code: public Long peek() throws InterruptedException {
          lock.lockInterruptibly();
          try {
              while (queue.size() == 0) {
                  notEmpty.await();
              }
  
              return queue.peek();
          } finally {
              lock.unlock();
          }
      }
docs: 从queue中获取第一个数据节点，不移除，等待ack信号进行移除
  
  @return
  @throws InterruptedException