在应用启动时检查读写外存权限，如没有则申请权限

将下面代码写入主窗体的 onCreate 事件中即可

private static String[] PERMISSIONS_STORAGE = {
		"android.permission.READ_EXTERNAL_STORAGE",
		"android.permission.WRITE_EXTERNAL_STORAGE" };
		
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
	int storagePermission = checkSelfPermission("android.permission.WRITE_EXTERNAL_STORAGE");
	//检测是否有权限，如果没有权限，就需要申请
	if (storagePermission != PackageManager.PERMISSION_GRANTED) {
		//申请权限
		requestPermissions(PERMISSIONS_STORAGE, 1);
	}
}