时间戳是指格林威治时间1970年01月01日00时00分00秒起至现在的总毫秒数，是所有时间的基础，其他时间可以通过时间戳转换得到。Java中本来已经有相关获取时间戳的方法，Java8后增加新的类Instant等专用于处理时间戳问题。
1 获取时间戳的方法和性能对比
1.1 获取时间戳方法
Java8以前可以使用System.currentTimeMillis() 、new Date().getTime() 、和Calendar.getInstance().getTimeInMillis()获取。Java8以后可以另外通过Instant.now().toEpochMilli()和Clock.systemUTC().millis()获取。
比较简单，下面直接看代码：
* 使用System获取时间戳
@Test
public
void
getEpochMilliWithSystem(){
long
s =
System.currentTimeMillis();
for
(
int
i = 0; i < 1000000; i++
) {
System.currentTimeMillis();
System.out.println(
"getEpochMilliWithSystem cost:"+(System.currentTimeMillis()-
s));
* 使用Date获取时间戳
@Test
public
void
getEpochMilliWithDate(){
long
s =
System.currentTimeMillis();
for
(
int
i = 0; i < 1000000; i++
) {
new
Date().getTime();
System.out.println(
"getEpochMilliWithDate cost:"+(System.currentTimeMillis()-
s));
* 使用Calendar获取时间戳
@Test
public
void
getEpochMilliWithCalendar(){
long
s =
System.currentTimeMillis();
for
(
int
i = 0; i < 1000000; i++
) {
Calendar.getInstance().getTimeInMillis();
System.out.println(
"getEpochMilliWithCalendar cost:"+(System.currentTimeMillis()-
s));
* 使用Instant获取时间戳
@Test
public
void
getEpochMilliWithInstant(){
long
s =
System.currentTimeMillis();
for
(
int
i = 0; i < 1000000; i++
) {
Instant.now().toEpochMilli();
System.out.println(
"getEpochMilliWithInstant cost:"+(System.currentTimeMillis()-
s));
* 使用Clock获取时间戳
@Test
public
void
getEpochMilliWithClock(){
long
s =
System.currentTimeMillis();
for
(
int
i = 0; i < 1000000; i++
) {
Clock.systemUTC().millis();
System.out.println(
"getEpochMilliWithClock cost:"+(System.currentTimeMillis()-
s));
查看过上面相关源码，基本都是通过System.currentTimeMillis() 来创建对象的。比如 new Date：
public Date() {
this(System.currentTimeMillis());
1.2 性能对比
上面代码执行输出：
getEpochMilliWithSystem cost:5
getEpochMilliWithDate cost:38
getEpochMilliWithCalendar cost:1094
getEpochMilliWithInstant cost:106
getEpochMilliWithClock cost:17
通过1.1 中的分析得知基本都是通过System.currentTimeMillis() 来创建对象的System.currentTimeMillis()最快，性能最好。
所以，性能排序：System > Clock > Date > Instant > Calendar 。
2.时间戳转换为其他类
2.1 时间戳和其他类型的转换
* 时间戳epochMilli毫秒转Date
* @param epochMilli
* @return
public static Date toDate(long epochMilli){
Objects.requireNonNull(epochMilli, "epochMilli");
return new Date(epochMilli);
* 时间戳epochMilli毫秒转LocalDateTime
* @param epochMilli
* @return
public static LocalDateTime toLocalDateTime(long epochMilli) {
Objects.requireNonNull(epochMilli, "epochMilli");
return LocalDateTime.ofInstant(Instant.ofEpochMilli(epochMilli), ZoneId.systemDefault());
* 时间戳epochMilli毫秒转LocalDate
* @param epochMilli
* @return
public static LocalDate toLocalDate(long epochMilli) {
Objects.requireNonNull(epochMilli, "epochMilli");
return toLocalDateTime(epochMilli).toLocalDate();
* 时间戳epochMilli毫秒转Instant
* @param epochMilli
* @return
public static Instant toInstant(long epochMilli) {
Objects.requireNonNull(epochMilli, "epochMilli");
return Instant.ofEpochMilli(epochMilli);
* 时间戳epochMilli毫秒转ZonedDateTime，时区为系统默认时区
* @param epochMilli
* @return
public static ZonedDateTime toZonedDateTime(long epochMilli) {
Objects.requireNonNull(epochMilli, "epochMilli");
return LocalDateTime.ofInstant(Instant.ofEpochMilli(epochMilli), ZoneId.systemDefault())
