protected function checkUserClass()
    {
        $userClass = $this->userClass;
        if (!class_exists($userClass)) {
            throw new Exception('User Class Invalid.');
        }
        if (!((new $userClass()) instanceof User)) {
            throw new Exception('User Class(' . $userClass . ') does not inherited from `\rhosocial\user\User`.');
        }
        return $userClass;
    }