code: private static String hex(byte[] arr) {
          StringBuffer sb = new StringBuffer();
          for (int i = 0; i < arr.length; ++i) {
              sb.append(Integer.toHexString((arr[i] & 0xFF) | 0x100).substring(1,
                      3));
          }
          return sb.toString();
      }
docs: 字节转换为16进制
  
  @param arr
  字节数组
  @return 16进制字 串