protected function registerRouteMiddleware()
    {
        // register route middleware.
        foreach ($this->routeMiddleware as $key => $middleware) {
            app('router')->aliasMiddleware($key, $middleware);
        }

        // register middleware group.
        foreach ($this->middlewareGroups as $key => $middleware) {
            app('router')->middlewareGroup($key, $middleware);
        }
    }