总体来讲，acquireQueued就是依靠前驱节点的状态来决定当前线程是否应该处于阻塞状态，如果前驱节点处于cancel状态，则丢弃这些节点，重新构建队列；
Unlock API详解流程类似lock api相关类的流程，这里讲主要的代码，unlock相对的比较简单
首先 ReentrantLock 调用 Sync的release接口也就是AbstractQueuedSynchronizer的release接口
/** * 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; if (h != null && h.waitStatus != 0) unparkSuccessor(h); return true; } return false; }
这个时候会先调用Sync的tryRelease，如果返回true，则释放锁成功
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; }
这个接口的作用很简单，如果不是获得锁的线程调用直接抛出异常，否则，如果当前state-releases==0也就是lock已经完全释放，返回true，清除资源；
这个返回free之后，release拿到head节点，进入以下代码：
/** * Wakes up node's successor, if one exists. * * @param node the node */ 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; if (ws < 0) compareAndSetWaitStatus(node, ws, 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. */ 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); }
这个作用即：当头结点的状态小于0，则将头结点的状态CAS为0，然后通过链表获取下一个节点，如果下一个节点为null或者不符合要求的状态，则从队尾遍历整个链表，直到遍历到离head节点最近的一个节点并且
等待状态符合预期，则将头结点的后继节点置为该节点；
对刚刚筛出来的符合要求的节点唤醒，也就是该节点获得 争夺 锁的权利；
这就是非公平锁的特点：在队列一直等待的线程不一定比后来的线程先获得锁，至此，unlock 已经解释完成；
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