Java 8 日期/时刻（Date Time）API攻略

Java 8日期/时刻（ Date/Time）API是开发人员最受追捧的改动之一，Java从一开端就没有对日期时刻处理的一同性办法，因而日期/时刻API也是除Java基地API以外另一项倍受等候的内容。

 

为何咱们需求新的Java日期/时刻API？

在开端研讨Java 8日期/时刻API之前，让咱们先来看一下为何咱们需求这样一个新的API。在Java中，现有的与日期和时刻有关的类存在许多疑问，其间有：Java的日期/时刻类的界说并纷歧同，在java.util和java.sql的包中都有日期类，此外用于格局化和解析的类在java.text包中界说。java.util.Date一同包括日期和时刻，而java.sql.Date仅包括日期，将其归入java.sql包并不合理。别的这两个类都有相同的姓名，这自身即是一个十分差劲的方案。关于时刻、时刻戳、格局化以及解析，并没有一些明断界说的类。关于格局化和解析的需求，咱们有java.text.DateFormat笼统类，但通常状况下，SimpleDateFormat类被用于此类需求。悉数的日期类都是可变的，因而他们都不是线程安全的，这是Java日期类最大的疑问之一。日期类并不供给世界化，没有时区支撑，因而Java引进了java.util.Calendar和java.util.TimeZone类，但他们相同存在上述悉数的疑问。在现有的日期和日历类中界说的办法还存在一些其他的疑问，但以上疑问现已很明晰地标明：Java需求一个强健的日期/时刻类。这也是为何Joda Time在Java日期/时刻需求中扮演了高质量更换的重要人物。

Java 8日期/时刻API

Java 8日期/时刻API是JSR-310的完结，它的完结方针是战胜旧的日期时刻完结中悉数的缺点，新的日期/时刻API的一些方案准则是：不变性：新的日期/时刻API中，悉数的类都是不行变的，这对多线程环境有利益。重视点别离：新的API将人可读的日期时刻和机器时刻（unix timestamp）明晰别离，它为日期（Date）、时刻（Time）、日期时刻（DateTime）、时刻戳（unix timestamp）以及时区界说了纷歧样的类。明晰：在悉数的类中，办法都被明断界说用以完结相同的举动。举个比方，要拿到其时实例咱们能够运用now()办法，在悉数的类中都界说了format()和parse()办法，而不是像早年那样专门有一个独立的类。为了十分好的处理疑问，悉数的类都运用了工厂办法和战略办法，一旦你运用了其间某个类的办法，与其他类协同运行并不艰难。有用操作：悉数新的日期/时刻API类都完结了一系列办法用以完结通用的使命，如：加、减、格局化、解析、从日期/时刻中获取独自有些，等等。可拓宽性：新的日期/时刻API是运行在ISO-8601日历体系上的，但咱们也能够将其运用在非IOS的日历上。

Java日期/时刻API包

Java日期/时刻API包括以下相应的包。java.time包：这是新的Java日期/时刻API的根底包，悉数的首要根底类都是这个包的一有些，如：LocalDate, LocalTime, LocalDateTime, Instant, Period, Duration等等。悉数这些类都是不行变的和线程安全的，在绝大大都状况下，这些类能够有用地处理一些公共的需求。java.time.chrono包：这个包为非ISO的日历体系界说了一些泛化的API，咱们能够拓宽AbstractChronology类来创立自个的日历体系。java.time.format包：这个包包括能够格局化和解析日期时刻方针的类，在绝大大都状况下，咱们不该当直接运用它们，由于java.time包中相应的类现已供给了格局化和解析的办法。java.time.temporal包：这个包包括一些时态方针，咱们能够用其找出关于日期/时刻方针的某个特定日期或时刻，比方说，能够找到某月的榜首天或终究一天。你能够十分简略地认出这些办法，由于它们都具有“withXXX”的格局。java.time.zone包：这个包包括支撑纷歧同区以及有关规矩的类。

