code: public long load() throws IOException {
          if (item.isStreamEnd)
              return readed;
          RingItem ri = item;
          while (!ri.isLoaded) {
              ri.load(ins);
              if (ri.max > 0)
                  readed += ri.max;
              ri = ri.next;
          }
          return readed;
      }
docs: 从当前节点的 next 开始，依次将所有可用的节点全部加载满
  
  @return 一共读去的字节数
  
  @throws IOException