LocalDateTime
java8使用了LocalDateTime和DateTimeFormatter。比之前的Date和Carlendar有所改进。
DateTimeFormatter是线程安全的。DateTimeFormatter中很多属性使用了final修饰。
LocalDate: 只能设置仅含年月日的格式，表示没有时区的日期, LocalDate是不可变并且线程安全的
LocalTime: 只能设置仅含时分秒的格式，表示没有时区的时间, LocalTime是不可变并且线程安全的
LocalDateTime: 可以设置含年月日时分秒的格式 ， 表示没有时区的日期时间, LocalDateTime是不可变并且线程安全的
ZoneId: 时区ID，用来确定Instant和LocalDateTime互相转换的规则
Instant: 用来表示时间线上的一个点（瞬时）
Clock: 用于访问当前时刻、日期、时间，用到时区
Duration: 用秒和纳秒表示时间的数量（长短），用于计算两个日期的“时间”间隔
构建LocalDateTime对象：
public void generateLocalDate() {LocalDate date = LocalDate.of(2000, 1, 8);LocalDateTime dateTime = LocalDateTime.of(2020, 1, 8, 15, 16, 17);System.out.println("date：" + date);System.out.println("dateTime:" + dateTime);}
获取LocalDateTime的年月日：
public void getLocalDateTimeMonth() {LocalDateTime dateTime = LocalDateTime.of(2020, 1, 8, 15, 16, 17);int year=dateTime.getYear();int month=dateTime.getMonth().getValue();int day=dateTime.getDayOfMonth();System.out.println("dateTime对应的年份为："+year);System.out.println("dateTime对应的月份为："+month);System.out.println("dateTime对应的天数为："+day);}
字符串String转LocalDateTime：
public void stringToTime() {DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");String datetimeText = "2020-04-01 23:59:59";LocalDateTime localDateTime = LocalDateTime.parse(datetimeText, formatter);// LocalDateTime的toString()方法，在日期和时间中间加入了"T"字符串System.out.println(localDateTime);}
LocalDateTime 转字符串String：
public void localDateTimeToString() {DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");LocalDateTime dateTime = LocalDateTime.of(2020, 1, 8, 15, 16, 17);String dateTimeStr = dateTime.format(formatter);System.out.println("dateTimeStr为：" + dateTime);}
LocalDateTime 转时间戳毫秒
public static long getMillsByLocalDateTime(LocalDateTime localDateTime) {long mills = localDateTime.toInstant(ZoneOffset.of("+8")).toEpochMilli();return mills;}
时间戳毫秒转LocalDateTime
public static LocalDateTime getLocalDateTimeByMills() {long mills = 1640419285000L;LocalDateTime result = new Date(mills).toInstant().atOffset(ZoneOffset.of("+8")).toLocalDateTime();return result;}
Date转Instant：
Instant instant= date.toInstant();
Instant转Date：
Date date = Date.from(instant);
Date转LocalDateTime ：
public static void dateToLocalDateTime(Date date) {Instant dateInstant = date.toInstant();LocalDateTime localDateTime = LocalDateTime.ofInstant(dateInstant, ZoneId.systemDefault());}
LocalDateTime转Date：
public static Date parse(LocalDateTime localDateTime) {return Date.from(localDateTime.atZone( ZoneId.systemDefault()).toInstant());}
LocalDateTime 获取LocalDate：
LocalDate localDate = localDateTime.toLocalDate();
LocalDateTime 获取LocalTime：
LocalTime localTime = localDateTime.toLocalTime();
LocalDate
Date转LocalDate：
public void dateToLocalDate(Date date) {Instant dateInstant = date.toInstant();LocalDate localDate = LocalDateTime.ofInstant(dateInstant, ZoneId.systemDefault()).toLocalDate();}
LocalDate转Date：
public static Date parse(LocalDate localDate) {return Date.from(localDate.atStartOfDay().atZone(ZoneId.systemDefault()).toInstant());}
localDate转String ：
public static String localDateToString(Date date) {Instant dateInstant = date.toInstant();LocalDate localDate = LocalDateTime.ofInstant(dateInstant, ZoneId.systemDefault()).toLocalDate();// localDate转String DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");String localDateStr = localDate.format(formatter);System.out.println(localDateStr);return localDateStr;}
String转LocalDate：
public static void stringToLocalDate() {DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");String datetimeText = "2020-04-01";LocalDate localDate = LocalDate.parse(datetimeText, formatter);System.out.println(localDate);}
LocalDate获取年月日:
int year = localDate.getYear();int monthValue = localDate.getMonthValue();int dayOfMonth = localDate.getDayOfMonth();LocalDate修改年月日：
public static void changeYearMonthDay() {LocalDate localDate = LocalDate.now();//修改年份LocalDate withYearResult = localDate.withYear(2020);//修改月份LocalDate withMonthResult = localDate.withMonth(5);//修改日LocalDate withDayResult = localDate.withDayOfMonth(20);System.out.println( withYearResult + "，" + withMonthResult+ "，"+ withDayResult);}
LocalDate比较日期大小compareTo：
compareTo()方法
在两边相等（=）的情况下返回 0
在左边大于（>）右边时返回 1
在左边小于（<）右边时返回 -1
public static void localDateCompareTo() {DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");String datetimeText = "2020-04-01";LocalDate localDate = LocalDate.parse(datetimeText, formatter);LocalDate nowDate = LocalDate.now();System.out.println(localDate);System.out.println("nowDate:"+nowDate);if (nowDate.compareTo(localDate) < 0) {System.out.println("nowDate小于"+ datetimeText);} else {System.out.println("nowDate大于等于"+ datetimeText);}}
LocalDate比较日期的前后：
after 只在大于（>）情况下才为true (相等时不会）
before 只在小于（<）情况下才为true (相等时不会）
public static void isBeforeAfter() {DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");String datetimeText = "2020-04-01";LocalDate localDate = LocalDate.parse(datetimeText, formatter);LocalDate nowDate = LocalDate.now();System.out.println(localDate);System.out.println("nowDate:"+nowDate);if (nowDate.isBefore(localDate)) {System.out.println(nowDate+ "在"+ datetimeText +"之前");} else {System.out.println(nowDate+ "在"+ datetimeText +"之后");}}
获取日期差距 / 获取日期时间间隔 ：
public static void durationDateTime() {LocalDateTime date3 = LocalDateTime.now();LocalDateTime date4 = LocalDateTime.of(2018, 1, 13, 22, 30, 10);Duration duration = Duration.between(date3, date4);System.out.println(date3 + " 与 " + date4 + " 间隔 " + "\n"+ " 天 :" + duration.toDays() + "\n"+ " 时 :" + duration.toHours() + "\n"+ " 分 :" + duration.toMinutes() + "\n"+ " 毫秒 :" + duration.toMillis() + "\n"+ " 纳秒 :" + duration.toNanos() + "\n");}
日期时间计算：
public void addMonthDay() {LocalDateTime dt = LocalDateTime.of(2020, 1, 26, 20, 30, 59);System.out.println(dt);// 加5天减3小时:LocalDateTime dt2 = dt.plusDays(5).minusHours(3);System.out.println(dt2); // 2019-10-31T17:30:59// 减1月:LocalDateTime dt3 = dt2.minusMonths(1);System.out.println(dt3); // 2019-09-30T17:30:59}
LocalTime
LocalTime获取时分秒：
//获取小时int hour = localTime.getHour();int hour1 = localTime.get(ChronoField.HOUR_OF_DAY);//获取分int minute = localTime.getMinute();int minute1 = localTime.get(ChronoField.MINUTE_OF_HOUR);//获取秒int second = localTime.getMinute();int second1 = localTime.get(ChronoField.SECOND_OF_MINUTE);
参考资料：
https://www.liaoxuefeng.com/wiki/1252599548343744/1303871087444002
https://blog.csdn.net/weixin_38405253/article/details/100765007
https://www.jianshu.com/p/048ee8580639