.atZone(ZoneId.systemDefault());
* 时间戳epochMilli转Timestamp
* @param epochMilli
* @return
public static Timestamp toTimestamp(long epochMilli){
return new Timestamp(epochMilli);
* Date转时间戳
* 从1970-01-01T00:00:00Z开始的毫秒值
* @param date
* @return
public static long toEpochMilli(Date date){
Objects.requireNonNull(date, "date");
return date.getTime();
* LocalDateTime转时间戳
* 从1970-01-01T00:00:00Z开始的毫秒值
* @param localDateTime
* @return
public static long toEpochMilli(LocalDateTime localDateTime){
return toInstant(localDateTime).toEpochMilli();
* LocalDate转时间戳
* 从1970-01-01T00:00:00Z开始的毫秒值
* @param localDate
* @return
public static long toEpochMilli(LocalDate localDate){
return toInstant(localDate).toEpochMilli();
* Instant转时间戳
* 从1970-01-01T00:00:00Z开始的毫秒值
* @param instant
* @return
public static long toEpochMilli(Instant instant){
Objects.requireNonNull(instant, "instant");
return instant.toEpochMilli();
* ZonedDateTime转时间戳，注意，zonedDateTime时区必须和当前系统时区一致，不然会出现问题
* 从1970-01-01T00:00:00Z开始的毫秒值
* @param zonedDateTime
* @return
public static long toEpochMilli(ZonedDateTime zonedDateTime) {
Objects.requireNonNull(zonedDateTime, "zonedDateTime");
return zonedDateTime.toInstant().toEpochMilli();
* Timestamp转时间戳
* 从1970-01-01T00:00:00Z开始的毫秒值
* @param timestamp
* @return
public static long toEpochMilli(Timestamp timestamp){
Objects.requireNonNull(timestamp, "timestamp");
return timestamp.getTime();
测试代码：
* 时间戳转换测试
@Test
public void epochMilliConverterTest(){
System.out.println("===================epochMilliConverterTest=====================");
Date date = new Date();
long epochMilli = date.getTime();
System.out.println("epochMilli:"+epochMilli);
System.out.println("===================ToOther=====================");
System.out.println(DateTimeConverterUtil.toDate(epochMilli));
System.out.println(DateTimeConverterUtil.toLocalDateTime(epochMilli));
System.out.println(DateTimeConverterUtil.toLocalDate(epochMilli));
System.out.println(DateTimeConverterUtil.toInstant(epochMilli));
System.out.println(DateTimeConverterUtil.toZonedDateTime(epochMilli));
System.out.println(DateTimeConverterUtil.toTimestamp(epochMilli));
System.out.println("===================toEpochMilli=====================");
System.out.println(DateTimeConverterUtil.toEpochMilli(new Date()));
System.out.println(DateTimeConverterUtil.toEpochMilli(LocalDateTime.now()));
// 另一种方式： +8 时区
System.out.println(LocalDateTime.now().toInstant(ZoneOffset.of("+8")).toEpochMilli());
System.out.println(DateTimeConverterUtil.toEpochMilli(LocalDate.now()));
System.out.println(DateTimeConverterUtil.toEpochMilli(Instant.now()));
System.out.println(DateTimeConverterUtil.toEpochMilli(ZonedDateTime.now()));
System.out.println(DateTimeConverterUtil.toEpochMilli(new Timestamp(System.currentTimeMillis())));
===================epochMilliConverterTest=====================
epochMilli:1587950768191
===================ToOther=====================
Mon Apr 27 09:26:08 CST 2020
2020-04-27T09:26:08.191
2020-04-27
2020-04-27T01:26:08.191Z
2020-04-27T09:26:08.191+08:00[Asia/Shanghai]
2020-04-27 09:26:08.191
===================toEpochMilli=====================
1587950768304
1587950768304
1587950768304
1587916800000
1587950768305
1587950768305
