Java8 日期/时间（Date Time）API指南

1. Java的日期/时间类的定义并不一致，在java.util和java.sql的包中都有日期类，此外用于格式化和解析的类在java.text包中定义。
2. java.util.Date同时包含日期和时间，而java.sql.Date仅包含日期，将其纳入java.sql包并不合理。另外这两个类都有相同的名字，这本身就是一个非常糟糕的设计。
3. 对于时间、时间戳、格式化以及解析，并没有一些明确定义的类。对于格式化和解析的需求，我们有java.text.DateFormat抽象类，但通常情况下，SimpleDateFormat类被用于此类需求。
4. 所有的日期类都是可变的，因此他们都不是线程安全的，这是Java日期类最大的问题之一。
5. 日期类并不提供国际化，没有时区支持，因此Java引入了java.util.Calendar和java.util.TimeZone类，但他们同样存在上述所有的问题。

1. 不变性：新的日期/时间API中，所有的类都是不可变的，这对多线程环境有好处。
2. 关注点分离：新的API将人可读的日期时间和机器时间（unix timestamp）明确分离，它为日期（Date）、时间（Time）、日期时间（DateTime）、时间戳（unix timestamp）以及时区定义了不同的类。
3. 清晰：在所有的类中，方法都被明确定义用以完成相同的行为。举个例子，要拿到当前实例我们可以使用now()方法，在所有的类中都定义了format()和parse()方法，而不是像以前那样专门有一个独立的类。为了更好的处理问题，所有的类都使用了工厂模式和策略模式，一旦你使用了其中某个类的方法，与其他类协同工作并不困难。
4. 实用操作：所有新的日期/时间API类都实现了一系列方法用以完成通用的任务，如：加、减、格式化、解析、从日期/时间中提取单独部分，等等。
5. 可扩展性：新的日期/时间API是工作在ISO-8601日历系统上的，但我们也可以将其应用在非IOS的日历上。

1. java.time包：这是新的Java日期/时间API的基础包，所有的主要基础类都是这个包的一部分，如：LocalDate, LocalTime, LocalDateTime, Instant, Period, Duration等等。所有这些类都是不可变的和线程安全的，在绝大多数情况下，这些类能够有效地处理一些公共的需求。
2. java.time.chrono包：这个包为非ISO的日历系统定义了一些泛化的API，我们可以扩展AbstractChronology类来创建自己的日历系统。
3. java.time.format包：这个包包含能够格式化和解析日期时间对象的类，在绝大多数情况下，我们不应该直接使用它们，因为java.time包中相应的类已经提供了格式化和解析的方法。
4. java.time.temporal包：这个包包含一些时态对象，我们可以用其找出关于日期/时间对象的某个特定日期或时间，比如说，可以找到某月的第一天或最后一天。你可以非常容易地认出这些方法，因为它们都具有“withXXX”的格式。
5. java.time.zone包：这个包包含支持不同时区以及相关规则的类。

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);      }    }

Current Date=2014-04-28  Specific Date=2014-01-01  Current Date in IST=2014-04-29  365th day from base date= 1971-01-01  100th day of 2014=2014-04-10

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);        }    }

Current Time=15:51:45.240  Specific Time of Day=12:20:25.000000040  Current Time in IST=04:21:45.276  10000th second time= 02:46:40

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);        }    }

Current DateTime=2014-04-28T16:00:49.455  Current DateTime=2014-04-28T16:00:49.493  Specific Date=2014-01-01T10:10:30  Current Date in IST=2014-04-29T04:30:49.493  10000th second time from 01/01/1970= 1970-01-01T02:46:40

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);      }    }

<span style="font-family: Georgia, 'Times New Roman', 'Bitstream Charter', Times, serif; font-size: 13px; line-height: 19px;">上述程序的输出是：</span>

Current Timestamp = 2014-04-28T23:20:08.489Z  Specific Time = 2014-04-28T23:20:08.489Z  PT720H

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());              }  }

Year 2014 is Leap Year? false  Today is before 01/01/2015? true  Current Time=2014-04-28T16:23:53.154  10 days after today will be 2014-05-08  3 weeks after today will be 2014-05-19  20 months after today will be 2015-12-28  10 days before today will be 2014-04-18  3 weeks before today will be 2014-04-07  20 months before today will be 2012-08-28  First date of this month= 2014-04-01  Last date of this year= 2014-12-31  Period Format= P8M3D  Months remaining in the year= 8

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，可以看到如下输出。

Default format of LocalDate=2014-04-28  28::Apr::2014  20140428  Default format of LocalDateTime=2014-04-28T16:25:49.341  28::Apr::2014 16::25::49  20140428  Default format of Instant=2014-04-28T23:25:49.342Z  Default format after parsing = 2014-04-27T21:39:48

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);        }    }

Date = 2014-04-28T16:28:54.340  2014-04-28T23:28:54.395Z  America/Los_Angeles  2014-04-28T16:28:54.404-07:00[America/Los_Angeles]  Mon Apr 28 16:28:54 PDT 2014  sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]]  java.util.GregorianCalendar[time=1398727734404,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOfWeek=2,minimalDaysInFirstWeek=4,ERA=1,YEAR=2014,MONTH=3,WEEK_OF_YEAR=18,WEEK_OF_MONTH=5,DAY_OF_MONTH=28,DAY_OF_YEAR=118,DAY_OF_WEEK=2,DAY_OF_WEEK_IN_MONTH=4,AM_PM=1,HOUR=4,HOUR_OF_DAY=16,MINUTE=28,SECOND=54,MILLISECOND=404,ZONE_OFFSET=-28800000,DST_OFFSET=3600000]