Zone parsing for GMT0

I have GMT0 as the default timezone in a system and it causes problem when I'm serializing it and deserializing it just after that.

System.setProperty("user.timezone","GMT0");
DateTimeFormatter zoneFormatter = new DateTimeFormatterBuilder()
.appendZoneOrOffsetId()
.toFormatter();
String formatted = zoneFormatter.format(ZonedDateTime.now());
System.out.println(formatted);
System.out.println(zoneFormatter.parse(formatted));


The first System.out.println prints GMT0 while the second throws the following problem.

System.out.println

GMT0

Exception in thread "main" java.time.format.DateTimeParseException: Text 'GMT0' could not be parsed, unparsed text found at index 3
at java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:1952)
at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1777)


is it an expected behavior? Is there a way to do that in a safe manner?

GMT0

,ISO,GMT0

GMT

UTC

0

2 Answers
2

As you noticed in the comments, that's a bug in JDK 8, fixed only in versions >= 9.

If you're using JDK 8 and can't/won't upgrade it, there's a workaround. You can treat the "GMT" part as a literal (the text "GMT" itself) and consider the 0 as the offset seconds, using the respective ChronoField:

"GMT"

ChronoField

DateTimeFormatter zoneParser = new DateTimeFormatterBuilder()
// text "GMT"
.appendLiteral("GMT")
// offset seconds
.appendValue(ChronoField.OFFSET_SECONDS)
.toFormatter();
System.out.println(zoneParser.parse("GMT0"));


Keep in mind that this works only for offset zero. For any other values (such as "GMT2" or "GMT-2") this won't work, because it'll consider the values "2" and "-2" as seconds, but they actually mean "hours".

Well, JDK 8 also can't handle one-digit offsets, and it always requires a signal, either + or -. So "GMT2" and "GMT-2" won't work with the current API.

+

-

There's a harder alternative, though: create your own TemporalField, representing "offset hours". All the details about how to do it are in the documentation, and I'm not sure if all methods are correctly implemented - I'm just sure about isSupportedBy, getFrom and adjustInto, the others maybe need some improvement/adjustment:

TemporalField

isSupportedBy

getFrom

adjustInto

public class OffsetHours implements TemporalField

@Override
public TemporalUnit getBaseUnit()
return ChronoUnit.HOURS;


@Override
public TemporalUnit getRangeUnit()
return ChronoUnit.FOREVER;


@Override
public ValueRange range()
return ValueRange.of(ZoneOffset.MIN.getTotalSeconds() / 3600, ZoneOffset.MAX.getTotalSeconds() / 3600);


@Override
public boolean isDateBased()
return false;


@Override
public boolean isTimeBased()
return true;


@Override
public boolean isSupportedBy(TemporalAccessor temporal)
return temporal.isSupported(ChronoField.OFFSET_SECONDS);


@Override
public ValueRange rangeRefinedBy(TemporalAccessor temporal)
ValueRange rangeInSecs = temporal.range(ChronoField.OFFSET_SECONDS);
return ValueRange.of(rangeInSecs.getMinimum() / 3600, rangeInSecs.getMaximum() / 3600);


@Override
public long getFrom(TemporalAccessor temporal)
return temporal.getLong(ChronoField.OFFSET_SECONDS) / 3600;


@Override
public <R extends Temporal> R adjustInto(R temporal, long newValue)
return (R) temporal.with(ChronoField.OFFSET_SECONDS, newValue * 3600);




Now you create an instance of this field and use it in your parser:

// the new field
OffsetHours offsetHoursField = new OffsetHours();
DateTimeFormatter zoneParser = new DateTimeFormatterBuilder()
// text "GMT"
.appendLiteral("GMT")
// offset hours
.appendValue(offsetHoursField)
.toFormatter();


I also recommend creating a TemporalQuery to convert the parsed result to a ZoneOffset:

TemporalQuery

ZoneOffset

// get hours and create offset from hours value
TemporalQuery<ZoneOffset> getOffsetFromHours = temporal ->
return ZoneOffset.ofHours((int) temporal.getLong(offsetHoursField));
;


Now you can parse it:

ZoneOffset offsetZero = zoneParser.parse("GMT0", getOffsetFromHours);
ZoneOffset offsetTwo = zoneParser.parse("GMT2", getOffsetFromHours);
ZoneOffset offsetMinusTwo = zoneParser.parse("GMT-2", getOffsetFromHours);


You can improve it letting the OffsetHours field to be a static instance (or maybe an enum), so you don't need to create it all the time.

OffsetHours

enum

0

GMT0

Since Java 8, the ZoneId is responsible for handling timezone names and aliases according to IANA Time Zone Database (often referred as TZ DB, zdata). In particular, ZoneId.html#of is used for that conversion.

Different TZ DB versions are shipped by Oracle with different JREs: see tzdata versions and overview of how to update tzdata

The list of TZ DB identifiers is available on wikipedia (TZ column), as for release release 2017c, GMT0 is already listed as deprecated in there, and canonical name for this time zone is Etc/GMT

GMT0

Etc/GMT

Despite there is no explicit instructions to use only canonical TZ names, it can be a best practice however, as deprecated aliases may be removed from further tzdata distributions (and thus from Zone.of support) without any special notice

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