ReentrantLock代码剖析之ReentrantLock.unlock

下面来看ReentrantLock.unlock，尝试在当前锁的锁定计数即state值上减1,而lock每次把锁定计数加1,这也是为什么lock和unlock必须成对出现，否则锁定计数就不能正常恢复到0，其它线程就不能尝试获取锁

/**
     * Attempts to release this lock.
     *
     * <p>If the current thread is the holder of this lock then the hold
     * count is decremented.  If the hold count is now zero then the lock
     * is released.  If the current thread is not the holder of this
     * lock then {@link IllegalMonitorStateException} is thrown.
     *
     * @throws IllegalMonitorStateException if the current thread does not
     *         hold this lock
     */
    public void unlock() {
        sync.release(1);
    }

AbstractQueuedsynchronizer.release(int arg)方法会在锁定数目上减去arg,若新锁定数目为0，表示锁被当前线程释放, 则试图唤醒等待队列中的下一个线程。请注意这里仅是唤醒，而非把锁的所有权交给下一个线程。该线程能否成功获取锁，还要看运气。

 

/**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryRelease} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            /*head的waitStatus不为零表示它的后继在等待唤醒，
             还记得AbstractQueuedSynchronizer.shouldParkAfterFailedAcquire中对waitStatus的操作么？waitStatus!=0表明或者处于CANCEL状态，或者是置SIGNAL表示下一个线程在等待其唤醒,CANCEL状态这里先不分析,可以认为这里!=0即表示SIGNAL状态*/
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

tryRelease定义在Sync类中

Sync.tryRelease会在当前锁定状态上减去要释放的锁定数，如果锁定状态为零则置当前持有锁线程为null,返回true,否则返回false

此外还会检查是否是占有锁的线程在调用，若不是则抛出IllegalMonitorStateException。

 

  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;
        }

再来看unparkSuccessor

 

    /**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * Try to clear status in anticipation of signalling.  It is
         * OK if this fails or if status is changed by waiting thread.
         */
//若当前节点waitStatus被置为SIGNAL则清零
        compareAndSetWaitStatus(node, Node.SIGNAL, 0);

        /*
         * 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.
         */
        // 若后续节点为空或已被cancel，则从尾部开始找到队列中第一个waitStatus<=0，即未被cancel的节点
        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);
    }

 

再回头看AbstractQueuedSynchronizer.acquireQueued,注释1,2,3,4表明了在线程被唤醒后的执行顺序

/**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     *
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        try {
            boolean interrupted = false;
            for (;;) {
                //2 因为等待队列只会从尾部插入，所以在被唤醒节点和头节点之间不会新加入等待节点，只会有节点被cancel
                //3 如果被唤醒节点不是头节点的直接后续，由唤醒算法可以保证其和头结点之间没有waitStatus<=0的节点,但有可能有waitStatus大于0,即被CANCEL的节点,参见注释4
                 // 5获取锁成功则把当前节点设置为头节点，返回，否则线程继续被禁止，直到下一次被唤醒
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return interrupted;
                }
                // 1 被lock.release唤醒，parkAndCheckInterrupt返回false,继续回到for循环开始
                  //4 被cancel的结点在shouldParkAfterFailedAcquire中会被清理掉,当前节点会成为头节点的直接后继,然后return false,在下次循环中即可尝试获取锁
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } catch (RuntimeException ex) {
            cancelAcquire(node);
            throw ex;
        }
    }