如何从 Instant 转换为 LocalDate

我有一个 Instant来自一个源,根据规格,应该是一个 LocalDate,但没有看到任何方法在 LocalDate类转换的 Instant到一个 LocalDate

最好的方法是什么?

102354 次浏览

You need to ask yourself at what zone offset you want to transform it to most probably and when you know the answer to that:

LocalDate.ofInstant(yourInstant, yourZoneOffSet)

EDIT

just realized that this is only possible since java-9, for a pre-java9 see the other answer

Java 9+

LocalDate.ofInstant(...) arrived in Java 9.

Instant instant = Instant.parse("2020-01-23T00:00:00Z");
ZoneId zone = ZoneId.of("America/Edmonton");
LocalDate date = LocalDate.ofInstant(instant, zone);

See code run live at IdeOne.com.

Notice the date is 22nd rather than 23rd as that time zone uses an offset several hours before UTC.

2020-01-22

Java 8

If you are using Java 8, then you could use ZonedDateTime's toLocalDate() method:

yourInstant.atZone(yourZoneId).toLocalDate()

Other answers provided the mechanics for the transformation, but I wanted to add some background on the meaning of such transformation which hopefully helps explain why it works the way it works.


LocalDate and Instant seem similar – they both hold date(/time) information without the time zone information. However, they have quite a different meaning.

Instant represents a point in time unambiguously. The representation does not explicitly contain any time zone, but implicitly it refers to the UTC time line.

LocalDateTime (and LocalDate) is ambiguous, because it represents a point in the local timeline, which implicitly refers to the local time zone.

So, in order to correctly transform an Instant into a LocalDateTime (conceptually – some of these steps are bundled together into a single operation in the implementation) you need to:
1. convert the Instant into a ZonedDateTime by applying the UTC time zone info
2. change the time zone from UTC to the local time zone (which implies applying the relevant time zone offset) which gives you another ZonedDateTime (with different time zone)
3. convert the ZonedDateTime into a LocalDateTime which makes the time zone implicit (local) by removing the time zone info.

Finally, you can drop the time-part of LocalDateTime and end up with the LocalDate.

If using java 8 you can do the following

Instant instantOfNow = Instant.now();
LocalDate localDate
= LocalDateTime.ofInstant(instantOfNow, ZoneOffset.UTC).toLocalDate();
 Instant instant = Instant.now();
LocalDate localDate = LocalDate.ofInstant(instant, ZoneOffset.UTC);

the above code worked for me.

Complete running example, Java 8 compatible:

import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneId;


class Scratch {


public static void main(String[] args) {
Instant now = Instant.now();
LocalDateTime ldt = now.atZone(ZoneId.systemDefault()).toLocalDateTime();
System.out.println(ldt);
}
}