public static function getInstance($configFile = null, $mode = self::APP_MODE_PROD) {
        $c = get_called_class();
        if (!isset(self::$_instances[$c])) {
            // Create an instance
            self::$_instances[$c] = new $c($configFile, $mode);
            // Setup application
            self::$_instances[$c]->setup();
        }

        return self::$_instances[$c];
    }