code: public static String read(InputStream is, int off, int len) throws IOException {
          byte[] bs = new byte[len];
          is.read(bs, off, len);
          return new String(bs);
      }
docs: 读取输入流，需手动关闭流
  
  @param is 输入流
  @param off 偏移
  @param len 长度
  
  @return 内容
  
  @throws IOException 异常