(转)ReentrantLock代码剖析之ReentrantLock.lockInterruptibly

ReentrantLock.lockInterruptibly允许在等待时由其它线程调用等待线程的Thread.interrupt方法来中断等待线程的等待而直接返回，这时不用获取锁，而会抛出一个InterruptedException。而ReentrantLock.lock方法不允许Thread.interrupt中断,即使检测到Thread.isInterrupted,一样会继续尝试获取锁，失败则继续休眠。只是在最后获取锁成功后再把当前线程置为interrupted状态。

那lockInterruptibly是如何做到这一点的？

   public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

这里调用了AbstractQueuedSynchronizer.acquireInterruptibly方法。如果线程已被中断则直接抛出异常，否则则尝试获取锁，失败则doAcquireInterruptibly

AbstractQueuedSynchronizer.acquireInterruptibly(int arg)
 /**
     * Acquires in exclusive mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking
     * at least once {@link #tryAcquire}, returning on
     * success.  Otherwise the thread is queued, possibly repeatedly
     * blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted.  This method can be
     * used to implement method {@link Lock#lockInterruptibly}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireInterruptibly(int arg) throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

AbstractQueuedSynchronizer.doAcquireInterruptibly大体上相当于前面的acquireQueued,关键的区别在于检测到interrupted后的处理，acquireQueued简单的记录下中断曾经发生，然后就象没事人似的去尝试获取锁，失败则休眠。而doAcquireInterruptibly检测到中断则直接退出循环，抛出InterruptedException异常。

AbstractQueuedSynchronizer.doAcquireInterruptibly(int arg)
/**
     * Acquires in exclusive interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return;
                }
                /*
                acquireQueued代码:
           if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
*/
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    break;
            }
        } catch (RuntimeException ex) {
            cancelAcquire(node);
            throw ex;
        }
        // Arrive here only if interrupted
        // 取消获取锁尝试，将当前节点从等待队列中移除
        cancelAcquire(node);
        throw new InterruptedException();
    }

在抛出异常之前，doAcquireInterruptibly还做了一件事情，cancelAcquire。cancelAcquire中有些细节值得玩味，参见代码中笔者注释。

AbstractQueuedSynchronizer.cancelAcquire(Node node)
/**
     * Cancels an ongoing attempt to acquire.
     *
     * @param node the node
     */
    private void cancelAcquire(Node node) {
    // Ignore if node doesn't exist
        if (node == null)
        return;

    node.thread = null;

    // Skip cancelled predecessors
    // 头节点一定不会是在等待状态，所以不会被cancel,所以这里一定能找到一个节点而不用担心null
    Node pred = node.prev;
    while (pred.waitStatus > 0)
        node.prev = pred = pred.prev;

    // Getting this before setting waitStatus ensures staleness
    Node predNext = pred.next;
    // Can use unconditional write instead of CAS here
    node.waitStatus = Node.CANCELLED;

    // If we are the tail, remove ourselves
    if (node == tail && compareAndSetTail(node, pred)) {
        compareAndSetNext(pred, predNext, null);
    } else {
        // If "active" predecessor found...
        if (pred != head
        && (pred.waitStatus == Node.SIGNAL
            || compareAndSetWaitStatus(pred, 0, Node.SIGNAL))
        && pred.thread != null) {

        // If successor is active, set predecessor's next link
        Node next = node.next;
        if (next != null && next.waitStatus <= 0)
            compareAndSetNext(pred, predNext, next);
        } else {
        /*这里如果不调用unparkSuccessor, 若在interrupted之后，执行到上面一句将waitStatus置CANCELLED之前，锁被释放，该线程被唤醒，则释放锁线程的unparkSuccessor不能起到预期作用，所以这里需要调用unparkSuccessor.即使此时持有锁的线程没有释放锁也不会有严重后果，被unpark的线程在获取锁失败后会继续park*/
        unparkSuccessor(node);
        }
        node.next = node; // help GC
    }
    }