前言

接着源码阅读:new Date之旅，补充几种关于获取当前时间的方式。

Date

在Java中，获取当前日期最简单的方法之一就是直接实例化位于Java包java.util的Date类。

Date date = new Date();

System.currentTimeMillis

获取标准时间可以通过System.currentTimeMillis()方法获取，此方法不受时区影响，得到的结果是时间戳格式的。

long time = System.currentTimeMillis();

源码：

  /*** Returns the current time in milliseconds.  Note that* while the unit of time of the return value is a millisecond,* the granularity of the value depends on the underlying* operating system and may be larger.  For example, many* operating systems measure time in units of tens of* milliseconds.** <p> See the description of the class <code>Date</code> for* a discussion of slight discrepancies that may arise between* "computer time" and coordinated universal time (UTC).** @return  the difference, measured in milliseconds, between*          the current time and midnight, January 1, 1970 UTC.* @see     java.util.Date*/public static native long currentTimeMillis();

Calendar

Calendar类，专门用于转换特定时刻和日历字段之间的日期和时间。

Calendar calendar = Calendar.getInstance();

源码：

/*** Gets a calendar using the default time zone and locale. The* <code>Calendar</code> returned is based on the current time* in the default time zone with the default* {@link Locale.Category#FORMAT FORMAT} locale.** @return a Calendar.*/public static Calendar getInstance(){return createCalendar(TimeZone.getDefault(), Locale.getDefault(Locale.Category.FORMAT));}private static Calendar createCalendar(TimeZone zone,Locale aLocale){CalendarProvider provider =LocaleProviderAdapter.getAdapter(CalendarProvider.class, aLocale).getCalendarProvider();if (provider != null) {try {return provider.getInstance(zone, aLocale);} catch (IllegalArgumentException iae) {// fall back to the default instantiation}}Calendar cal = null;if (aLocale.hasExtensions()) {String caltype = aLocale.getUnicodeLocaleType("ca");if (caltype != null) {switch (caltype) {case "buddhist":cal = new BuddhistCalendar(zone, aLocale);break;case "japanese":cal = new JapaneseImperialCalendar(zone, aLocale);break;case "gregory":cal = new GregorianCalendar(zone, aLocale);break;}}}if (cal == null) {// If no known calendar type is explicitly specified,// perform the traditional way to create a Calendar:// create a BuddhistCalendar for th_TH locale,// a JapaneseImperialCalendar for ja_JP_JP locale, or// a GregorianCalendar for any other locales.// NOTE: The language, country and variant strings are interned.if (aLocale.getLanguage() == "th" && aLocale.getCountry() == "TH") {cal = new BuddhistCalendar(zone, aLocale);} else if (aLocale.getVariant() == "JP" && aLocale.getLanguage() == "ja"&& aLocale.getCountry() == "JP") {cal = new JapaneseImperialCalendar(zone, aLocale);} else {cal = new GregorianCalendar(zone, aLocale);}}return cal;}

LocalDateTime

LocalDateTime，是Java中最常用的Date / Time类，代表前两个类的组合 – 即日期和时间的值。

LocalDateTime dateTime = LocalDateTime.now();

源码：

  /*** Obtains the current date-time from the system clock in the default time-zone.* <p>* This will query the {@link Clock#systemDefaultZone() system clock} in the default* time-zone to obtain the current date-time.* <p>* Using this method will prevent the ability to use an alternate clock for testing* because the clock is hard-coded.** @return the current date-time using the system clock and default time-zone, not null*/public static LocalDateTime now() {return now(Clock.systemDefaultZone());}/*** Obtains the current date-time from the specified clock.* <p>* This will query the specified clock to obtain the current date-time.* Using this method allows the use of an alternate clock for testing.* The alternate clock may be introduced using {@link Clock dependency injection}.** @param clock  the clock to use, not null* @return the current date-time, not null*/public static LocalDateTime now(Clock clock) {Objects.requireNonNull(clock, "clock");final Instant now = clock.instant();  // called onceZoneOffset offset = clock.getZone().getRules().getOffset(now);return ofEpochSecond(now.getEpochSecond(), now.getNano(), offset);}/*** Obtains an instance of {@code LocalDateTime} using seconds from the* epoch of 1970-01-01T00:00:00Z.* <p>* This allows the {@link ChronoField#INSTANT_SECONDS epoch-second} field* to be converted to a local date-time. This is primarily intended for* low-level conversions rather than general application usage.** @param epochSecond  the number of seconds from the epoch of 1970-01-01T00:00:00Z* @param nanoOfSecond  the nanosecond within the second, from 0 to 999,999,999* @param offset  the zone offset, not null* @return the local date-time, not null* @throws DateTimeException if the result exceeds the supported range,*  or if the nano-of-second is invalid*/public static LocalDateTime ofEpochSecond(long epochSecond, int nanoOfSecond, ZoneOffset offset) {Objects.requireNonNull(offset, "offset");NANO_OF_SECOND.checkValidValue(nanoOfSecond);long localSecond = epochSecond + offset.getTotalSeconds();  // overflow caught laterlong localEpochDay = Math.floorDiv(localSecond, SECONDS_PER_DAY);int secsOfDay = (int)Math.floorMod(localSecond, SECONDS_PER_DAY);LocalDate date = LocalDate.ofEpochDay(localEpochDay);LocalTime time = LocalTime.ofNanoOfDay(secsOfDay * NANOS_PER_SECOND + nanoOfSecond);return new LocalDateTime(date, time);}

结果

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java当前时间获取

前言

Date

System.currentTimeMillis

Calendar

LocalDateTime

结果

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