java當前時間獲取

前言

接着源碼閱讀:new Date之旅,補充幾種關於獲取當前時間的方式。

Date

在Java中,獲取當前日期最簡單的方法之一就是直接實例化位於Java包java.util的Date類。

Date date = new Date();

System.currentTimeMillis

獲取標準時間可以通過System.currentTimeMillis()方法獲取,此方法不受時區影響,得到的結果是時間戳格式的。

long time = System.currentTimeMillis();

源碼:

	 /**
     * 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類,專門用於轉換特定時刻和日曆字段之間的日期和時間。

Calendar calendar = Calendar.getInstance(); 

源碼:

/**
     * 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類,代表前兩個類的組合 – 即日期和時間的值。

LocalDateTime dateTime = LocalDateTime.now(); 

源碼:

	 /**
     * 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 once
        ZoneOffset 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 later
        long 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);
    }

結果

當前時間獲取對比
先贊後看,養成習慣。歡迎收看一個行走的熊貓程序猿,下期再見

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