并发编程 - AQS 源码

1. AQS 源码

public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable {
    private static final long serialVersionUID = 7373984972572414691L;
 
    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance
     * with initial synchronization state of zero.
     */
    protected AbstractQueuedSynchronizer() {
    }
 
    /**
     * Wait queue node class.
     * <p>
     * 不管是条件队列，还是CLH等待队列
     * 都是基于Node类
     * <p>
     * AQS当中的同步等待队列也称CLH队列，CLH队列是Craig、Landin、Hagersten三人
     * 发明的一种基于双向链表数据结构的队列，是FIFO先入先出线程等待队列，Java中的
     * CLH队列是原CLH队列的一个变种,线程由原自旋机制改为阻塞机制。
     */
    static final class Node {
        /**
         * 标记节点未共享模式
         */
        static final Node SHARED = new Node();
        /**
         * 标记节点为独占模式
         */
        static final Node EXCLUSIVE = null;
 
        /**
         * 在同步队列中等待的线程等待超时或者被中断，需要从同步队列中取消等待
         */
        static final int CANCELLED = 1;
        /**
         * 后继节点的线程处于等待状态，而当前的节点如果释放了同步状态或者被取消，
         * 将会通知后继节点，使后继节点的线程得以运行。
         */
        static final int SIGNAL = -1;
        /**
         * 节点在等待队列中，节点的线程等待在Condition上，当其他线程对Condition调用了signal()方法后，
         * 该节点会从等待队列中转移到同步队列中，加入到同步状态的获取中
         */
        static final int CONDITION = -2;
        /**
         * 表示下一次共享式同步状态获取将会被无条件地传播下去
         */
        static final int PROPAGATE = -3;
 
        /**
         * 标记当前节点的信号量状态 (1,0,-1,-2,-3)5种状态
         * 使用CAS更改状态，volatile保证线程可见性，高并发场景下，
         * 即被一个线程修改后，状态会立马让其他线程可见。
         */
        volatile int waitStatus;
 
        /**
         * 前驱节点，当前节点加入到同步队列中被设置
         */
        volatile Node prev;
 
        /**
         * 后继节点
         */
        volatile Node next;
 
        /**
         * 节点同步状态的线程
         */
        volatile Thread thread;
 
        /**
         * 等待队列中的后继节点，如果当前节点是共享的，那么这个字段是一个SHARED常量，
         * 也就是说节点类型(独占和共享)和等待队列中的后继节点共用同一个字段。
         */
        Node nextWaiter;
 
        /**
         * Returns true if node is waiting in shared mode.
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }
 
        /**
         * 返回前驱节点
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }
 
        //空节点，用于标记共享模式
        Node() {    // Used to establish initial head or SHARED marker
        }
 
        //用于同步队列CLH
        Node(Thread thread, Node mode) {     // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }
 
        //用于条件队列
        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }
 
    /**
     * 指向同步等待队列的头节点
     */
    private transient volatile Node head;
 
    /**
     * 指向同步等待队列的尾节点
     */
    private transient volatile Node tail;
 
    /**
     * 同步资源状态
     */
    private volatile int state;
 
    protected final int getState() {
        return state;
    }
 
    protected final void setState(int newState) {
        state = newState;
    }
 
    protected final boolean compareAndSetState(int expect, int update) {
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }
 
    static final long spinForTimeoutThreshold = 1000L;
 
    /**
     * 节点加入CLH同步队列
     */
    private Node enq(final Node node) {
        for (; ; ) {
            Node t = tail;
            if (t == null) { // Must initialize
                //队列为空需要初始化，创建空的头节点
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                //set尾部节点
                if (compareAndSetTail(t, node)) {//当前节点置为尾部
                    t.next = node; //前驱节点的next指针指向当前节点
                    return t;
                }
            }
        }
    }
 
    private Node addWaiter(Node mode) {
        // 1. 将当前线程构建成Node类型
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        // 2. 1当前尾节点是否为null？
        if (pred != null) {
            // 2.2 将当前节点尾插入的方式
            node.prev = pred;
            // 2.3 CAS将节点插入同步队列的尾部
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }
 
