * 
     * @return 等待队列中的所有线程集合
     */
    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;
    }
    
    /**
     * 获取独占模式下等待队列中的所有线程集合
     * 
     * @return 独占模式下等待队列中的所有线程集合
     */
    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;
    }
    
    /**
     * 获取共享模式下等待队列中的所有线程集合
     * 
     * @return 共享模式下等待队列中的所有线程集合
     */
    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;
    }
    
    /**
     * toString方法
     */
    public String toString() {
        int s = getState();
        String q  = hasQueuedThreads() ? "non" : "";
        return super.toString() +
            "[State = " + s + ", " + q + "empty queue]";
    }
    
    // 需要子类实现的抽象方法
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }
    
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }
    
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }
    
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }
    
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }
}

5. 条件队列相关方法

/*
 * AQS条件队列相关方法
 */
public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    
    /**
     * ConditionObject内部类 - 条件对象
     * 实现了Condition接口
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        
        // 条件队列的第一个节点
        private transient Node firstWaiter;
        
        // 条件队列的最后一个节点
        private transient Node lastWaiter;
        
        /**
         * 构造方法
         */
        public ConditionObject() { }
        
        /**
         * 添加等待节点到条件队列
         * 
         * @return 新创建的节点
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            // 如果尾节点状态不是CONDITION，清理取消的节点
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            // 创建新节点并添加到条件队列
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }
        
        /**
         * 从条件队列转移到同步队列
         * 
         * @param first 要转移的节点
         * @return 如果成功转移返回true
         */
        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }
        
        /**
         * 唤醒所有等待节点
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }
        
        /**
         * 清理取消的等待节点
         */
        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;
            }
        }
        
        /**
         * 唤醒一个等待线程
         */
        @Override
        public final void signal() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);
        }
        
        /**
         * 唤醒所有等待线程
         */
        @Override
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }
        
        /**
         * 等待条件满足
         */
        @Override
        public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            
            // 添加到条件队列
            Node node = addConditionWaiter();
            // 释放锁
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            // 在同步队列中等待
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            // 重新获取锁
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }
        
        // 其他await方法的实现...
    }
    
    /**
     * 完全释放锁
     * 
     * @param node 节点
     * @return 释放前的锁状态
     */
    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;
        }
    }
    
    /**
     * 判断节点是否在同步队列中
     * 
     * @param node 节点
     * @return 如果在同步队列中返回true
     */
    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;
        return findNodeFromTail(node);
    }
    
    /**
     * 从尾部查找节点
     * 
     * @param node 要查找的节点
     * @return 如果找到返回true
     */
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }
    
    /**
     * 转移节点到同步队列
     * 
     * @param node 要转移的节点
     * @return 如果成功转移返回true
     */
    final boolean transferForSignal(Node node) {
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;
        
        Node p = enq(node);
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }
}

6. 使用示例

/**
 * 简单的互斥锁实现示例
 */
public class SimpleLock extends AbstractQueuedSynchronizer {
    
    /**
     * 尝试获取锁
     */
    @Override
    protected boolean tryAcquire(int arg) {
        // CAS操作：如果状态为0（未锁定），则设置为1（锁定）
        return compareAndSetState(0, 1);
    }
    
    /**
     * 尝试释放锁
     */
    @Override
    protected boolean tryRelease(int arg) {
        // 设置状态为0（未锁定）
        setState(0);
        return true;
    }
    
    /**
     * 判断是否被当前线程独占
     */
    @Override
    protected boolean isHeldExclusively() {
        return getState() == 1;
    }
    
    /**
     * 获取锁的公共方法
     */
    public void lock() {
        acquire(1);
    }
    
    /**
     * 释放锁的公共方法
     */
    public void unlock() {
        release(1);
    }
}

/**
 * 简单的信号量实现示例
 */
public class SimpleSemaphore extends AbstractQueuedSynchronizer {
    
    /**
     * 构造方法
     * 
     * @param permits 许可数量
     */
    public SimpleSemaphore(int permits) {
        setState(permits);
    }
    
    /**
     * 尝试获取许可
     */
    @Override
    protected int tryAcquireShared(int acquires) {
        for (;;) {
            int available = getState();
            int remaining = available - acquires;
            // 如果剩余许可数小于0，获取失败
            if (remaining < 0 ||
                compareAndSetState(available, remaining))
                return remaining;
        }
    }
    
    /**
     * 尝试释放许可
     */
    @Override
    protected boolean tryReleaseShared(int releases) {
        for (;;) {
            int current = getState();
            int next = current + releases;
            if (next < current) // overflow
                throw new Error("Maximum permit count exceeded");
            if (compareAndSetState(current, next))
                return true;
        }
    }
    
    /**
     * 获取许可
     */
    public void acquire() throws InterruptedException {
        acquireSharedInterruptibly(1);
    }
    
    /**
     * 释放许可
     */
    public void release() {
        releaseShared(1);
    }
}

/**
 * AQS使用示例
 */
public class AQSExample {
    
    public static void main(String[] args) {
        // 测试简单锁
        testSimpleLock();
        
        // 测试简单信号量
        testSimpleSemaphore();
    }
    
    private static void testSimpleLock() {
        SimpleLock lock = new SimpleLock();
        
        // 启动多个线程竞争锁
        for (int i = 0; i < 5; i++) {
            final int threadId = i;
            new Thread(() -> {
                lock.lock();
                try {
                    System.out.println("Thread " + threadId + " acquired lock");
                    Thread.sleep(1000);
                    System.out.println("Thread " + threadId + " releasing lock");
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                } finally {
                    lock.unlock();
                }
            }).start();
        }
    }
    
    private static void testSimpleSemaphore() {
        SimpleSemaphore semaphore = new SimpleSemaphore(2); // 2个许可
        
        // 启动多个线程竞争许可
        for (int i = 0; i < 5; i++) {
            final int threadId = i;
            new Thread(() -> {
                try {
                    semaphore.acquire();
                    System.out.println("Thread " + threadId + " acquired permit");
                    Thread.sleep(2000);
                    System.out.println("Thread " + threadId + " releasing permit");
                    semaphore.release();
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            }).start();
        }
    }
}