public function validateRouteCallback(array $routes)
    {
        foreach ($routes as $route) {
            $callback = $route[2];
            if (is_array($callback)) {
                $class = $callback[0];
                $method = $callback[1];
                if (!class_exists($class, true)) {
                    throw new Exception("Controller {$class} does not exist.");
                }
                // rebless a controller (extract this to common method)
                $controller = new $class();
                if (!method_exists($controller, $method)) {
                    throw new Exception("Method $method not found in controller $class.");
                }
            }
        }
    }