confused when using createController and render

<<<<Yii 2.0>>>>

in vendor/yiisoft/yii2/Yii.php ,I add a function like

    static private function mytest(){

        $cc = Yii::$app->createController('site/show');





while in frontend/controllers/SiteContoller.php,action is such defined

        public function actionShow(){

                return $this->render('show');


and in frontend/themes/default/site/show.php,it,s just,

<?php echo "ssssssss";?>

question is ,when call,Yii::mytest,i just got a white page with nothing on it!

is there anything wrong on the function mytest()??