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()))); }
输出：
===================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)); }
输出：
===================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。
5.总结
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。
