java - How to parse ZonedDateTime with default zone?

Question

How to parse ZoneDateTime from string that doesn't contain zone and others fields?

Here is test in Spock to reproduce:

import spock.lang.Specification
import spock.lang.Unroll

import java.time.ZoneId
import java.time.ZoneOffset
import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter

@Unroll
class ZonedDateTimeParsingSpec extends Specification {
    def "DateTimeFormatter.ISO_ZONED_DATE_TIME parsing incomplete date: #value #expected"() {
        expect:
        ZonedDateTime.parse(value, DateTimeFormatter.ISO_ZONED_DATE_TIME) == expected
        where:
        value                           | expected
        '2014-04-23T04:30:45.123Z'      | ZonedDateTime.of(2014, 4, 23, 4, 30, 45, 123_000_000, ZoneOffset.UTC)
        '2014-04-23T04:30:45.123+01:00' | ZonedDateTime.of(2014, 4, 23, 4, 30, 45, 123_000_000, ZoneOffset.ofHours(1))
        '2014-04-23T04:30:45.123'       | ZonedDateTime.of(2014, 4, 23, 4, 30, 45, 123_000_000, ZoneId.systemDefault())
        '2014-04-23T04:30'              | ZonedDateTime.of(2014, 4, 23, 4, 30, 0, 0, ZoneId.systemDefault())
        '2014-04-23'                    | ZonedDateTime.of(2014, 4, 23, 0, 0, 0, 0, ZoneId.systemDefault())
    }
}

First two test passed, all others failed with DateTimeParseException:

• '2014-04-23T04:30:45.123' could not be parsed at index 23
• '2014-04-23T04:30' could not be parsed at index 16
• '2014-04-23' could not be parsed at index 10

How can I parse incomplete dates with time and zone setted to default?

Answer 1

Since the ISO_ZONED_DATE_TIME formatter expects zone or offset information, parsing fails. You'll have to make a DateTimeFormatter that has optional parts for both the zone information and the time part. It's not too hard reverse engineering the ZonedDateTimeFormatter and adding optional tags.

Then you parse the String using the parseBest() method of the formatter. Then, for suboptimal parse results you can create the ZonedDateTime using any default you want.

DateTimeFormatter formatter = new DateTimeFormatterBuilder()
        .parseCaseInsensitive()
        .append(ISO_LOCAL_DATE)
        .optionalStart()           // time made optional
        .appendLiteral('T')
        .append(ISO_LOCAL_TIME)
        .optionalStart()           // zone and offset made optional
        .appendOffsetId()
        .optionalStart()
        .appendLiteral('[')
        .parseCaseSensitive()
        .appendZoneRegionId()
        .appendLiteral(']')
        .optionalEnd()
        .optionalEnd()
        .optionalEnd()
        .toFormatter();

TemporalAccessor temporalAccessor = formatter.parseBest(value, ZonedDateTime::from, LocalDateTime::from, LocalDate::from);
if (temporalAccessor instanceof ZonedDateTime) {
    return ((ZonedDateTime) temporalAccessor);
}
if (temporalAccessor instanceof LocalDateTime) {
    return ((LocalDateTime) temporalAccessor).atZone(ZoneId.systemDefault());
}
return ((LocalDate) temporalAccessor).atStartOfDay(ZoneId.systemDefault());