java8 时间类
LocalTime (包括了小时 分钟 秒 纳秒)四个属性
LocalTime now = LocalTime.now();
LocalDate 是一个日期类
LocalDate now2=LocalDate.now();
WeekFields weekFields = WeekFields.ISO;
int weekNumber = now2.get(weekFields.weekOfWeekBasedYear());
LocalDateTime
LocalDateTime now3=LocalDateTime.now();
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String aa=now3.format(formatter);