1587950768305
3 Timestamp
Timestamp是Java8以前处理时间戳的类。Timestamp和其他类型的转换
* Timestamp转LocalDateTime
* @param timestamp
* @return
public static LocalDateTime toLocalDateTime(Timestamp timestamp) {
Objects.requireNonNull(timestamp, "timestamp");
return timestamp.toLocalDateTime();
* Timestamp转Instant
* @param timestamp
* @return
public static Instant toInstant(Timestamp timestamp) {
Objects.requireNonNull(timestamp, "timestamp");
return timestamp.toInstant();
* Timestamp转时间戳
* 从1970-01-01T00:00:00Z开始的毫秒值
* @param timestamp
* @return
public static long toEpochMilli(Timestamp timestamp){
Objects.requireNonNull(timestamp, "timestamp");
return timestamp.getTime();
* Date转Timestamp
* @param date
* @return
public static Timestamp toTimestamp(Date date){
Objects.requireNonNull(date, "date");
return new Timestamp(date.getTime());
* LocalDateTime转Timestamp
* @param localDateTime
* @return
public static Timestamp toTimestamp(LocalDateTime localDateTime){
Objects.requireNonNull(localDateTime, "localDateTime");
return Timestamp.valueOf(localDateTime);
* Instant转Timestamp
* @param instant
* @return
public static Timestamp toTimestamp(Instant instant){
Objects.requireNonNull(instant, "instant");
return Timestamp.from(instant);
* 时间戳epochMilli转Timestamp
* @param epochMilli
* @return
public static Timestamp toTimestamp(long epochMilli){
return new Timestamp(epochMilli);
测试代码：
* Timestamp转换测试
@Test
public void timestampConverterTest(){
System.out.println("===================timestampConverterTest=====================");
Timestamp timestamp = new Timestamp(System.currentTimeMillis());
long epochMilli = timestamp.getTime();
System.out.println("epochMilli:"+epochMilli);
System.out.println("===================ToOther=====================");
System.out.println(DateTimeConverterUtil.toLocalDateTime(timestamp));
System.out.println(DateTimeConverterUtil.toInstant(timestamp));
System.out.println(DateTimeConverterUtil.toEpochMilli(timestamp));
System.out.println("===================toEpochMilli=====================");
System.out.println(DateTimeConverterUtil.toTimestamp(new Date()));
System.out.println(DateTimeConverterUtil.toTimestamp(LocalDateTime.now()));
System.out.println(DateTimeConverterUtil.toTimestamp(Instant.now()));
System.out.println(DateTimeConverterUtil.toTimestamp(epochMilli));
===================timestampConverterTest=====================
epochMilli:1587950937677
===================ToOther=====================
2020-04-27T09:28:57.677
2020-04-27T01:28:57.677Z
1587950937677
===================toEpochMilli=====================
2020-04-27 09:28:57.774
2020-04-27 09:28:57.781
2020-04-27 09:28:57.782
2020-04-27 09:28:57.677
4 Instant
Java8可以使用Instant方便的获取时间戳相关的信息。
4.1 创建方式
public static Instant now() {
return Clock.systemUTC().instant();
Instant now方法默认使用的是UTC，协调世界时。now(Clock clock)方法 clock参数可以设置时区信息，就可以获取不同时区的Instant，比如 Clock systemDefaultZone()
或指定时区 Clock system(ZoneId zone)等。
4.2 获取时间戳
public long toEpochMilli() 获取时间戳 单位为毫秒。
public long getEpochSecond() 获取时间戳 单位为秒。
通过上面可以看出，时间戳是所有时间创建和转换的基础，通过简单的System.currentTimeMillis()获取到，但时间戳只是一个简单的数字，不转换为其他时间类没有意义，比如 年、月、日、时、分、秒、星期、闰年、时区、夏令时等，更多相关的比如各种节假日，星座等附加意义的信息。
源代码地址：https://github.com/xkzhangsan/xk-time
