将即时转换为LocalDate并返回

Question

我在Instant和LocalDate之间转换日期并返回时遇到问题。我需要将当前日期更改为当前星期的星期一（如果是星期三，那么我要更改为星期一）：

public static Instant getStartDateAsMonday(Instant startTime) {
    LocalDate monday = startTime.atZone(ZoneId.systemDefault()).toLocalDate()
        .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
    startTime = monday.atStartOfDay(ZoneId.systemDefault()).toInstant();
    return startTime;
}

我测试时...

@Test
public void testGetStartDateAsMonday() {
    Instant instant = DateHelperService.getStartDateAsBeginningOnMonday(
        Instant.parse("2020-05-27T00:00:00Z"));
    assertThat(instant).isEqualTo("2020-05-25T00:00:00Z");
}

...测试未通过并提供输出：

Expected : 2020-05-25T00:00:00Z
Actual   : 2020-05-24T22:00:00Z

我的系统默认时区为GMT + 2。当我执行atStartOfDay(ZoneId.of("UTC"))时测试通过，但是我不明白为什么我不能在该转换中使用系统默认值。

Answer 1

让我们逐个检查它：

LocalDate monday = startTime // startTime is 2020-05-27 00:00:00 as Instant

    // 2020-05-27 02:00:00, but with GMT+2 information as it is a ZonedDateTime now
    .atZone(ZoneId.systemDefault())

    // (1) 2020-05-27 02:00:00 without zone information
    // (2) 2020-05-27 00:00:00 because hour is dropped as it's a LocalDate here
    .toLocalDate()

    // changed to 2020-05-25 00:00:00, which is what you want
    .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));

    startTime = monday // monday is 2020-05-25 00:00:00 as LocalDate

    // you say this is at GMT+2 and you want the start of day
    // you get a ZonedDateTime 2020-05-25 00:00:00 with information about GMT+2
    .atStartOfDay(ZoneId.systemDefault())

    // Instant has no zone offset, so this is what the ZonedDateTime needs to remove
    // 2020-05-25 00:00:00 becomes 2020-05-24 22:00:00 where the +2 offset is subtracted
    .toInstant();

[当您使用ZoneId.of("UTC")时，则不会发生任何问题，因为ZoneId.of("UTC")的偏移量为零（为简便起见，忽略了夏令时的内容）。日期时间减去零小时是相同的日期时间。