code: @Override
      @Transactional(readOnly = true)
      public void validateToken(E userToken) throws Exception {
  
          // call super
          super.validateToken(userToken);
  
          // check if user is associated
          if (userToken.getUser() == null) {
              throw new Exception("There is no user associated with this token.");
          }
  
      }
docs: If the passed token is null or expired or if there is no user associated
  with the token, this method will throw an {@link Exception}.
  
  @param userToken
  @throws Exception if the token is not valid (e.g. because it is expired)