code: public static String toHexString(byte[] b) {
          if (b == null)
              return null;
  
          StringBuilder sb = new StringBuilder(b.length * 2);
          for (int i = 0; i < b.length; i++) {
              int v = b[i] & 0xFF;
              String hv = Integer.toHexString(v);
              if (hv.length() < 2) {
                  sb.append(0);
              }
              sb.append(hv);
          }
          return sb.toString();
      }
docs: 将 byte[] 数组转换成16进制字符串
  
  @param b
  @return 传入参数为null, 将返回null