java - 基于周的模式无法按预期工作

Question

我希望这两个格式化程序是等效的:

DateTimeFormatter fromBuilder = new DateTimeFormatterBuilder()
    .appendValue(IsoFields.WEEK_BASED_YEAR, 4)
    .appendLiteral('-')
    .appendValue(IsoFields.WEEK_OF_WEEK_BASED_YEAR, 2)
    .toFormatter();
DateTimeFormatter fromPattern = DateTimeFormatter.ofPattern("YYYY-ww");

但是他们没有给出相同的结果:

LocalDate date = LocalDate.of(2017, 1, 1);
System.out.printf("from builder: %s%n", fromBuilder.format(date));  // prints 'from builder: 2016-52'
System.out.printf("from pattern: %s%n", fromPattern.format(date));  // prints 'from pattern: 2017-01'

我错过了什么？

Answer 1

Y和w模式correspond to a localized version of week-fields ，使用 JVM 的默认区域设置 ( java.util.Locale )。第二个格式化程序相当于:

// localized week fields (using default Locale)
WeekFields weekFields = WeekFields.of(Locale.getDefault());
// equivalent to YYYY-ww
DateTimeFormatter fmt = new DateTimeFormatterBuilder()
    .appendValue(weekFields.weekBasedYear(), 4)
    .appendLiteral('-')
    .appendValue(weekFields.weekOfWeekBasedYear(), 2)
    .toFormatter();

由于这取决于区域设置，因此它可以或不能像 IsoFields 那样工作。 。 WeekFields根据 JVM 的默认区域设置，上面创建的将具有不同的行为。

IsoFields另一方面，遵循 ISO-8601 定义来定义基于周的字段，如 described in the javadoc :

The first week of a week-based-year is the first Monday-based week of the standard ISO year that has at least 4 days in the new year.

• If January 1^st is Monday then week 1 starts on January 1^st
• If January 1^st is Tuesday then week 1 starts on December 31^st of the previous standard year
• If January 1^st is Wednesday then week 1 starts on December 30^th of the previous standard year
• If January 1^st is Thursday then week 1 starts on December 29^th of the previous standard year
• If January 1^st is Friday then week 1 starts on January 4^th
• If January 1^st is Saturday then week 1 starts on January 3^rd
• If January 1^st is Sunday then week 1 starts on January 2^nd

如2017-01-01是星期日，它对应于上面的最后一行:第 1 周从 2017 年 1 月 2 日开始，因此 2017 年 1 月 1 日 现在还是2016年的最后一周。

---

您可以查看您的WeekFields实例不同于 IsoFields通过调用方法 getFirstDayOfWeek()和getMinimalDaysInFirstWeek() - 习惯 calculate the values of the respecitive week-based fields :

A week is defined by:

• The first day-of-week. For example, the ISO-8601 standard considers Monday to be the first day-of-week.
• The minimal number of days in the first week. For example, the ISO-8601 standard counts the first week as needing at least 4 days.

Together these two values allow a year or month to be divided into weeks.

在我使用的 JVM 中，默认区域设置是 pt_BR ，以及WeekFields创建的一周的第一天为星期日，第一周的最少天数为 1 。检查你的，你会发现它也不同于 IsoFields .

---

您可以使用 constant WeekFields.ISO 检查 ISO 的定义。 :getFirstDayOfWeek()周一返回，getMinimalDaysInFirstWeek()返回4 .

---

另外，请注意 IsoFields 之间存在细微差别。和WeekFields.ISO 。引用JodaStephen's comment in this thread :

The only observable difference was that WeekFields operates on all calendar systems (by converting to ISO) whereas IsoFields only operates on ISO (and rejects other calendar systems)