Marshalling LocalDate using JAXB

I’m building a series of linked classes whose instances I want to be able to marshall to XML so I can save them to a file and read them in again later.

At present I’m using the following code as a test case:

import javax.xml.bind.annotation.*;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;

import java.time.LocalDate;

public class LocalDateExample
{
  @XmlRootElement
  private static class WrapperTest {
    public LocalDate startDate;
  }

  public static void main(String[] args) throws JAXBException
  {
    WrapperTest wt = new WrapperTest();
    LocalDate ld = LocalDate.of(2016, 3, 1);
    wt.startDate = ld;
    marshall(wt);
  }

  public static void marshall(Object jaxbObject) throws JAXBException
  {
    JAXBContext context = JAXBContext.newInstance(jaxbObject.getClass());
    Marshaller marshaller = context.createMarshaller();
    marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
    marshaller.marshal(jaxbObject, System.out);
  }
}

The XML output is:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<wrapperTest>
    <startDate/>
</wrapperTest>

Is there a reason why the startDate element is empty? I would like it to contain the string representation of the date (i.e. toString()). Do I need to write some code of my own in order to do this?

The output of java -version is:

openjdk version "1.8.0_66-internal"
OpenJDK Runtime Environment (build 1.8.0_66-internal-b17)
OpenJDK 64-Bit Server VM (build 25.66-b17, mixed mode)

Answer

You will have to create an XmlAdapter like this:

public class LocalDateAdapter extends XmlAdapter<String, LocalDate> {
    public LocalDate unmarshal(String v) throws Exception {
        return LocalDate.parse(v);
    }

    public String marshal(LocalDate v) throws Exception {
        return v.toString();
    }
}

And annotate your field using

 @XmlJavaTypeAdapter(value = LocalDateAdapter.class)

See also javax.xml.bind.annotation.adapters.XmlJavaTypeAdapters if you want to define your adapters on a package level.