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }
 
    private void unparkSuccessor(Node node) {
        //获取wait状态
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);// 将等待状态waitStatus设置为初始值0
 
        /**
         * 若后继结点为空，或状态为CANCEL（已失效），则从后尾部往前遍历找到最前的一个处于正常阻塞状态的结点
         * 进行唤醒
         */
        Node s = node.next; //head.next = Node1 ,thread = T3
        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;
        }
        if (s != null)
            LockSupport.unpark(s.thread);//唤醒线程,T3唤醒
    }
 
    /**
     * 把当前结点设置为SIGNAL或者PROPAGATE
     * 唤醒head.next(B节点)，B节点唤醒后可以竞争锁，成功后head->B，然后又会唤醒B.next，一直重复直到共享节点都唤醒
     * head节点状态为SIGNAL，重置head.waitStatus->0，唤醒head节点线程，唤醒后线程去竞争共享锁
     * head节点状态为0，将head.waitStatus->Node.PROPAGATE传播状态，表示需要将状态向后继节点传播
     */
    private void doReleaseShared() {
        for (; ; ) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {//head是SIGNAL状态
                    /* head状态是SIGNAL，重置head节点waitStatus为0，E这里不直接设为Node.PROPAGAT,
                     * 是因为unparkSuccessor(h)中，如果ws < 0会设置为0，所以ws先设置为0，再设置为PROPAGATE
                     * 这里需要控制并发，因为入口有setHeadAndPropagate跟release两个，避免两次unpark
                     */
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue; //设置失败，重新循环
                    /* head状态为SIGNAL，且成功设置为0之后,唤醒head.next节点线程
                     * 此时head、head.next的线程都唤醒了，head.next会去竞争锁，成功后head会指向获取锁的节点，
                     * 也就是head发生了变化。看最底下一行代码可知，head发生变化后会重新循环，继续唤醒head的下一个节点
                     */
                    unparkSuccessor(h);
                    /*
                     * 如果本身头节点的waitStatus是出于重置状态（waitStatus==0）的，将其设置为“传播”状态。
                     * 意味着需要将状态向后一个节点传播
                     */
                } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head) //如果head变了，重新循环
                break;
        }
    }
 
    /**
     * 把node节点设置成head节点，且Node.waitStatus->Node.PROPAGATE
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; //h用来保存旧的head节点
        setHead(node);//head引用指向node节点
        /* 这里意思有两种情况是需要执行唤醒操作
         * 1.propagate > 0 表示调用方指明了后继节点需要被唤醒
         * 2.头节点后面的节点需要被唤醒（waitStatus<0），不论是老的头结点还是新的头结点
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
                (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())//node是最后一个节点或者 node的后继节点是共享节点
                /* 如果head节点状态为SIGNAL，唤醒head节点线程，重置head.waitStatus->0
                 * head节点状态为0(第一次添加时是0)，设置head.waitStatus->Node.PROPAGATE表示状态需要向后继节点传播 */
                doReleaseShared();
        }
    }
 
    /**
     * 终结掉正在尝试去获取锁的节点
     *
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;
 
        node.thread = null;
 
        // 剔除掉一件被cancel掉的节点
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;
 
        Node predNext = pred.next;
 
        node.waitStatus = Node.CANCELLED;
 
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            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 {
                unparkSuccessor(node);
            }
 
            node.next = node; // help GC
        }
    }
 
    /**
     *
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            //若前驱结点的状态是SIGNAL，意味着当前结点可以被安全地park
            return true;
        if (ws > 0) {
            //前驱节点状态如果被取消状态，将被移除出队列
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /* 当前驱节点waitStatus为 0 or PROPAGATE状态时
             * 将其设置为SIGNAL状态，然后当前结点才可以可以被安全地park */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }
 
    /**
     * 中断当前线程
     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }
 
    /**
     * 阻塞当前节点，返回当前Thread的中断状态
     * LockSupport.park 底层实现逻辑调用系统内核功能 pthread_mutex_lock 阻塞线程
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);//阻塞
        return Thread.interrupted();
    }
 
