AQS(AbstractQueuedSynchronizer)抽象队列同步器及工作原理解析

	private Node addWaiter(Node mode) {
     Node node = new Node(Thread.currentThread(), mode);
     // Try the fast path of enq; backup to full enq on failure
     Node pred = tail;// 将tail的前指针赋值过去
     if (pred != null) { //一开始的时候并没有指向所以位null，因此条件不成立不走里面的语句
         node.prev = pred;
         if (compareAndSetTail(pred, node)) {//CAS
             pred.next = node;
             return node;
         }
     }
     enq(node);//一开始会来走这个enq()这个方法。
     return node;
 }

	private Node enq(final Node node) {
     for (;;) {
         Node t = tail;
         if (t == null) { // Must initialize
             if (compareAndSetHead(new Node()))
                 tail = head;
         } else {
             node.prev = t;
             if (compareAndSetTail(t, node)) {
                 t.next = node;
                 return t;
             }
         }
     }
 }

	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()) //第一次的时候会阻塞在前面的条件并将哨兵的waitState置为-1，第二次的话第一个条件为真，所以会走第二个判断条件，会将访问的线程给阻塞掉等待unpark();
                 interrupted = true;
         }
     } finally {
         if (failed)
             cancelAcquire(node);
     }
 }

	private final boolean parkAndCheckInterrupt() {
     LockSupport.park(this); //将访问次方法的线程给阻塞掉，等待unpark()方法唤醒
     return Thread.interrupted();
 }

	private void unparkSuccessor(Node node) {
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);
        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); //释放锁唤醒等待队列中的等待线程
    }