详解tryAcquire()、addWaiter()、acquireQueued()

final boolean nonfairTryAcquire(int acquires) {
      final Thread current = Thread.currentThread();
      int c = getState();
      if (c == 0) {
        if (compareAndSetState(0, acquires)) {
          setExclusiveOwnerThread(current);
          return true;
        }
      }
      else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
          throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
      }
      return false;
    }

/**
   * Creates and enqueues node for current thread and given mode.
   *
   * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
   * @return the new node
   */
  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;
    if (pred != null) {
      node.prev = pred;
      if (compareAndSetTail(pred, node)) {
        pred.next = node;
        return node;
      }
    }
    enq(node);
    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())
          interrupted = true;
      }
    } finally {
      if (failed)
        cancelAcquire(node);
    }
  }