    /**
     * 已经在队列当中的Thread节点，准备阻塞等待获取锁
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (; ; ) {//死循环
                final Node p = node.predecessor();//找到当前结点的前驱结点
                if (p == head && tryAcquire(arg)) {//如果前驱结点是头结点，才tryAcquire，其他结点是没有机会tryAcquire的。
                    setHead(node);//获取同步状态成功，将当前结点设置为头结点。
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                /**
                 * 如果前驱节点不是Head，通过shouldParkAfterFailedAcquire判断是否应该阻塞
                 * 前驱节点信号量为-1，当前线程可以安全被parkAndCheckInterrupt用来阻塞线程
                 */
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
 
    /**
     * 与acquireQueued逻辑相似，唯一区别节点还不在队列当中需要先进行入队操作
     */
    private void doAcquireInterruptibly(int arg) throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);//以独占模式放入队列尾部
        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);
        }
    }
 
    /**
     * 独占模式定时获取
     */
    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);//加入队列
        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;//超时直接返回获取失败
                if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
                    //阻塞指定时长，超时则线程自动被唤醒
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())//当前线程中断状态
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
 
    /**
     * 尝试获取共享锁
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);//入队
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (; ; ) {
                final Node p = node.predecessor();//前驱节点
                if (p == head) {
                    int r = tryAcquireShared(arg); //非公平锁实现，再尝试获取锁
                    //state==0时tryAcquireShared会返回>=0(CountDownLatch中返回的是1)。
                    // state为0说明共享次数已经到了，可以获取锁了
                    if (r >= 0) {//r>0表示state==0,前继节点已经释放锁，锁的状态为可被获取
                        //这一步设置node为head节点设置node.waitStatus->Node.PROPAGATE，然后唤醒node.thread
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                //前继节点非head节点，将前继节点状态设置为SIGNAL，通过park挂起node节点的线程
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
 
    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);
        }
    }
 
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (; ; ) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
 
    // Main exported methods
 
    /**
     * 尝试获取独占锁，可指定锁的获取数量
     */
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }
 
    /**
     * 尝试释放独占锁，在子类当中实现
     */
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }
 
    /**
     * 共享式：共享式地获取同步状态。对于独占式同步组件来讲，同一时刻只有一个线程能获取到同步状态，
     * 其他线程都得去排队等待，其待重写的尝试获取同步状态的方法tryAcquire返回值为boolean，这很容易理解；
     * 对于共享式同步组件来讲，同一时刻可以有多个线程同时获取到同步状态，这也是“共享”的意义所在。
     * 本方法待被之类覆盖实现具体逻辑
     * 1.当返回值大于0时，表示获取同步状态成功，同时还有剩余同步状态可供其他线程获取；
     * <p>
     * 　2.当返回值等于0时，表示获取同步状态成功，但没有可用同步状态了；
     * 　3.当返回值小于0时，表示获取同步状态失败。
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }
 
    /**
     * 释放共享锁，具体实现在子类当中实现
     */
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }
 
    /**
     * 当前线程是否持有独占锁
     */
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }
 
    /**
     * 获取独占锁
     */
    public final void acquire(int arg) {
        //尝试获取锁
        if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//独占模式
            selfInterrupt();
    }
 
    public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }
 
    /**
     * 获取独占锁，设置最大等待时间
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) ||
                doAcquireNanos(arg, nanosTimeout);
    }
 
    /**
     * 释放独占模式持有的锁
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {//释放一次锁
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);//唤醒后继结点
            return true;
        }
        return false;
    }
 
    /**
     * 请求获取共享锁
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)//返回值小于0，获取同步状态失败，排队去；获取同步状态成功，直接返回去干自己的事儿。
            doAcquireShared(arg);
    }
 
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
 
    public final boolean hasQueuedThreads() {
        return head != tail;
    }
 
    public final boolean hasContended() {
        return head != null;
    }
 
    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }
 
    private Thread fullGetFirstQueuedThread() {
        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;
 
        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;
    }
 
    /**
     * 判断当前线程是否在队列当中
     */
    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;
    }
 
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null;
    }
 
    /**
     * 判断当前节点是否有前驱节点
     */
    public final boolean hasQueuedPredecessors() {
        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());
    }
 
    /**
     * 同步队列长度
     */
    public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null)
                ++n;
        }
        return n;
    }
 
    /**
     * 获取队列等待thread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;
    }
 
    /**
     * 获取独占模式等待thread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;
    }
 
    /**
     * 获取共享模式等待thread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;
    }
 
    /**
     * 判断节点是否在同步队列中
     */
    final boolean isOnSyncQueue(Node node) {
        //快速判断1：节点状态或者节点没有前置节点
        //注：同步队列是有头节点的，而条件队列没有
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        //快速判断2：next字段只有同步队列才会使用，条件队列中使用的是nextWaiter字段
        if (node.next != null) // If has successor, it must be on queue
            return true;
        //上面如果无法判断则进入复杂判断
        return findNodeFromTail(node);
    }
 
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (; ; ) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }
 
    /**
     * 将节点从条件队列当中移动到同步队列当中，等待获取锁
     */
    final boolean transferForSignal(Node node) {
        /*
         * 修改节点信号量状态为0，失败直接返回false
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;
 
        /*
         * 加入同步队列尾部当中，返回前驱节点
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        //前驱节点不可用 或者 修改信号量状态失败
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread); //唤醒当前节点
        return true;
    }
 
    final boolean transferAfterCancelledWait(Node node) {
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
 
        while (!isOnSyncQueue(node))
            Thread.yield();
        return false;
    }
 
    /**
     * 入参就是新创建的节点，即当前节点
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            //这里这个取值要注意，获取当前的state并释放，这从另一个角度说明必须是独占锁
            //可以考虑下这个逻辑放在共享锁下面会发生什么？
            int savedState = getState();
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                //如果这里释放失败，则抛出异常
                throw new IllegalMonitorStateException();
            }
        } finally {
            /**
             * 如果释放锁失败，则把节点取消，由这里就能看出来上面添加节点的逻辑中
             * 只需要判断最后一个节点是否被取消就可以了
             */
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }
 
    public final boolean hasWaiters(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.hasWaiters();
    }
 
    /**
     * 获取条件队列长度
     */
    public final int getWaitQueueLength(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitQueueLength();
    }
 
    /**
     * 获取条件队列当中所有等待的thread集合
     */
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitingThreads();
    }
 
    /**
     * 条件对象，实现基于条件的具体行为
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /**
         * First node of condition queue.
         */
        private transient Node firstWaiter;
        /**
         * Last node of condition queue.
         */
        private transient Node lastWaiter;
 
        public ConditionObject() {
        }
 
        /**
         * 1.与同步队列不同，条件队列头尾指针是firstWaiter跟lastWaiter
         * 2.条件队列是在获取锁之后，也就是临界区进行操作，因此很多地方不用考虑并发
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            //如果最后一个节点被取消，则删除队列中被取消的节点
            //至于为啥是最后一个节点后面会分析
            if (t != null && t.waitStatus != Node.CONDITION) {
                //删除所有被取消的节点
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            //创建一个类型为CONDITION的节点并加入队列，由于在临界区，所以这里不用并发控制
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }
 
        /**
         * 发信号，通知遍历条件队列当中的节点转移到同步队列当中，准备排队获取锁
         */
        private void doSignal(Node first) {
            do {
                if ((firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) && //转移节点
                    (first = firstWaiter) != null);
        }
 
        /**
         * 通知所有节点移动到同步队列当中，并将节点从条件队列删除
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }
 
        /**
         * 删除条件队列当中被取消的节点
         */
        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;
            }
        }
 
        /**
         * 发新号，通知条件队列当中节点到同步队列当中去排队
         */
        public final void signal() {
            if (!isHeldExclusively())//节点不能已经持有独占锁
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
            /**
             * 发信号通知条件队列的节点准备到同步队列当中去排队
             */
                doSignal(first);
        }
 
        /**
         * 唤醒所有条件队列的节点转移到同步队列当中
         */
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }
 
