Android不能在定时器中执行UI操作

timer = new Timer("randomShelfUploadTimer");
TimerTask timerTask = new TimerTask() {
    @Override
    public void run() {
        isTimer = true;
        selectRandomShelfSQLiteDatabase(0);
        selectScanRecordAndUpload(0);
    }
};

if (isTimer) {//这样运行会抛出异常
    Toast.makeText(this, "有" + cursor.getCount() + "条数据正在上传中...", Toast.LENGTH_SHORT).show();//在定时器任务中不能显示Toast等UI操作
}

if (!isTimer) {//这样不会抛出异常
    Toast.makeText(this, "有" + cursor.getCount() + "条数据正在上传中...", Toast.LENGTH_SHORT).show();
}