如何将 JSR-310 日期解析为 Instant？

Question

I'm trying to parse lets say "2020-01-12+01:00" with JSR-310 time.

I read it via DateTimeFormatter.ofPattern("yyyy-MM-ddVV") , however now if I want to transform that into a Instant via Instant.from(DateTimeFormatter.ofPattern("yyyy-MM-ddVV").parse("...") , it throws where it complains that time is null.

Which granted it is, but, I'd like to get Instant from that, ie epochMillis, so I can serialize the long into a database.

Is there a way around it? Basically I'd like to extend the "2020-01-12+01:00" to "2020-01-12T00:00.000+01:00" and parse that to Instant as usual

Answer 1

You need to use DateTimeFormatterBuilder , specifying ISO_DATE format and a default time-of-day (midnight ¹ ):

DateTimeFormatter formatter = new DateTimeFormatterBuilder()
        .append(DateTimeFormatter.ISO_DATE)
        .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
        .toFormatter();
Instant instant = Instant.from(formatter.parse("2020-01-12+01:00"));
System.out.println(instant);

^{1) The ChronoField can be any time- of-day field, ie HOUR_OF_DAY , CLOCK_HOUR_OF_DAY , MINUTE_OF_DAY , SECOND_OF_DAY , MILLI_OF_DAY , MICRO_OF_DAY , or NANO_OF_DAY .}

Output

2020-01-11T23:00:00Z

---

If you want to retain the time zone offset, you need to use OffsetDateTime (or ZonedDateTime ) instead of Instant :

OffsetDateTime dateTime = OffsetDateTime.parse("2020-01-12+01:00", formatter);
System.out.println(dateTime);
System.out.println(dateTime.format(DateTimeFormatter.ISO_DATE));

Output (from both OffsetDateTime and ZonedDateTime)

2020-01-12T00:00+01:00
2020-01-12+01:00

Answer 2

You can use LocalDate.parse(dateString, formatter) using the Formatter you've made above to give you a LocalDate instance.

LocalDate can then give you a LocalDateTime at any time in that day, but (for example) you can get the start of day from it.

LocalDateTime has a toInstant method to give you an Instant .

Instant has a toEpochMilli method to get your long .

Answer 3

It's easy enough when you know how. The formatter we need is built in. There's a complication in the fact that there isn't a type to parse the string into, no OffsetDate . I present two options for tackling this.

    String s = "2020-01-12+01:00";

    TemporalAccessor parsed = DateTimeFormatter.ISO_OFFSET_DATE.parse(s);
    LocalDate date = LocalDate.from(parsed);
    ZoneOffset offset = ZoneOffset.from(parsed);
    Instant result = date.atStartOfDay(offset).toInstant();

    System.out.println(result);

Output from this snippet is:

2020-01-11T23:00:00Z

We seldom need to use the TemporalAccessor interface directly, and it's considered low-level. It also isn't the only way to go here. The other good option is to define a default time of day so we can parse directly into an Instant :

    DateTimeFormatter formatter = new DateTimeFormatterBuilder()
            .append(DateTimeFormatter.ISO_OFFSET_DATE)
            .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
            .toFormatter();
    Instant result = formatter.parse(s, Instant::from);

The result is the same as before.