【java并发编程实战6】AQS之独占锁ReentrantLock实现前言关于AQS锁的实现

  public void lock() {
        sync.lock();
   }

    /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync{..}
    /**
     * Sync object for fair locks
     */
    static final class FairSync extends Sync{..}

    final void lock() {
            acquire(1);
        }

/**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) 
            selfInterrupt();
    }

   protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();//获取当前线程
            int c = getState();  //获取父类AQS中的标志位
            if (c == 0) {
                if (!hasQueuedPredecessors() && 
                    //如果队列中没有其他线程  说明没有线程正在占有锁！
                    compareAndSetState(0, acquires)) { 
                    //修改一下状态位，注意：这里的acquires是在lock的时候传递来的，从上面的图中可以知道，这个值是写死的1
                    setExclusiveOwnerThread(current);
                    //如果通过CAS操作将状态为更新成功则代表当前线程获取锁，因此，将当前线程设置到AQS的一个变量中，说明这个线程拿走了锁。
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
             //如果不为0 意味着，锁已经被拿走了，但是，因为ReentrantLock是重入锁，
             //是可以重复lock,unlock的，只要成对出现行。一次。这里还要再判断一次 获取锁的线程是不是当前请求锁的线程。
                int nextc = c + acquires;//如果是的，累加在state字段上就可以了。
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

   /**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    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) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                // 一个空的节点，通常代表获取锁的线程
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                // 获取当前节点前驱结点
                final Node p = node.predecessor();
                // 如果前驱节点是head，那么它就是等待队列中的第一个线程
                // 因为我们知道head就是获取线程的节点，那么它就有机会再次获取锁
                if (p == head && tryAcquire(arg)) {
                    //成功后，将上图中的黄色节点移除，Node1变成头节点。 也证实了head就是获取锁的线程的节点。
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 1、检查前一个节点的状态，判断是否要挂起
                // 2、如果需要挂起，则通过JUC下的LockSopport类的静态方法park挂起当前线程，直到被唤醒。
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            // 如果发生异常
            if (failed)
                // 取消请求，也就是将当前节点重队列中移除。
                cancelAcquire(node);
        }
    }

       /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3;

public void unlock() {
    sync.release(1);
}

  public final boolean release(int arg) {
        // 尝试释放锁
        if (tryRelease(arg)) {
            // 释放锁成功，获取当前队列的头节点
            Node h = head;
            if (h != null && h.waitStatus != 0)
                // 唤醒当前节点的下一个节点
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    protected final boolean tryRelease(int releases) {
        
            int c = getState() - releases;
            // 当前线程不是获取锁的线程 抛出异常
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            // 因为是重入的关系，不是每次释放锁c都等于0，直到最后一次释放锁时，才通知AQS不需要再记录哪个线程正在获取锁。
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
    }

 private void unparkSuccessor(Node node) {
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }
}

static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }
}