1、unlock()

  public void unlock() {
        sync.release(1);
    }

unlock()呼叫的release()方法，方法中引數傳入1。跟獲取鎖一樣，每呼叫一次unlock()，鎖的狀態減1，當鎖的狀態等於0時，將會喚醒佇列中的下一個節點。

2、release()

 public final boolean release(int arg) {
        if (tryRelease(arg)) {//釋放鎖，如果鎖的狀態為0時，返回true
            Node h = head;
            if (h != null && h.waitStatus != 0)//如果鎖狀態為0時，喚醒下一個節點
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
 

 protected final boolean tryRelease(int releases) {
            int c = getState() - releases;//獲取鎖的狀態
            if (Thread.currentThread() != getExclusiveOwnerThread())//如果釋放鎖的執行緒不是持有鎖的執行緒，將會丟擲異常
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {//執行緒持有的鎖都釋放完了
                free = true;
                setExclusiveOwnerThread(null);//將持有鎖的執行緒置為空
            }
            setState(c);//重新設定鎖的狀態
            return free;
        }
 

4、unparkSuccessor()：喚醒下一個執行緒

  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;//頭節點的下一個節點
        if (s == null || s.waitStatus > 0) {//如果頭節點的下一個節點為空或者已經取消
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)//從尾節點往前迴圈查詢最靠近頭節點且狀態為SINGLE的節點
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);//喚醒節點
    }