Java日期/时刻API示例

咱们现已阅览了Java日期/时刻API的大大都重要有些，如今是时分依据示例细心看一下最重要的一些类了。1. java.time.LocalDate：LocalDate是一个不行变的类，它标明默许格局(yyyy-MM-dd)的日期，咱们能够运用now()办法得到其时时刻，也能够供给输入年份、月份和日期的输入参数来创立一个LocalDate实例。该类为now()办法供给了重载办法，咱们能够传入ZoneId来取得指定时区的日期。该类供给与java.sql.Date相同的功用，关于怎么运用该类，咱们来看一个简略的比方。

package com.journaldev.java8.time;import java.time.LocalDate;import java.time.Month;import java.time.ZoneId;/** * LocalDate Examples * @author pankaj * */public class LocalDateExample {    public static void main(String[] args) {        //Current Date        LocalDate today = LocalDate.now();        System.out.println("Current Date="+today);        //Creating LocalDate by providing input arguments        LocalDate firstDay_2014 = LocalDate.of(2014, Month.JANUARY, 1);        System.out.println("Specific Date="+firstDay_2014);        //Try creating date by providing invalid inputs        //LocalDate feb29_2014 = LocalDate.of(2014, Month.FEBRUARY, 29);        //Exception in thread "main" java.time.DateTimeException:         //Invalid date 'February 29' as '2014' is not a leap year        //Current date in "Asia/Kolkata", you can get it from ZoneId javadoc        LocalDate todayKolkata = LocalDate.now(ZoneId.of("Asia/Kolkata"));        System.out.println("Current Date in IST="+todayKolkata);        //java.time.zone.ZoneRulesException: Unknown time-zone ID: IST        //LocalDate todayIST = LocalDate.now(ZoneId.of("IST"));        //Getting date from the base date i.e 01/01/1970        LocalDate dateFromBase = LocalDate.ofEpochDay(365);        System.out.println("365th day from base date= "+dateFromBase);        LocalDate hundredDay2014 = LocalDate.ofYearDay(2014, 100);        System.out.println("100th day of 2014="+hundredDay2014);    }}

示例办法的详解都包括在注释内，当咱们运行程序时，能够得到以下输出：

Current Date=2014-04-28Specific Date=2014-01-01Current Date in IST=2014-04-29365th day from base date= 1971-01-01100th day of 2014=2014-04-10

2. java.time.LocalTime：LocalTime是一个不行变的类，它的实例代表一个契合人类可读格局的时刻，默许格局是hh:mm:ss.zzz。像LocalDate相同，该类也供给了时区支撑，一同也能够传入小时、分钟和秒等输入参数创立实例，咱们来看一个简略的程序，演示该类的运用办法。

package com.journaldev.java8.time;import java.time.LocalTime;import java.time.ZoneId;/** * LocalTime Examples * @author pankaj * */public class LocalTimeExample {    public static void main(String[] args) {        //Current Time        LocalTime time = LocalTime.now();        System.out.println("Current Time="+time);        //Creating LocalTime by providing input arguments        LocalTime specificTime = LocalTime.of(12,20,25,40);        System.out.println("Specific Time of Day="+specificTime);        //Try creating time by providing invalid inputs        //LocalTime invalidTime = LocalTime.of(25,20);        //Exception in thread "main" java.time.DateTimeException:         //Invalid value for HourOfDay (valid values 0 - 23): 25        //Current date in "Asia/Kolkata", you can get it from ZoneId javadoc        LocalTime timeKolkata = LocalTime.now(ZoneId.of("Asia/Kolkata"));        System.out.println("Current Time in IST="+timeKolkata);        //java.time.zone.ZoneRulesException: Unknown time-zone ID: IST        //LocalTime todayIST = LocalTime.now(ZoneId.of("IST"));        //Getting date from the base date i.e 01/01/1970        LocalTime specificSecondTime = LocalTime.ofSecondOfDay(10000);        System.out.println("10000th second time= "+specificSecondTime);    }}

当运行以上程序时，能够看到如下输出。

Current Time=15:51:45.240Specific Time of Day=12:20:25.000000040Current Time in IST=04:21:45.27610000th second time= 02:46:40

