java8时间API

LocalDate

只会获取年月日

• 创建LocalDate

  //获取当前年月日
  LocalDate localDate = LocalDate.now();
  //构造指定的年月日
  LocalDate localDate1 = LocalDate.of(2019, 9, 10);  

• 获取年、月、日、星期几

  int year = localDate.getYear();
  int year1 = localDate.get(ChronoField.YEAR);
  Month month = localDate.getMonth();
  int month1 = localDate.get(ChronoField.MONTH_OF_YEAR);
  int day = localDate.getDayOfMonth();
  int day1 = localDate.get(ChronoField.DAY_OF_MONTH);
  DayOfWeek dayOfWeek = localDate.getDayOfWeek();
  int dayOfWeek1 = localDate.get(ChronoField.DAY_OF_WEEK);

LocalTime

只会获取几点几分几秒

• 创建LocalTime

  LocalTime localTime = LocalTime.of(13, 51, 10);
  LocalTime localTime1 = LocalTime.now();

• 获取时分秒

  //获取小时
  int hour = localTime.getHour();
  int hour1 = localTime.get(ChronoField.HOUR_OF_DAY);
  //获取分
  int minute = localTime.getMinute();
  int minute1 = localTime.get(ChronoField.MINUTE_OF_HOUR);
  //获取秒
  int second = localTime.getSecond();
  int second1 = localTime.get(ChronoField.SECOND_OF_MINUTE);

LocalDateTime

获取年月日时分秒，等于LocalDate+LocalTime

• 创建LocalDateTime

  LocalDateTime localDateTime = LocalDateTime.now();
  LocalDateTime localDateTime1 = LocalDateTime.of(2019, Month.SEPTEMBER, 10, 14, 46, 56);
  LocalDateTime localDateTime2 = LocalDateTime.of(localDate, localTime);
  LocalDateTime localDateTime3 = localDate.atTime(localTime);
  LocalDateTime localDateTime4 = localTime.atDate(localDate);

• 获取LocalDate

  LocalDate localDate2 = localDateTime.toLocalDate();

• 获取LocalTime

  LocalTime localTime2 = localDateTime.toLocalTime();

Instant

获取秒数

• 创建Instant对象

  Instant instant = Instant.now();

• 获取秒数

  long currentSecond = instant.getEpochSecond();

• 获取毫秒数

  long currentMilli = instant.toEpochMilli();

如果只是为了获取秒数或者毫秒数，使用System.currentTimeMillis()来得更为方便

修改LocalDate、LocalTime、LocalDateTime、Instant

LocalDate、LocalTime、LocalDateTime、Instant为不可变对象，修改这些对象对象会返回一个副本，所以是线程安全的

• 增加、减少年数、月数、天数等 以LocalDateTime为例

  LocalDateTime localDateTime = LocalDateTime.of(2019, Month.SEPTEMBER, 10, 14, 46, 56);
  //增加一年
  localDateTime = localDateTime.plusYears(1);
  localDateTime = localDateTime.plus(1, ChronoUnit.YEARS);
  //减少一个月
  localDateTime = localDateTime.minusMonths(1);
  localDateTime = localDateTime.minus(1, ChronoUnit.MONTHS);

• 通过with修改某些值

  //修改年为2019
  localDateTime = localDateTime.withYear(2020);
  //修改为2022
  localDateTime = localDateTime.with(ChronoField.YEAR, 2022);

  还可以修改月、日

时间计算

LocalDate localDate = LocalDate.now();
LocalDate localDate1 = localDate.with(firstDayOfYear());

格式化时间

LocalDate localDate = LocalDate.of(2019, 9, 10);
String s1 = localDate.format(DateTimeFormatter.BASIC_ISO_DATE);
String s2 = localDate.format(DateTimeFormatter.ISO_LOCAL_DATE);
//自定义格式化
DateTimeFormatter dateTimeFormatter =   DateTimeFormatter.ofPattern("dd/MM/yyyy");
String s3 = localDate.format(dateTimeFormatter);

DateTimeFormatter默认提供了多种格式化方式，如果默认提供的不能满足要求，可以通过DateTimeFormatter的ofPattern方法创建自定义格式化方式

解析时间

LocalDate localDate1 = LocalDate.parse("20190910", DateTimeFormatter.BASIC_ISO_DATE);
LocalDate localDate2 = LocalDate.parse("2019-09-10", DateTimeFormatter.ISO_LOCAL_DATE);

和SimpleDateFormat相比，DateTimeFormatter是线程安全的