JAVA SE 8 学习笔记（四）日期和时间API

・所有的java.time对象都是不可变的

・一个瞬间(Instant)是时间线上的一个点

・在Java事件中，每天都是86400秒

・持续时间(Duration)是两个瞬间之间的时间

・LocalXXX 没有时区信息

・TemporalAdjuster可以处理常用日历计算美丽如找到某个月的第一个星期二

1.时间线

Instant对象为时间轴上的一个点，原点为1970.1.1日午夜

Duration表示两个瞬时点之间的时间量。

两者提供多个plus/minus/multipliedBy/dividedBy方法以供计算

也可以使用toNano方法以long值进行计算

保存Api链接随时查阅：

Instant  https://docs.oracle.com/javase/8/docs/api/java/time/Instant.html

Duration  https://docs.oracle.com/javase/8/docs/api/java/time/Duration.html

2.本地日期

在新的Java API中，包含两种人类时间：本地日期/带时区的日期

由于有些操作不需要时区，或者为了排除夏令时等的影响，使用本地日期进行计算更为适合

LocalDate包含年月日的信息：

LocalDate today = LocalDate.now();

LocalDate birthday = LocalDate.of(1991,2,3);

LocalDate birthday = LocalDate.of(1991, Month.JUNE, 14);

LocalDate：https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html

两个Instant之间的距离是Duration，对于本地日期，两个LocalDate之间的距离就是Period(时段)

Period： https://docs.oracle.com/javase/8/docs/api/java/time/Period.html

例如增加一年的时间，如果使用day.plus(Duration.ofDays(365))在闰年的时候便会错误，正确的方法是birthday.plusYear(1)或者birthday.plus(Period.ofYear(1))

注意：LocalDate的getDayOfWeek与Calendar不同，周一为1，周日为7（Calendar周日为1，周六为7）

3.日期校正器

TemporalAdjuster提供许多静态方法进行常用校正。

例如将校正方法的结果传递给LocalDate的with方法，计算某个月的第一个周二

LocalDate.of(year, month, 1).with(TemporalAdjuster.nextOrSame(DayOfWeek.TUESDAY));

也可以实现TemporalAdjuster接口创建自己的校正

TemporalAdjuster NEXT_WORKDAY =TemporalAdjusters.ofDateAdjuster(w -> {

LocalDate = result =w;

//  计算日期..

return result;

});

TemporalAdjuster: https://docs.oracle.com/javase/jp/8/docs/api/java/time/temporal/TemporalAdjusters.html

4.本地时间

与本地日期相对，LocalTime表示一天中某个时间。LocalDateTime表示一个日期和时间

LocalTime.now()

LocalTime.of(22,30)

LocalTime.of(22,30, 10);

LocalTime： https://docs.oracle.com/javase/8/docs/api/java/time/LocalTime.html

LocalDateTime：https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html

5.带时区的时间

每个时区都有一个ID，比如America/New_York。想要获得所有时区可以调用ZoneId.getAvailableIds

根据指定的id,ZoneId.of(id)会返回一个ZoneId对象

根据ZoneId，可以调用LocalDateTime的atZone(ZoneId)方法，获取一个ZonedDateTime对象

或者调用静态方法ZoneDateTime.of(year, month, day, hour, minute, second, nano, zoneId)

ZonedDateTime代表一个特定的时间点，可以调用toInstant获得一个Instant对象

反过来，instant.atZone(ZoneId.of("UTC"))可以获得格林威治皇家天文台为标准的ZonedDateTime对象

ZonedDateTime与LocalDateTime许多方法都一样，只是夏令时带来了一些复杂性

ZonedDateTime： https://docs.oracle.com/javase/8/docs/api/java/time/ZonedDateTime.html

6.格式化和解析

DateTimeFormatter用以代替java.util.Dateformat,如果需要转换，可以使用formatter.toFormat()

DateTimeFormatter类提供三种格式化方法：

1. 预定义标准格式

DateTimeFormatter.ISO_DATE_TIME.format(datetime);