3. java.time.LocalDateTime：LocalDateTime是一个不行变的日期-时刻方针，它标明一组日期-时刻，默许格局是yyyy-MM-dd-HH-mm-ss.zzz。它供给了一个工厂办法，接纳LocalDate和LocalTime输入参数，创立LocalDateTime实例。咱们来看一个简略的比方。

package com.journaldev.java8.time;import java.time.LocalDate;import java.time.LocalDateTime;import java.time.LocalTime;import java.time.Month;import java.time.ZoneId;import java.time.ZoneOffset;public class LocalDateTimeExample {    public static void main(String[] args) {        //Current Date        LocalDateTime today = LocalDateTime.now();        System.out.println("Current DateTime="+today);        //Current Date using LocalDate and LocalTime        today = LocalDateTime.of(LocalDate.now(), LocalTime.now());        System.out.println("Current DateTime="+today);        //Creating LocalDateTime by providing input arguments        LocalDateTime specificDate = LocalDateTime.of(2014, Month.JANUARY, 1, 10, 10, 30);        System.out.println("Specific Date="+specificDate);        //Try creating date by providing invalid inputs        //LocalDateTime feb29_2014 = LocalDateTime.of(2014, Month.FEBRUARY, 28, 25,1,1);        //Exception in thread "main" java.time.DateTimeException:         //Invalid value for HourOfDay (valid values 0 - 23): 25        //Current date in "Asia/Kolkata", you can get it from ZoneId javadoc        LocalDateTime todayKolkata = LocalDateTime.now(ZoneId.of("Asia/Kolkata"));        System.out.println("Current Date in IST="+todayKolkata);        //java.time.zone.ZoneRulesException: Unknown time-zone ID: IST        //LocalDateTime todayIST = LocalDateTime.now(ZoneId.of("IST"));        //Getting date from the base date i.e 01/01/1970        LocalDateTime dateFromBase = LocalDateTime.ofEpochSecond(10000, 0, ZoneOffset.UTC);        System.out.println("10000th second time from 01/01/1970= "+dateFromBase);    }}

在悉数这三个比方中，咱们现已看到假定咱们供给了无效的参数去创立日期/时刻，那么体系会抛出java.time.DateTimeException，这是一种运行时反常，咱们并不需求显式地捕获它。一同咱们也看到，能够经过传入ZoneId得到日期/时刻数据，你能够从它的Javadoc中得到支撑的Zoneid的列表，当运行以上类时，能够得到以下输出。

Current DateTime=2014-04-28T16:00:49.455Current DateTime=2014-04-28T16:00:49.493Specific Date=2014-01-01T10:10:30Current Date in IST=2014-04-29T04:30:49.49310000th second time from 01/01/1970= 1970-01-01T02:46:40

4. java.time.Instant：Instant类是用在机器可读的时刻格局上的，它以Unix时刻戳的办法存储日期时刻，咱们来看一个简略的程序。

package com.journaldev.java8.time;import java.time.Duration;import java.time.Instant;public class InstantExample {    public static void main(String[] args) {        //Current timestamp        Instant timestamp = Instant.now();        System.out.println("Current Timestamp = "+timestamp);        //Instant from timestamp        Instant specificTime = Instant.ofEpochMilli(timestamp.toEpochMilli());        System.out.println("Specific Time = "+specificTime);        //Duration example        Duration thirtyDay = Duration.ofDays(30);        System.out.println(thirtyDay);    }}

5. 日期API东西，还有其他的东西办法能够运用TemporalAdjuster调整，并算出两日时期的周期。

package com.journaldev.java8.time;import java.time.LocalDate;import java.time.LocalTime;import java.time.Period;import java.time.temporal.TemporalAdjusters;public class DateAPIUtilities {    public static void main(String[] args) {        LocalDate today = LocalDate.now();        //Get the Year, check if it's leap year        System.out.println("Year "+today.getYear()+" is Leap Year? "+today.isLeapYear());        //Compare two LocalDate for before and after        System.out.println("Today is before 01/01/2015? "+today.isBefore(LocalDate.of(2015,1,1)));        //Create LocalDateTime from LocalDate        System.out.println("Current Time="+today.atTime(LocalTime.now()));        //plus and minus operations        System.out.println("10 days after today will be "+today.plusDays(10));        System.out.println("3 weeks after today will be "+today.plusWeeks(3));        System.out.println("20 months after today will be "+today.plusMonths(20));        System.out.println("10 days before today will be "+today.minusDays(10));        System.out.println("3 weeks before today will be "+today.minusWeeks(3));        System.out.println("20 months before today will be "+today.minusMonths(20));        //Temporal adjusters for adjusting the dates        System.out.println("First date of this month= "+today.with(TemporalAdjusters.firstDayOfMonth()));        LocalDate lastDayOfYear = today.with(TemporalAdjusters.lastDayOfYear());        System.out.println("Last date of this year= "+lastDayOfYear);        Period period = today.until(lastDayOfYear);        System.out.println("Period Format= "+period);        System.out.println("Months remaining in the year= "+period.getMonths());            }}

