code: protected ID convertCookieUserKeyToUserId(String userKey) {
          final ID userId;
          try {
              userId = toTypedUserId(userKey); // as default (override if it needs)
          } catch (NumberFormatException e) {
              throw new LoginFailureException("Invalid user key (not ID): " + userKey, e);
          }
          return userId;
      }
docs: Convert user key to user ID by your rule. #change_user_key <br>
  The default rule is simple conversion to number, you can change it by overriding.
  @param userKey The string key for the login user, same as remember-me key. (NotNull)
  @return The ID for the login user. (NotNull)