code: public static String convert2String(long time, String format) {
          if (time > 0l) {
              if (StringUtil.isEmpty(format))
                  format = dateTimeFormat;
              SimpleDateFormat sf = new SimpleDateFormat(format);
              Date date = new Date(time);
              return sf.format(date);
          }
          return "";
      }
docs: 将长整型数字转换为日期格式的字符串