ReentrantLock线程未获取到锁进入等待队列原理

公平锁　　

非公平锁　都会调用acquire方法

    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

1 使用tryacquire获取锁 2 未获取到锁，则将当前线程添加到等到队列

acquireQueued(addWaiter(Node.EXCLUSIVE), arg))原理剖析

1. 线程A获取到锁，但是未释放
2. 线程B尝试获取锁，但是锁未被释放，因此进入等待队列
3. 线程C尝试获取锁，但是锁未被释放，因此进入等待队列

 AbstractQueuedSynchronizer

    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

 当前线程的前驱不是head节点（空节点，不存储线程相关信息），且获取锁失败(tryAcquire)，则判断是否需要阻塞当前线程 shouldParkAfterFailedAcquire

    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * 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 {
                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.
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

 判断当前线程是否需要被阻塞（加入等待队列）流程图 shouldParkAfterFailedAcquire