Java8新特性系列-Date Time API

Java 8 - New Date/Time API

在 Java 8 中，引入了一个新的日期时间 API，以弥补旧日期时间 API 的以下缺点。

不是线程安全的 - java.util.Date 不是线程安全的，因此开发人员在使用日期时必须处理并发问题。

新的日期时间 API 是不可变的，并且没有 setter 方法。

糟糕的设计 - 默认日期从 1900 开始，月份从 1 开始，日从 0 开始，因此没有统一性。

旧 API 对日期操作的直接方法较少。新的 API 为此类操作提供了许多实用方法。

时区处理困难 - 开发人员必须编写大量代码来处理时区问题。新 API 的开发考虑了特定领域的设计。

Java 8 在 java.time 包下引入了一个新的日期时间 API。以下是 java.time 包中引入的一些重要类。

Local - 简化的日期时间 API，没有时区处理的复杂性。

Zoned - 处理各种时区的专用日期时间 API。

Local Date-Time API

LocalDate/LocalTime 和 LocalDateTime 类简化了不需要时区的开发。让我们看看他们的行动。

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.LocalDateTime;
import java.time.Month;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testLocalDateTime();
   }
	
   public void testLocalDateTime() {
      // Get the current date and time
      LocalDateTime currentTime = LocalDateTime.now();
      System.out.println("Current DateTime: " + currentTime);
		
      LocalDate date1 = currentTime.toLocalDate();
      System.out.println("date1: " + date1);
		
      Month month = currentTime.getMonth();
      int day = currentTime.getDayOfMonth();
      int seconds = currentTime.getSecond();
		
      System.out.println("Month: " + month +"day: " + day +"seconds: " + seconds);
		
      LocalDateTime date2 = currentTime.withDayOfMonth(10).withYear(2012);
      System.out.println("date2: " + date2);
		
      //12 december 2014
      LocalDate date3 = LocalDate.of(2014, Month.DECEMBER, 12);
      System.out.println("date3: " + date3);
		
      //22 hour 15 minutes
      LocalTime date4 = LocalTime.of(22, 15);
      System.out.println("date4: " + date4);
		
      //parse a string
      LocalTime date5 = LocalTime.parse("20:15:30");
      System.out.println("date5: " + date5);
   }
}

输出结果

Current DateTime: 2022-02-10T21:53:59.395
date1: 2022-02-10
Month: FEBRUARYday: 10seconds: 59
date2: 2012-02-10T21:53:59.395
date3: 2014-12-12
date4: 22:15
date5: 20:15:30

Zoned Date-Time API

在考虑时区时，将使用分区日期时间 API。让我们看看他们的行动。

import java.time.ZonedDateTime;
import java.time.ZoneId;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testZonedDateTime();
   }
	
   public void testZonedDateTime() {
      // Get the current date and time
      ZonedDateTime date1 = ZonedDateTime.parse("2007-12-03T10:15:30+05:30[Asia/Karachi]");
      System.out.println("date1: " + date1);
		
      ZoneId id = ZoneId.of("Europe/Paris");
      System.out.println("ZoneId: " + id);
		
      ZoneId currentZone = ZoneId.systemDefault();
      System.out.println("CurrentZone: " + currentZone);
   }
}

验证结果

date1: 2007-12-03T10:15:30+05:00[Asia/Karachi]
ZoneId: Europe/Paris
CurrentZone: Asia/Shanghai

Chrono Units Enum

Java 8 中添加了 java.time.temporal.ChronoUnit 枚举，以替换旧 API 中用于表示日、月等的整数值。让我们看看它们的实际应用。

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testChromoUnits();
   }
	
   public void testChromoUnits() {
      //Get the current date
      LocalDate today = LocalDate.now();
      System.out.println("Current date: " + today);
		
      //add 1 week to the current date
      LocalDate nextWeek = today.plus(1, ChronoUnit.WEEKS);
      System.out.println("Next week: " + nextWeek);
		
      //add 1 month to the current date
      LocalDate nextMonth = today.plus(1, ChronoUnit.MONTHS);
      System.out.println("Next month: " + nextMonth);
		