详见下表：

Formatter	Description	Example
BASIC_ISO_DATE	Basic ISO date	'20111203'
ISO_LOCAL_DATE	ISO Local Date	'2011-12-03'
ISO_OFFSET_DATE	ISO Date with offset	'2011-12-03+01:00'
ISO_DATE	ISO Date with or without offset	'2011-12-03+01:00'; '2011-12-03'
ISO_LOCAL_TIME	Time without offset	'10:15:30'
ISO_OFFSET_TIME	Time with offset	'10:15:30+01:00'
ISO_TIME	Time with or without offset	'10:15:30+01:00'; '10:15:30'
ISO_LOCAL_DATE_TIME	ISO Local Date and Time	'2011-12-03T10:15:30'
ISO_OFFSET_DATE_TIME	Date Time with Offset	2011-12-03T10:15:30+01:00'
ISO_ZONED_DATE_TIME	Zoned Date Time	'2011-12-03T10:15:30+01:00[Europe/Paris]'
ISO_DATE_TIME	Date and time with ZoneId	'2011-12-03T10:15:30+01:00[Europe/Paris]'
ISO_ORDINAL_DATE	Year and day of year	'2012-337'
ISO_WEEK_DATE	Year and Week	2012-W48-6'
ISO_INSTANT	Date and Time of an Instant	'2011-12-03T10:15:30Z'
RFC_1123_DATE_TIME	RFC 1123 / RFC 822	'Tue, 3 Jun 2008 11:05:30 GMT'

2. 语言环境相关格式

可以通过静态方法创建这类格式

Formatter	Description	Example
ofLocalizedDate(dateStyle)	Formatter with date style from the locale	'2011-12-03'
ofLocalizedTime(timeStyle)	Formatter with time style from the locale	'10:15:30'
ofLocalizedDateTime(dateTimeStyle)	Formatter with a style for date and time from the locale	'3 Jun 2008 11:05:30'
ofLocalizedDateTime(dateStyle,timeStyle)	Formatter with date and time styles from the locale	'3 Jun 2008 11:05'

DateTimeFormatter.ofLocalizedDateTime(FormatStype.Long).format(datetime);

Enum Constants

Enum Constant and Description
FULL Full text style, with the most detail.
LONG Long text style, with lots of detail.
MEDIUM Medium text style, with some detail.
SHORT Short text style, typically numeric.

这些方法使用默认的语言环境，可以通过withLocal方法设定语言环境

DateTimeFormatter.ofLocalizedDateTime(FormatStyle.Long).withLocal(Locale.FRENCH).format(datetime);

3. 指定模式

DateTimeFormatter.ofPattern("E yyyy-MM-dd HH:mm")

常用格式化符号：

All letters 'A' to 'Z' and 'a' to 'z' are reserved as pattern letters. The following pattern letters are defined:

  Symbol  Meaning                     Presentation      Examples
  ------  -------                     ------------      -------
   G       era                         text              AD; Anno Domini; A
   u       year                        year              2004; 04
   y       year-of-era                 year              2004; 04
   D       day-of-year                 number            189
   M/L     month-of-year               number/text       7; 07; Jul; July; J
   d       day-of-month                number            10

   Q/q     quarter-of-year             number/text       3; 03; Q3; 3rd quarter
   Y       week-based-year             year              1996; 96
   w       week-of-week-based-year     number            27
   W       week-of-month               number            4
   E       day-of-week                 text              Tue; Tuesday; T
   e/c     localized day-of-week       number/text       2; 02; Tue; Tuesday; T
   F       week-of-month               number            3

   a       am-pm-of-day                text              PM
   h       clock-hour-of-am-pm (1-12)  number            12
   K       hour-of-am-pm (0-11)        number            0
   k       clock-hour-of-am-pm (1-24)  number            0

   H       hour-of-day (0-23)          number            0
   m       minute-of-hour              number            30
   s       second-of-minute            number            55
   S       fraction-of-second          fraction          978
   A       milli-of-day                number            1234
   n       nano-of-second              number            987654321
   N       nano-of-day                 number            1234000000

   V       time-zone ID                zone-id           America/Los_Angeles; Z; -08:30
   z       time-zone name              zone-name         Pacific Standard Time; PST
   O       localized zone-offset       offset-O          GMT+8; GMT+08:00; UTC-08:00;
   X       zone-offset 'Z' for zero    offset-X          Z; -08; -0830; -08:30; -083015; -08:30:15;
   x       zone-offset                 offset-x          +0000; -08; -0830; -08:30; -083015; -08:30:15;
   Z       zone-offset                 offset-Z          +0000; -0800; -08:00;

   p       pad next                    pad modifier      1

   '       escape for text             delimiter
   ''      single quote                literal           '
   [       optional section start
   ]       optional section end
   #       reserved for future use
   {       reserved for future use
   }       reserved for future use

DateTimeFormatter：https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html

FormatStyle：https://docs.oracle.com/javase/8/docs/api/java/time/format/FormatStyle.html

7.与遗留代码互操作

Date -> ZonedDateTime

ZonedDateTime zonedDateTime = ZonedDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault());