code: protected String readString(InputStream in) throws IOException {
          int length = readInt(in);
          if (length < 0) {
              return null;
          } else if (length == 0) {
              return StringUtils.EMPTY;
          } else {
              byte[] value = new byte[length];
              in.read(value);
              return StringSerializer.decode(value);
          }
      }
docs: 读取一个字符串
  
  @param in 输入流程
  @return 字符串
  @throws IOException 读取异常