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Java 8日期/時間（ Date/Time）API是開發(fā)人員最受追捧的變化之一，Java從一開始就沒有對日期時間處理的一致性方法，因此日期/時間API也是除Java核心API以外另一項倍受歡迎的內(nèi)容。 為什么我們需要新的Java日期/時間API？在開始研究Java 8日期/時間API之前，讓我們先來看一下為什么我們需要這樣一個新的API。在Java中，現(xiàn)有的與日期和時間相關的類存在諸多問題，其中有：
在現(xiàn)有的日期和日歷類中定義的方法還存在一些其他的問題，但以上問題已經(jīng)很清晰地表明：Java需要一個健壯的日期/時間類。這也是為什么Joda Time在Java日期/時間需求中扮演了高質(zhì)量替換的重要角色。
Java 8日期/時間APIJava 8日期/時間API是JSR-310的實現(xiàn)，它的實現(xiàn)目標是克服舊的日期時間實現(xiàn)中所有的缺陷，新的日期/時間API的一些設計原則是：
Java日期/時間API包Java日期/時間API包含以下相應的包。
Java日期/時間API示例我們已經(jīng)瀏覽了Java日期/時間API的大多數(shù)重要部分，現(xiàn)在是時候根據(jù)示例仔細看一下最重要的一些類了。
1. java.time.LocalDate：LocalDate是一個不可變的類，它表示默認格式(yyyy-MM-dd)的日期，我們可以使用now()方法得到當前時間，也可以提供輸入年份、月份和日期的輸入?yún)?shù)來創(chuàng)建一個LocalDate實例。該類為now()方法提供了重載方法，我們可以傳入ZoneId來獲得指定時區(qū)的日期。該類提供與java.sql.Date相同的功能，對于如何使用該類，我們來看一個簡單的例子。
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);
}
}
示例方法的詳解都包含在注釋內(nèi)，當我們運行程序時，可以得到以下輸出：
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 2. java.time.LocalTime：LocalTime是一個不可變的類，它的實例代表一個符合人類可讀格式的時間，默認格式是hh:mm:ss.zzz。像LocalDate一樣，該類也提供了時區(qū)支持，同時也可以傳入小時、分鐘和秒等輸入?yún)?shù)創(chuàng)建實例，我們來看一個簡單的程序，演示該類的使用方法。
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);
}
}
當運行以上程序時，可以看到如下輸出。
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 3. java.time.LocalDateTime：LocalDateTime是一個不可變的日期-時間對象，它表示一組日期-時間，默認格式是yyyy-MM-dd-HH-mm-ss.zzz。它提供了一個工廠方法，接收LocalDate和LocalTime輸入?yún)?shù)，創(chuàng)建LocalDateTime實例。我們來看一個簡單的例子。
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);
}
}
在所有這三個例子中，我們已經(jīng)看到如果我們提供了無效的參數(shù)去創(chuàng)建日期/時間，那么系統(tǒng)會拋出java.time.DateTimeException，這是一種運行時異常，我們并不需要顯式地捕獲它。
同時我們也看到，能夠通過傳入ZoneId得到日期/時間數(shù)據(jù)，你可以從它的Javadoc中得到支持的Zoneid的列表，當運行以上類時，可以得到以下輸出。
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 4. java.time.Instant：Instant類是用在機器可讀的時間格式上的，它以Unix時間戳的形式存儲日期時間，我們來看一個簡單的程序。
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);
}
}
<span style="font-family: Georgia, 'Times New Roman', 'Bitstream Charter', Times, serif; font-size: 13px; line-height: 19px;">上述程序的輸出是：</span> Current Timestamp = 2014-04-28T23:20:08.489Z Specific Time = 2014-04-28T23:20:08.489Z PT720H 5. 日期API工具：我們早些時候提到過，大多數(shù)日期/時間API類都實現(xiàn)了一系列工具方法，如：加/減天數(shù)、周數(shù)、月份數(shù)，等等。還有其他的工具方法能夠使用TemporalAdjuster調(diào)整日期，并計算兩個日期間的周期。
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());
}
}
上述程序的輸出是：
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 6. 解析和格式化：將一個日期格式轉(zhuǎn)換為不同的格式，之后再解析一個字符串，得到日期時間對象，這些都是很常見的。我們來看一下簡單的例子。
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);
}
}
當運行以上程序時，可以看到如下輸出。 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 7. 舊的日期時間支持：舊的日期/時間類已經(jīng)在幾乎所有的應用程序中使用，因此做到向下兼容是必須的。這也是為什么會有若干工具方法幫助我們將舊的類轉(zhuǎn)換為新的類，反之亦然。我們來看一下簡單的例子。
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);
}
}
當運行以上程序時，可以看到如下輸出。
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] 你可以看到，舊的TimeZone和GregorianCalendar類的toString()方法太啰嗦了，一點都不友好。
這就是所有的Java 8 日期/時間API的內(nèi)容，我非常喜歡這個API，它易于使用，同時它采取了某項工作，使相似的方法也易于尋找，雖然從舊的類轉(zhuǎn)移到新的日期時間類需要消耗一定的時間，但我相信這是值得的。
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