Convert between LocalDate and XMLGregorianCalendar

What's the best way to convert between LocalDate from Java 8 and XMLGregorianCalendar?

Converting from LocalDate to XMLGregorianCalendar:

LocalDate date = LocalDate.now();
GregorianCalendar gcal = GregorianCalendar.from(date.atStartOfDay(ZoneId.systemDefault()));
XMLGregorianCalendar xcal = DatatypeFactory.newInstance().newXMLGregorianCalendar(gcal);

Converting back is simpler:

xcal.toGregorianCalendar().toZonedDateTime().toLocalDate();

The LocalDate stores only year/month/day information. There is no time nor time-zone information in it. The XMLGregorianCalendar stores date (year/month/day) + optionally time and optionally time zone information.

So converting from LocalDate to XMLGregorianCalendar is simple:

LocalDate in;
XMLGregorianCalendar out;
in = LocalDate.parse("1999-11-11");
out = DatatypeFactory.newInstance().newXMLGregorianCalendar(in.toString());

Converting from XMLGregorianCalendar to LocalDate might be not so simple, because XMLGregorianCalendar may have time and time-zone information which you simply can't store in LocalDate.

However, I guess that if you are converting from XMLGregorianCalendar to LocalDate then the XMLGregorianCalendar is resulting from a nontimezoned xsd:date element (represented as YYYY-MM-DD in the xml). In that case you should convert it like this:

XMLGregorianCalendar in;
LocalDate out;
in = DatatypeFactory.newInstance().newXMLGregorianCalendar("2011-11-11");
out = LocalDate.parse(in.toXMLFormat());

Whole example:

	{
		LocalDate in;
		XMLGregorianCalendar out;
		in = LocalDate.parse("1999-11-11");
		out = DatatypeFactory.newInstance().newXMLGregorianCalendar(in.toString());
		System.out.println("in: " + in.toString());
		System.out.println("out: " + out.toXMLFormat());
	}
	{
		XMLGregorianCalendar in;
		LocalDate out;
		in = DatatypeFactory.newInstance().newXMLGregorianCalendar("2011-11-11");
		out = LocalDate.parse(in.toXMLFormat());
		System.out.println("in: " + in.toXMLFormat());
		System.out.println("out: " + out.toString());
	}

To convert from LocalDate to XMLGregorianCalendar you can use

LocalDate localDate = ...;
GregorianCalendar calender = new GregorianCalendar();
Date utilDate = Date.from( localDate.atStartOfDay( ZoneId.systemDefault() ).toInstant() );
calender.setTime(utilDate);		
XMLGregorianCalendar xmlCal = DatatypeFactory.newInstance().newXMLGregorianCalendar(calender);

And to convert XMLGregorianCalendar back to LocalDate:

XMLGregorianCalendar xmlCal = ...;
Date utilDate = xmlCal.toGregorianCalendar().getTime();
LocalDate localDate = LocalDateTime.ofInstant( utilDate.toInstant(), ZoneId.systemDefault() ).toLocalDate();

The following is a simple way to convert from LocalDate to XMLGregorianCalendar which both preserves the undefined fields (hours, timezone, etc.) and is efficient (i.e. no conversion to/from String). Unlike some of the other solutions this results in XML dates without timezones, e.g. <date>2018-11-06</date> instead of <date>2018-11-06+01:00</date>.

LocalDate date = ...;
XMLGregorianCalendar xmlCal = DatatypeFactory.newInstance().newXMLGregorianCalendar();
xmlCal.setYear(date.getYear());
xmlCal.setMonth(date.getMonthValue());
xmlCal.setDay(date.getDayOfMonth());

Converting back is a bit simpler:

XMLGregorianCalendar xmlCal = ...
LocalDate date = LocalDate.of(xmlCal.getYear(), xmlCal.getMonth(), xmlCal.getDay());