java8日期函数

java8之前的日期函数是java.util.Date,再使用过程中，体验是非常不理想的。尤其是当对创建好的时间对象进行改变的时候（日期加减等），会改变这个创建好的对象。

java.util.Date

public class DateTest {
public static void main(String[] args) {
    Date date =new Date();
    System.out.println(date);// Mon Aug 05 09:26:03 CST 2019
    
    // 日期操作
    date.setYear(date.getYear()+1);
    System.out.println(date); //Wed Aug 05 09:26:03 CST 2020
}
}

输出相同的date对象，得到的结果却被改变了。

java.time.LocalDate、java.time.LocalDateTime;

import java.time.LocalDate;
import java.time.LocalDateTime;

public class DateTest {
public static void main(String[] args) {
    LocalDate ldate =LocalDate.now();
    System.out.println(ldate); // 2019-08-05
    
    LocalDate newDate = ldate.plusDays(1);
    System.out.println(ldate);// 2019-08-05
    System.out.println(newDate);// 2019-08-06
    
    LocalDateTime ldt =LocalDateTime.now();
    System.out.println(ldt); // 2019-08-05T09:32:04.273
}
}

观察二者的API会发现java.time.LocalDate是一个final类。

---

日期类LocalDate

设置固定日期，并对该日期进行操作。

 @Test
    public void test(){

        LocalDate date = LocalDate.of(2017, 11, 23);

        System.out.println("年份:"+date.getYear());

        Month month = date.getMonth();
        System.out.println("月份(单词):"+month);
        int monthValue = date.getMonthValue();
        System.out.println("月份(数字)"+monthValue);

        System.out.println("多少号:"+date.getDayOfMonth());


        System.out.println("返回由此日期表示的月份的长度:"+date.lengthOfMonth());
        System.out.println("返回由此日期表示的年份的长度:"+date.lengthOfYear());

        DayOfWeek dayOfWeek = date.getDayOfWeek();
        System.out.println("星期几(单词):"+dayOfWeek);
        System.out.println("星期几(数字):"+dayOfWeek.getValue());


        System.out.println("一年中的第几天:"+date.getDayOfYear());


        System.out.println("是否是闰年:"+ date.isLeapYear());

        LocalDate a = LocalDate.of(2012, 6, 30);
        System.out.println("检查此日期是否在指定日期之后:"+ date.isAfter(a));
        System.out.println("检查此日期是否在指定日期之前:"+date.isBefore(a));
        System.out.println("检查此日期是否等于指定的日期。"+date.isEqual(a));

        System.out.println("将日期输出:"+date.toString());


    }

年份:2017
月份(单词):NOVEMBER
月份(数字)11
多少号:23
返回由此日期表示的月份的长度:30
返回由此日期表示的年份的长度:365
星期几(单词):THURSDAY
星期几(数字):4
一年中的第几天:327
是否是闰年:false
检查此日期是否在指定日期之后:true
检查此日期是否在指定日期之前:false
检查此日期是否等于指定的日期。false

将日期输出:2017-11-23

---

时间类LocalTime

对当前时间操作(使用工厂方法 now 从系统时钟中获取当前的日期)

public static void main(String[] args) {
        LocalTime now = LocalTime.now();

        int hour = now.getHour();
        int minute = now.getMinute();
        int second = now.getSecond();
        System.out.println("小时:" + hour + '\t' + "分钟:" + minute + '\t' + "秒:" + second);

        System.out.println(now.toString());
    }

---

日期类时间类格式化

将字符串转为日期或时分秒（注意字符串参数格式，格式不对java.time.format.DateTimeParseException）

public static void main(String[] args) {
        String date = "2019-08-05";
        String time = "15:51:30";
        LocalDate localDate = LocalDate.parse(date);
        LocalTime localTime = LocalTime.parse(time);
        System.out.println(localDate);
        System.out.println(localTime);
    }

2019-08-05
15:51:3022

//时间转字符串格式化
 DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyyMMddHHmmssSSS");
 String dateTime = LocalDateTime.now(ZoneOffset.of("+8")).format(formatter);
 
//字符串转时间
String dateTimeStr = "2018-07-28 14:11:15";
DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
LocalDateTime dateTime = LocalDateTime.parse(dateTimeStr, df);

---

日期时间类LocalDateTime

public static void main(String[] args) {
        LocalDateTime ldt =LocalDateTime.now();
        System.out.println(ldt);
    }

2019-08-05T10:08:23.545

---

LocalDate、LocalTime、LocalDateTime三者之间转换

public static void main(String[] args) {
        String date = "2019-08-17";
        String time = "15:51:30";
        LocalDate localDate = LocalDate.parse(date);
        LocalTime localTime = LocalTime.parse(time);

        LocalDateTime localDateTime = LocalDateTime.of(2017, 11, 23, 16, 01, 30, 888);
        LocalDateTime localDateTime1 = LocalDateTime.of(localDate, localTime);

        // localDateTime-->LocalDate,LocalTime
        LocalDate localDate1 = localDateTime.toLocalDate();
        LocalTime localTime1 = localDateTime.toLocalTime();

        // LocalDate,LocalTime --> LocalDateTime
        LocalDateTime localDateTime2 = localDate.atTime(16, 02, 30);
        LocalDateTime localDateTime3 = localTime.atDate(localDate);
        System.out.println("localDate:"+localDate);
        System.out.println("localTime:"+localTime);
        System.out.println("localDateTime:"+localDateTime);
        System.out.println("localDateTime1:"+localDateTime1);
        System.out.println("localDate1:"+localDate1);
        System.out.println("localTime1:"+localTime1);
    }

localDate:2019-08-17
localTime:15:51:30
localDateTime:2017-11-23T16:01:30.000000888
localDateTime1:2019-08-17T15:51:30
localDate1:2017-11-23
localTime1:16:01:30.000000888

---

Period获取两个日期之间的日期差

public static void main(String[] args) {
        Period p = Period.between(LocalDate.of(2014, 3, 18),LocalDate.now());
        System.out.printf("目标日期距离今天的时间差：%d 年 %d 个月 %d 天\n", p.getYears(), p.getMonths(), p.getDays());
    }

---

Duration获取时间差

Duration d1 = Duration.between(time1, time2);
Duration d1 = Duration.between(dateTime1, dateTime2);
Duration d2 = Duration.between(instant1, instant2);

---

获取秒数和毫秒数

//获取秒数
Long second = LocalDateTime.now().toEpochSecond(ZoneOffset.of("+8"));
//获取毫秒数
Long milliSecond = LocalDateTime.now().toInstant(ZoneOffset.of("+8")).toEpochMilli();

---

获取时间戳

public static void main(String[] args) {
        LocalDateTime localDateTime = LocalDateTime.now();
        System.out.println(localDateTime);
        //将localDateTime转换成时间戳    
        System.out.println("localDateTime:"+java.sql.Timestamp.valueOf(localDateTime).getTime());
        //使用System获取时间戳
        System.out.println("system:"+System.currentTimeMillis());
    }

2019-08-05T10:43:15.328
localDateTime:1564972995328
system:1564972995332

---

时间戳转换成LocalDateTime

Long timestamp = LocalDateTime.now().toInstant(ZoneOffset.of("+8")).toEpochMilli();
LocalDateTime time2 =LocalDateTime.ofEpochSecond(timestamp/1000,0,ZoneOffset.ofHours(8));