上述程序的输出是：

Year 2014 is Leap Year? falseToday is before 01/01/2015? trueCurrent Time=2014-04-28T16:23:53.15410 days after today will be 2014-05-083 weeks after today will be 2014-05-1920 months after today will be 2015-12-2810 days before today will be 2014-04-183 weeks before today will be 2014-04-0720 months before today will be 2012-08-28First date of this month= 2014-04-01Last date of this year= 2014-12-31Period Format= P8M3DMonths remaining in the year= 8

6. 解析和格局化：将一个日期格局改换为纷歧样的格局，往后再解析一个字符串，得到日期时刻方针，这些都是许多见的。咱们来看一下简略的比方。

package com.journaldev.java8.time;import java.time.Instant;import java.time.LocalDate;import java.time.LocalDateTime;import java.time.format.DateTimeFormatter;public class DateParseFormatExample {    public static void main(String[] args) {        //Format examples        LocalDate date = LocalDate.now();        //default format        System.out.println("Default format of LocalDate="+date);        //specific format        System.out.println(date.format(DateTimeFormatter.ofPattern("d::MMM::uuuu")));        System.out.println(date.format(DateTimeFormatter.BASIC_ISO_DATE));        LocalDateTime dateTime = LocalDateTime.now();        //default format        System.out.println("Default format of LocalDateTime="+dateTime);        //specific format        System.out.println(dateTime.format(DateTimeFormatter.ofPattern("d::MMM::uuuu HH::mm::ss")));        System.out.println(dateTime.format(DateTimeFormatter.BASIC_ISO_DATE));        Instant timestamp = Instant.now();        //default format        System.out.println("Default format of Instant="+timestamp);        //Parse examples        LocalDateTime dt = LocalDateTime.parse("27::Apr::2014 21::39::48",                DateTimeFormatter.ofPattern("d::MMM::uuuu HH::mm::ss"));        System.out.println("Default format after parsing = "+dt);    }}

当运行以上程序时，能够看到如下输出。

Default format of LocalDate=2014-04-2828::Apr::201420140428Default format of LocalDateTime=2014-04-28T16:25:49.34128::Apr::2014 16::25::4920140428Default format of Instant=2014-04-28T23:25:49.342ZDefault format after parsing = 2014-04-27T21:39:48

7. 旧的日期时刻支撑：旧的日期/时刻类现已在简直悉数的运用程序中运用，因而做到向下兼容是有必要的。这也是为何会有若干东西办法帮忙咱们将旧的类改换为新的类，反之亦然。咱们来看一下简略的比方。

package com.journaldev.java8.time;import java.time.Instant;import java.time.LocalDateTime;import java.time.ZoneId;import java.time.ZonedDateTime;import java.util.Calendar;import java.util.Date;import java.util.GregorianCalendar;import java.util.TimeZone;public class DateAPILegacySupport {    public static void main(String[] args) {        //Date to Instant        Instant timestamp = new Date().toInstant();        //Now we can convert Instant to LocalDateTime or other similar classes        LocalDateTime date = LocalDateTime.ofInstant(timestamp,                         ZoneId.of(ZoneId.SHORT_IDS.get("PST")));        System.out.println("Date = "+date);        //Calendar to Instant        Instant time = Calendar.getInstance().toInstant();        System.out.println(time);        //TimeZone to ZoneId        ZoneId defaultZone = TimeZone.getDefault().toZoneId();        System.out.println(defaultZone);        //ZonedDateTime from specific Calendar        ZonedDateTime gregorianCalendarDateTime = new GregorianCalendar().toZonedDateTime();        System.out.println(gregorianCalendarDateTime);        //Date API to Legacy classes        Date dt = Date.from(Instant.now());        System.out.println(dt);        TimeZone tz = TimeZone.getTimeZone(defaultZone);        System.out.println(tz);        GregorianCalendar gc = GregorianCalendar.from(gregorianCalendarDateTime);        System.out.println(gc);    }}

当运行以上程序时，能够看到如下输出。


这即是悉数的Java 8 日期/时刻API的内容，我十分喜爱这个API，它易于运用，一同它采纳了某项运行，使类似的办法也易于寻觅，尽管从旧的类转移到新的日期时刻类需求耗费必定的时刻，但我信赖这是值得的。