concurrent-4-AQS-ReentrantLock-Lock

    可重入锁源码解析。

AbstractQueuedSynchronizer#Node 节点信息

tatic final class Node {
        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();   //共享模式
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;   //独占模式

        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1;  ////因为超时或者中断，node会被设置成取消状态，被取消的节点时不会参与到竞争中的，会一直保持取消状态不会转变为其他状态； 
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL    = -1; //若节点的后继节点被阻塞，当前节点释放锁或者取消的时候需要唤醒后继者。 
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;   //表明线程在等待中
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3;   //该状态表示下一次节点如果是Shared的，则无条件获取锁。

        volatile int waitStatus;   //节点的状态值

        volatile Node prev;   //前节点

        volatile Node next;  //后节点  

        volatile Thread thread;  //节点关联线程  

        Node nextWaiter;   //连接到下一个节点等待条件，或特殊值共享

lock获取资源

ReenTrantLock#lock

//加锁
 public void lock() {
        sync.lock();   //实际为封装AQS实现类
    }

ReenTrantLock#NonfairSync#lock

   final void lock() {
            if (compareAndSetState(0, 1))   //尝试cas操作，抢占锁
                setExclusiveOwnerThread(Thread.currentThread());   //标记独占锁为被当前线程
            else
                acquire(1);     //尝试获取锁(AQS方法)
        }

AbstractQueuedSynchronizer#acquire

public final void acquire(int arg) {
        if (!tryAcquire(arg) &&  //尝试直接去获取资源，如果成功则直接返回
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            //addWaiter：线程加入等待队列的尾部，并标记为独占模式
            //acquireQueued 尝试获取资源，如果获取了资源被且没有被标记为中断则返回false，否则返回true
            selfInterrupt();  //等待中标记中断，获取资源后自我中断
    }

ReenTrantLock#NonfairSync#tryAcquire

protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);   //非公平锁的获取资源方式
        }

ReenTrantLock#NonfairSync#nonfairTryAcquire

 final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();  //获取状态
            if (c == 0) {
                //如果是0，则尝试cas操作加锁，无须进入队列排队，可能被其它线程抢占
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {   //如果已经被抢占，判断是否为当前线程抢占
                int nextc = c + acquires;    //增加抢占锁的次数  （可重入）
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);    //设置状态
                return true;
            }
            return false;
        }

AbstractQueuedSynchronizer#addWaiter

 private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {   //如果pred 不为null 则非初始化状态 尝试cas操作，失败则enq进行循环cas操作
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {  //尝试入队列
                pred.next = node;
                return node;
            }
        }
        enq(node);  //循环cas入sync队列
        return node;
    }

AbstractQueuedSynchronizer#enq

private Node enq(final Node node) {
        for (;;) {
            Node t = tail;   //指定 t 为 尾部节点
            if (t == null) { // Must initialize   //初始化
                if (compareAndSetHead(new Node()))   //将head位设置为新节点
                    tail = head;
            } else {
                node.prev = t;    //将node 节点的prev 标记为 tail节点（可能有其它线程在重复改变tail）
                if (compareAndSetTail(t, node)) {   //如果cas操作成功，那么将node节点标记为tail节点
                    t.next = node;     //同时将t（上一次的tail节点）的下一个节点标记为node（新的tail节点）
                    return t;
                }
            }
        }
    }

AbstractQueuedSynchronizer#acquireQueued

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;  //标记是否成功获取资源
        try {
            boolean interrupted = false; //标记是否被中断
            for (;;) {
                final Node p = node.predecessor();  //获取前节点
                if (p == head && tryAcquire(arg)) {   //如果前节点是head，则尝试获取资源！！！！！队列中头部节点先获取资源
                    setHead(node);   //获取成功，将本节点设置为head节点
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;  //成功获取资源并且未被中断
                }
                //shouldParkAfterFailedAcquire：判断在获取资源失败后，是否应该睡眠（根据前节点的waitStatus）。
                //parkAndCheckInterrupt：睡眠的同时，判断是否有中断状态
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);  //如果失败则取消获取资源
        }
    }

AbstractQueuedSynchronizer#shouldParkAfterFailedAcquire

 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;  //下一个节点的等待状态
        if (ws == Node.SIGNAL)    //signal 状态则正常放回
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
        return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {   //如要将所有>0 即取消的节点都排除
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
            //cas 操作将节点的waitStatus标记为signal状态
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

UnLock

ReenTrantLock#unLock

 public void unlock() {
        sync.release(1);   //释放资源
    }

AbstractQueuedSynchronizer#release

public final boolean release(int arg) {
        if (tryRelease(arg)) {  //尝试释放锁成功（可能重入了多次锁）
            Node h = head;
            //如果没有队列外的线程获取锁，那么获取锁资源的一定是队列头，此处需要唤醒其它线程
            //从队尾开始唤醒，因为队尾可能一直会新加入线程，双端队列容易死循环
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

ReenTrantLock#tryRelease

protected final boolean tryRelease(int releases) {
            int c = getState() - releases;  //获取当前线程状态值
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {   //如果线程状态为0，则表示已经释放了锁
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);   //设置状态
            return free;
        }

AbstractQueuedSynchronizer#unparkSuccessor

private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;  //获取线程的下一个节点的等待状态
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);  //标记该节点线程已经被释放

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        //如果下一个节点为null，或者状态为取消
        if (s == null || s.waitStatus > 0) {
            s = null;
            //从尾部开始寻找一个可以被唤醒的节点，尾部节点是稳定的
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }