（1）在jdk8之前,一般使用的是Date操作时间,获取Date当前的时间戳为:
public static void main(String[] args) {
Date date = new Date();
System.out.println(date.getTime());
}
备注:Date中的getDate()等方法随着jdk升级也过时了;
(2)一般格式化Date使用的是SimpleDateFormat,代码为：
static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
public static void main(String[] args) {
Date date = new Date();
String time = simpleDateFormat.format(date);
System.out.println(time);
}
（3）SimpleDateFormat作为全局变量或者在多线程竞争下，是线程不安全的,
private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(10);
for (int i = 0; i < 1_00_00; i++) {
service.execute(() -> {
System.out.println(dateStrFormat("2022-07-26 12:02:12"));
});
}
service.shutdown();
}
public static Date dateStrFormat(String dateStr) {
Date date = null;
try {
date = simpleDateFormat.parse(dateStr);
} catch (ParseException e) {
e.printStackTrace();
}
return date;
}
会报错:
(4)解决SimpleDateFormat线程安全问题,使用ThreadLocal,代码为:
private static ThreadLocal<SimpleDateFormat> simpleDateFormat = ThreadLocal.withInitial(()-> new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"));
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(10);
for (int i = 0; i < 1_00_00; i++) {
service.execute(() -> {
System.out.println(dateStrFormat("2022-07-26 12:02:12"));
});
}
service.shutdown();
}
public static Date dateStrFormat(String dateStr) {
Date date = null;
try {
date = simpleDateFormat.get().parse(dateStr);
} catch (ParseException e) {
e.printStackTrace();
}
return date;
}
(5) jdk8提供了LocalDateTime和LocalDate以及LocalTime操作时间,Date在不格式化的情况下,可读性很差,区别在于创建LocalDate 只获取某年某月,创建LocalTime只能获取时分秒,而LocalDateTime = LocalDate + LocalTime；
(6)LocalTime获取时间:
public static void main(String[] args) {
LocalTime localTime = LocalTime.now();
System.out.println(localTime);
}
(6)LocalDate获取时间：
public static void main(String[] args) {
LocalDate localDate = LocalDate.now();
System.out.println(localDate);
}
(7)LocalDate转时间戳:
public static void main(String[] args) {
LocalDate localDate = LocalDate.now();
long timestamp = localDate.atStartOfDay(ZoneOffset.ofHours(8)).toInstant().toEpochMilli();
System.out.println(timestamp);
}
(8)时间戳转LocalDate：
public static void main(String[] args) {
long timestamp = System.currentTimeMillis();
LocalDate localDate = Instant.ofEpochMilli(timestamp).atZone(ZoneOffset.ofHours(8))
.toLocalDate();
System.out.println(localDate);
}
(9)LocalDateTime获取时间：
public static void main(String[] args) {
LocalDateTime localDateTime = LocalDateTime.now();
System.out.println(localDateTime);
}
（10）LocalDateTime转时间戳:
public static void main(String[] args) {
LocalDateTime localDateTime = LocalDateTime.now();
long timestamp = localDateTime.toInstant(ZoneOffset.ofHours(8)).toEpochMilli();
System.out.println(timestamp);
}
(11)时间戳转LocalDateTime:
public static void main(String[] args) {
long timestamp = System.currentTimeMillis();
LocalDateTime localDateTime = Instant.ofEpochMilli(timestamp).atZone(ZoneOffset.ofHours(8))
.toLocalDateTime();
System.out.println(localDateTime);
}
(12)Date转LocalDateTime:
Date date = new Date();
System.out.println(date);
LocalDateTime localDateTime = date.toInstant().atZone(ZoneOffset.ofHours(8)).toLocalDateTime();
System.out.println(localDateTime);
(13)Date转LocalDate:
public static void main(String[] args) {
Date date = new Date();
System.out.println(date);
LocalDate localDate = date.toInstant().atZone(ZoneOffset.ofHours(8)).toLocalDate();
System.out.println(localDate);
}
(14)LocalDateTime转Date:
LocalDateTime localDateTime = LocalDateTime.now();
Date date = Date.from(localDateTime.atZone(ZoneOffset.ofHours(8)).toInstant());
System.out.println(date);
(15)LocalDateTime格式化时间:
public static void main(String[] args) {
LocalDateTime now = LocalDateTime.now();
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String formatDateStr = now.format(dateTimeFormatter);
System.out.println(formatDateStr);
}
(16)时间串转换为LocalDateTime:
public static void main(String[] args) {
String time = "2022-07-23 13:44:31";
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
LocalDateTime localDateTime = LocalDateTime.parse(time, dateTimeFormatter);
System.out.println(localDateTime);
}
(17)LocalDate等提供很多方法操作,如:当前时间减一天:
public static void main(String[] args) {
LocalDate localDate = LocalDate.now();
LocalDate localDate1 = localDate.minusDays(1);
System.out.println(localDate1);
}
(18)LocalDate计算时间差,代码如下:
LocalDate now1 = LocalDate.of(2022, 1, 1);
LocalDate now2 = LocalDate.of(2022, 2, 1);
Period period = Period.between(now1, now2);
System.out.println(period.getYears() + "-" + period.getMonths() + "-" + period.getDays());
(19)顺道一提,之前也有Calendar操作时间,代码如下：
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
System.out.println(calendar);
int year = calendar.get(Calendar.YEAR);
System.out.println(year);
int month = calendar.get(Calendar.MONTH) + 1;
System.out.println(month);
int days = calendar.get(Calendar.DAY_OF_YEAR);
System.out.println(days);
}
(20)在解析中yyyy-MM-dd HH:mm:ss中yyyy和YYYY，以及HH和hh要注意一下,24小时制要用大写的HH,12小时的用hh,yyyy是Year,YYYY表示的是Week year,Week year 意思是当天所在的周属于的年份，一周从周日开始，周六结束，只要本周跨年，那么这周就算入下一年;
(21)总之,jdk8提供的LocalDateTime等都有很方便处理时间的方法,可以使用看看