        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();
        }
 
        /**
         * 该模式表示在退出等待时重新中断
         */
        private static final int REINTERRUPT = 1;
        /**
         * 异常中断
         */
        private static final int THROW_IE = -1;
 
        /**
         * 这里的判断逻辑是：
         * 1.如果现在不是中断的，即正常被signal唤醒则返回0
         * 2.如果节点由中断加入同步队列则返回THROW_IE，由signal加入同步队列则返回REINTERRUPT
         */
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0;
        }
 
        /**
         * 根据中断时机选择抛出异常或者设置线程中断状态
         */
        private void reportInterruptAfterWait(int interruptMode)
                throws InterruptedException {
            if (interruptMode == THROW_IE)
                throw new InterruptedException();
            else if (interruptMode == REINTERRUPT)
                selfInterrupt();
        }
 
        /**
         * 加入条件队列等待，条件队列入口
         */
        public final void await() throws InterruptedException {
 
            //T2进来
            //如果当前线程被中断则直接抛出异常
            if (Thread.interrupted())
                throw new InterruptedException();
            //把当前节点加入条件队列
            Node node = addConditionWaiter();
            //释放掉已经获取的独占锁资源
            int savedState = fullyRelease(node);//T2释放锁
            int interruptMode = 0;
            //如果不在同步队列中则不断挂起
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);//T1被阻塞
                //这里被唤醒可能是正常的signal操作也可能是中断
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            /**
             * 走到这里说明节点已经条件满足被加入到了同步队列中或者中断了
             * 这个方法很熟悉吧？就跟独占锁调用同样的获取锁方法，从这里可以看出条件队列只能用于独占锁
             * 在处理中断之前首先要做的是从同步队列中成功获取锁资源
             */
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            //走到这里说明已经成功获取到了独占锁，接下来就做些收尾工作
            //删除条件队列中被取消的节点
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            //根据不同模式处理中断
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }
 
        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;
        }
 
        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }
 
        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;
        }
 
        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;
        }
 
        /**
         * 得到同步队列当中所有在等待的Thread集合
         */
        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    }
 
    /**
     * unsafe魔法类，直接绕过虚拟机内存管理机制，修改内存
     */
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    //偏移量
    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 {
            //状态偏移量
            stateOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
            //head指针偏移量，head指向CLH队列的头部
            headOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("next"));
 
        } catch (Exception ex) {
            throw new Error(ex);
        }
    }
 
    /**
     * CAS 修改头部节点指向. 并发入队时使用.
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }
 
    /**
     * CAS 修改尾部节点指向. 并发入队时使用.
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }
 
    /**
     * CAS 修改信号量状态.
     */
    private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset,
                expect, update);
    }
 
    /**
     * 修改节点的后继指针.
     */
    private static final boolean compareAndSetNext(Node node, Node expect, Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
    }
}

2. AQS 框架具体实现 - 独占锁实现 ReentrantLock 源码

实现 ReentrantLock 的三大核心原理：

• LocksSuport：加锁解锁
• 自旋：如果没有加锁成功就一直自旋
• CAS：保证只能有一个线程可以加锁成功
• queue 队列：用容器保存上面未加锁成功的阻塞线程，要解锁的时候从容器中拿出线程进行解锁（为什么选择队列？因为ReentrantLock要实现公平与非公平锁，所以选择先进先出的队列）

public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /**
     * 内部调用AQS的动作，都基于该成员属性实现
     */
    private final Sync sync;
 
    /**
     * ReentrantLock锁同步操作的基础类,继承自AQS框架.
     * 该类有两个继承类，1、NonfairSync 非公平锁，2、FairSync公平锁
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;
 
        /**
         * 加锁的具体行为由子类实现
         */
        abstract void lock();
 
        /**
         * 尝试获取非公平锁
         */
        final boolean nonfairTryAcquire(int acquires) {
            //acquires = 1
            final Thread current = Thread.currentThread();
            int c = getState();
            /**
             * 不需要判断同步队列（CLH）中是否有排队等待线程
             * 判断state状态是否为0，不为0可以加锁
             */
            if (c == 0) {
                //unsafe操作，cas修改state状态
                if (compareAndSetState(0, acquires)) {
                    //独占状态锁持有者指向当前线程
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            /**
             * state状态不为0，判断锁持有者是否是当前线程，
             * 如果是当前线程持有 则state+1
             */
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            //加锁失败
            return false;
        }
 
