/**
     * Terminates the application.
     * This method replaces the `exit()` function by ensuring the application life cycle is completed
     * before terminating the application.
     * @param integer $status the exit status (value 0 means normal exit while other values mean abnormal exit).
     * @param Response $response the response to be sent. If not set, the default application [[response]] component will be used.
     * @throws ExitException if the application is in testing mode
*/

这是注释。意思就是end()为确保程序的生命周期的完成。