code: public static String hmac(String string) throws NoSuchAlgorithmException, InvalidKeyException {
          String code = "";
          KeyGenerator keyGenerator = KeyGenerator.getInstance("HmacMD5");
          SecretKey secretKey = keyGenerator.generateKey();
          SecretKey secretKey2 = new SecretKeySpec(secretKey.getEncoded(), "HmacMD5");
          Mac mac = Mac.getInstance(secretKey.getAlgorithm());
          mac.init(secretKey2);
          return code;
      }
docs: HMAC加密，单向
  
  @param string {@link String}
  
  @return {@link String}
  
  @throws NoSuchAlgorithmException 异常
  @throws InvalidKeyException 异常