并发之父

Doug Lea(小名：李二狗)

Java并发编程核心在于java.util.concurrent包而juc当中的大多数同步器实现都是围绕着共同的基础行为，比如等待队列、条件队列、独占获取、共享获取等，而这个行为的抽象就是基于AbstractQueuedSynchronizer简称AQS，AQS定义了一套多线程访问共享资源的同步器框架，是一个依赖状态(state)的同步器。

ReentrantLock

ReentrantLock是一种基于AQS框架的应用实现，是JDK中的一种线程并发访问的同步手段，它的功能类似于synchronized是一种互斥锁，可以保证线程安全。而且它具有比synchronized更多的特性，比如它支持手动加锁与解锁，支持加锁的公平性。

在ReentrantLock内部定义了一个Sync的内部类，该类继承AbstractQueuedSynchronized，对该抽象类的部分方法做了实现；并且还定义了两个子类： 1、FairSync 公平锁的实现 2、NonfairSync 非公平锁的实现 这两个类都继承自Sync，也就是间接继承了AbstractQueuedSynchronized，所以这一个ReentrantLock同时具备公平与非公平特性。 上面主要涉及的设计模式：模板模式-子类根据需要做具体业务实现

AQS具备特性

• 阻塞等待队列
• 共享/独占
• 公平/非公平
• 可重入
• 允许中断

除了Lock外，Java.util.concurrent当中同步器的实现如Latch,Barrier,BlockingQueue等，都是基于AQS框架实现

• 一般通过定义内部类Sync继承AQS
• 将同步器所有调用都映射到Sync对应的方法

AQS内部维护属性volatile int state (32位)

• state表示资源的可用状态

State三种访问方式
getState()、setState()、compareAndSetState()
AQS定义两种资源共享方式

• Exclusive-独占，只有一个线程能执行，如ReentrantLock
• Share-共享，多个线程可以同时执行，如Semaphore/CountDownLatch

AQS定义两种队列

• 同步等待队列
• 条件等待队列

不同的自定义同步器争用共享资源的方式也不同。自定义同步器在实现时只需要实现共享资源state的获取与释放方式即可，至于具体线程等待队列的维护（如获取资源失败入队/唤醒出队等），AQS已经在顶层实现好了。自定义同步器实现时主要实现以下几种方法：

• isHeldExclusively()：该线程是否正在独占资源。只有用到condition才需要去实现它。
• tryAcquire(int)：独占方式。尝试获取资源，成功则返回true，失败则返回false。
• tryRelease(int)：独占方式。尝试释放资源，成功则返回true，失败则返回false。
• tryAcquireShared(int)：共享方式。尝试获取资源。负数表示失败；0表示成功，但没有剩余可用资源；正数表示成功，且有剩余资源。
• tryReleaseShared(int)：共享方式。尝试释放资源，如果释放后允许唤醒后续等待结点返回true，否则返回false。

  同步等待队列

  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。
  

  条件等待队列

  Condition是一个多线程间协调通信的工具类，使得某个，或者某些线程一起等待某个条件（Condition）,只有当该条件具备时，这些等待线程才会被唤醒，从而重新争夺锁

  AQS源码分析

  public abstract class AbstractQueuedSynchronizer
  extends AbstractOwnableSynchronizer
  implements java.io.Serializable {

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    private static final long serialVersionUID = 7373984972572414691L;
protected AbstractQueuedSynchronizer() { }
  /**
     * Wait queue node class.
     *
     * 不管是条件队列，还是CLH等待队列
     * 都是基于Node类
     * 
     * 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;
    /**
     * 
     * @return current state value
     */
    protected final int getState() {
        return state;
    }
    protected final void setState(int newState) {
        state = newState;
    }
    //cas替换
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }
    // Queuing utilities
    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;
    }
    //释放锁
    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(-1)或者PROPAGATE(-3)
     * 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，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;
        // 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.
        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.
        node.waitStatus = Node.CANCELLED;
        // If we are the tail, remove ourselves.
        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.
            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
        }
    }
    //设置waitStatus 状态
    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);
        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);
        }
    }
    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    /**
     * Acquires in shared timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    /**
     * 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；
     *
     *　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);
    }
    /**
     * Releases in shared mode.  Implemented by unblocking one or more
     * threads if {@link #tryReleaseShared} returns true.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryReleaseShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    // Queue inspection methods
    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();
    }
    /**
     * Version of getFirstQueuedThread called when fastpath fails
     */
    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;
        }
        /*
         * 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;
    }
    /**
     * 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;
        /**
         * Creates a new {@code ConditionObject} instance.
         */
        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);
        }
        /**
         * Implements uninterruptible condition wait.
         * <ol>
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * </ol>
         */
        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);
        }
        /**
         * Implements timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        public final boolean await(long time, TimeUnit unit)
                throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }
        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }
        /**
         * Queries whether any threads are waiting on this condition.
         * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
         *
         * @return {@code true} if there are any waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        protected final boolean hasWaiters() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }
        /**
         * Returns an estimate of the number of threads waiting on
         * this condition.
         * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
         *
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        protected final int getWaitQueueLength() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }
        /**
         * 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;
        }
    }
    /**
     * Setup to support compareAndSet. We need to natively implement
     * this here: For the sake of permitting future enhancements, we
     * cannot explicitly subclass AtomicInteger, which would be
     * efficient and useful otherwise. So, as the lesser of evils, we
     * natively implement using hotspot intrinsics API. And while we
     * are at it, we do the same for other CASable fields (which could
     * otherwise be done with atomic field updaters).
     * 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);
    }
}