      //add 1 year to the current date
      LocalDate nextYear = today.plus(1, ChronoUnit.YEARS);
      System.out.println("Next year: " + nextYear);
		
      //add 10 years to the current date
      LocalDate nextDecade = today.plus(1, ChronoUnit.DECADES);
      System.out.println("Date after ten year: " + nextDecade);
   }
}

输出结果

Current date: 2022-02-10
Next week: 2022-02-17
Next month: 2022-03-10
Next year: 2023-02-10
Date after ten year: 2032-02-10

Period and Duration

在 Java 8 中，引入了两个专门的类来处理时间差异。

Period - 它处理基于日期的时间量。

Duration - 它处理基于时间的时间量。

让我们看看他们的行动。

import java.time.temporal.ChronoUnit;

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.Duration;
import java.time.Period;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testPeriod();
      java8tester.testDuration();
   }
	
   public void testPeriod() {
      //Get the current date
      LocalDate date1 = LocalDate.now();
      System.out.println("Current date: " + date1);
		
      //add 1 month to the current date
      LocalDate date2 = date1.plus(1, ChronoUnit.MONTHS);
      System.out.println("Next month: " + date2);
      
      Period period = Period.between(date2, date1);
      System.out.println("Period: " + period);
   }
	
   public void testDuration() {
      LocalTime time1 = LocalTime.now();
      Duration twoHours = Duration.ofHours(2);
		
      LocalTime time2 = time1.plus(twoHours);
      Duration duration = Duration.between(time1, time2);
		
      System.out.println("Duration: " + duration);
   }
}

输出结果

Current date: 2022-02-10
Next month: 2022-03-10
Period: P-1M
Duration: PT-22H

Temporal Adjusters

TemporalAdjuster 用于执行日期数学运算。例如，获取“每月的第二个星期六”或“下星期二”。让我们看看他们的行动。

import java.time.LocalDate;
import java.time.temporal.TemporalAdjusters;
import java.time.DayOfWeek;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testAdjusters();
   }
	
   public void testAdjusters() {
      //Get the current date
      LocalDate date1 = LocalDate.now();
      System.out.println("Current date: " + date1);
		
      //get the next tuesday
      LocalDate nextTuesday = date1.with(TemporalAdjusters.next(DayOfWeek.TUESDAY));
      System.out.println("Next Tuesday on : " + nextTuesday);
		
      //get the second saturday of next month
      LocalDate firstInYear = LocalDate.of(date1.getYear(),date1.getMonth(), 1);
      LocalDate secondSaturday = firstInYear.with(TemporalAdjusters.nextOrSame(
         DayOfWeek.SATURDAY)).with(TemporalAdjusters.next(DayOfWeek.SATURDAY));
      System.out.println("Second Saturday on : " + secondSaturday);
   }
}

输出结果

Current date: 2022-02-10
Next Tuesday on : 2022-02-15
Second Saturday on : 2022-02-12

Backward Compatibility

一个 toInstant() 方法被添加到原始的 Date 和 Calendar 对象中，可用于将它们转换为新的 Date-Time API。使用 ofInstant(Insant,ZoneId) 方法获取 LocalDateTime 或 ZonedDateTime 对象。让我们看看他们的行动。

import java.time.LocalDateTime;
import java.time.ZonedDateTime;

import java.util.Date;

import java.time.Instant;
import java.time.ZoneId;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testBackwardCompatability();
   }
	
   public void testBackwardCompatability() {
      //Get the current date
      Date currentDate = new Date();
      System.out.println("Current date: " + currentDate);
		
      //Get the instant of current date in terms of milliseconds
      Instant now = currentDate.toInstant();
      ZoneId currentZone = ZoneId.systemDefault();
		
      LocalDateTime localDateTime = LocalDateTime.ofInstant(now, currentZone);
      System.out.println("Local date: " + localDateTime);
		
      ZonedDateTime zonedDateTime = ZonedDateTime.ofInstant(now, currentZone);
      System.out.println("Zoned date: " + zonedDateTime);
   }
}

输出结果

Current date: Thu Feb 10 22:08:27 CST 2022
Local date: 2022-02-10T22:08:27.730
Zoned date: 2022-02-10T22:08:27.730+08:00[Asia/Shanghai]