//获取状态
public final void acquire(int arg) {if (!tryAcquire(arg) &&acquireQueued(addWaiter(Node.EXCLUSIVE), arg))selfInterrupt();}
//该方法由子类去实现（改变state的值）
protected boolean tryAcquire(int arg) {throw new UnsupportedOperationException();}
//为snyc队列添加节点
private Node addWaiter(Node mode) {Node node = new Node(Thread.currentThread(), mode);// Try the fast path of enq; backup to full enq on failureNode pred = tail;if (pred != null) {node.prev = pred;//CAS更新尾节点if (compareAndSetTail(pred, node)) {pred.next = node;return node;}}enq(node);return node;}
//判断有没有释放状态的线程并且尝试获取状态，或者将线程挂起
final boolean acquireQueued(final Node node, int arg) {boolean failed = true;try {boolean interrupted = false;for (;;) {final Node p = node.predecessor();//当前线程所在节点的前驱节点为头节点，说明前驱节点所代表的线程正在执行，这个时候尝试获取状态来验证前驱节点的线程有没有执行完，if (p == head && tryAcquire(arg)) {setHead(node);//当前线程已获得状态，执行当前线程。将当前线程设置为头节点p.next = null; // help GCfailed = false;return interrupted;//退出当前循环
                }//将当前线程挂起if (shouldParkAfterFailedAcquire(p, node) &&parkAndCheckInterrupt())interrupted = true;}} finally {if (failed)cancelAcquire(node);}}

//线程执行完后释放持有的状态
public final boolean release(int arg) {if (tryRelease(arg)) {Node h = head;if (h != null && h.waitStatus != 0)unparkSuccessor(h);return true;}return false;}
//子类实现（改变状态的值）
protected boolean tryRelease(int arg) {throw new UnsupportedOperationException();}
//将头节点的后继节点的线程恢复，这样后继节点就可以继续执行之前的循环，获取状态。
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);}