应用程序必须运行完所有前台线程后才能退出； 对于后台线程，应用程序可以不考虑其是否已运行完毕而直接退出，所有后台线程在应用程序退出时都会自动结束。-The application must run to completion all foreground threads to exit background thread, the application can not be considered its run is finished and exit all background th