calendar中set方法和静态属性带来的坑

private String getDataCollectionName() {
       Calendar calendar = Calendar.getInstance();
       try {
            calendar.set(Calendar.DATE, -day);
            return "alarm_" + ToolUtils.dateToFormatStrDate(calendar.getTime(), "yyyy_MM");
        } catch (Exception e) {
        logger.error("{}: data transformating failed,{}", this.getClass().getName(), e.getMessage());
        }
       return null;
 }