        /**
         * 释放锁
         */
        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }
 
        /**
         * 判断持有独占锁的线程是否是当前线程
         */
        protected final boolean isHeldExclusively() {
            return getExclusiveOwnerThread() == Thread.currentThread();
        }
 
        //返回条件对象
        final ConditionObject newCondition() {
            return new ConditionObject();
        }
 
 
        final Thread getOwner() {
            return getState() == 0 ? null : getExclusiveOwnerThread();
        }
 
        final int getHoldCount() {
            return isHeldExclusively() ? getState() : 0;
        }
 
        final boolean isLocked() {
            return getState() != 0;
        }
 
        private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }
 
    /**
     * 非公平锁
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
 
        /**
         * 加锁行为
         */
        final void lock() {
            /**
             * 第一步：直接尝试加锁
             * 与公平锁实现的加锁行为一个最大的区别在于，此处不会去判断同步队列(CLH队列)中
             * 是否有排队等待加锁的节点，上来直接加锁（判断state是否为0,CAS修改state为1）
             * ，并将独占锁持有者 exclusiveOwnerThread 属性指向当前线程
             * 如果当前有人占用锁，再尝试去加一次锁
             */
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                //AQS定义的方法,加锁
                acquire(1);
        }
 
        /**
         * 父类AbstractQueuedSynchronizer.acquire()中调用本方法
         */
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }
 
    /**
     * 公平锁
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;
 
        final void lock() {
            acquire(1);
        }
 
        /**
         * 重写aqs中的方法逻辑
         * 尝试加锁，被AQS的acquire()方法调用
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                /**
                 * 与非公平锁中的区别，需要先判断队列当中是否有等待的节点
                 * 如果没有则可以尝试CAS获取锁
                 */
                if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) {
                    //独占线程指向当前线程
                    setExclusiveOwnerThread(current);
                    return true;
                }
            } else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }
 
    /**
     * 默认构造函数，创建非公平锁对象
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }
 
    /**
     * 根据要求创建公平锁或非公平锁
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }
 
    /**
     * 加锁
     */
    public void lock() {
        sync.lock();
    }
 
    /**
     * 尝试获去取锁，获取失败被阻塞，线程被中断直接抛出异常
     */
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }
 
    /**
     * 尝试加锁
     */
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }
 
    /**
     * 指定等待时间内尝试加锁
     */
    public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }
 
    /**
     * 尝试去释放锁
     */
    public void unlock() {
        sync.release(1);
    }
 
    /**
     * 返回条件对象
     */
    public Condition newCondition() {
        return sync.newCondition();
    }
 
    /**
     * 返回当前线程持有的state状态数量
     */
    public int getHoldCount() {
        return sync.getHoldCount();
    }
 
    /**
     * 查询当前线程是否持有锁
     */
    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }
 
    /**
     * 状态表示是否被Thread加锁持有
     */
    public boolean isLocked() {
        return sync.isLocked();
    }
 
    /**
     * 是否公平锁？是返回true 否则返回 false
     */
    public final boolean isFair() {
        return sync instanceof FairSync;
    }
 
    /**
     * 获取持有锁的当前线程
     */
    protected Thread getOwner() {
        return sync.getOwner();
    }
 
    /**
     * 判断队列当中是否有在等待获取锁的Thread节点
     */
    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }
 
    /**
     * 当前线程是否在同步队列中等待
     */
    public final boolean hasQueuedThread(Thread thread) {
        return sync.isQueued(thread);
    }
 
    /**
     * 获取同步队列长度
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }
 
    /**
     * 返回Thread集合，排队中的所有节点Thread会被返回
     */
    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }
 
    /**
     * 条件队列当中是否有正在等待的节点
     */
    public boolean hasWaiters(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject) condition);
    }
 
}