Thanks guys for your answers.
Chris Dail – I tried your approach, and it didn’t really do what I wanted. JAXB was still ignoring my null values, in spite of defining a default value for my fields.
I did stumble across the answer after somebody in the Jersey forums pointed me to documentation section 2.2.12.8 No Value.
Basically, all I had to do was to add the following to my fields :
@XmlElement(nillable = true)
Once I added that, JAXB would show up those fields when marshalling them to XML like this:
...
<num>5</num>
<date xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
....