Question

I have an XML document that looks something like the following:

Note that I cannot change the schema because it is part of a standard XML Schema (Library of Congress METS).

<amdSec ID="AMDSEC001">
    <digiprovMD ID="DMD001">
        <mdWrap MDTYPE="OBJECT">
            <xmlData>
                <object xsi:type="file">
                    .....
                </object>
            </xmlData>
        </mdWrap>
    </digiprovMD>
    <digiprovMD ID="DMD001_EVENT">
        <mdWrap MDTYPE="EVENT">
            <xmlData>
                <event xsi:type="event">
                    .....
                </event>
            </xmlData>
        </mdWrap>
    </digiprovMD>
</amdSec>

As you can see, the inner element <mdWrap> can contain elements of different types; in this case they’re <event> and <object>, but it isn’t constrained to just those two types. Creating two classes (like below), marshals okay, but this doesn’t work for unmarshalling.

class ObjectMDWrap {
    @XmlElementWrapper(name = "xmlData")
    @XmlElement(name = "object")
    List<MyObject> object; //Wrapped in list to use @XmlElementWrapper
}

class EventMDWrap {
    @XmlElementWrapper(name = "xmlData")
    @XmlElement(name = "event")
    List<MyEvent> event; //Wrapped in list to use @XmlElementWrapper
}

What can I do so that JAXB unmarshals the correct “type” of MDWrap?

Answer 1

I was able to figure out the solution, and it’s much simpler than I initially thought (which speaks to my relative inexperience with XML and JAXB). By creating my MDWrap class in the following way

class MDWrap {
    @XmlAnyElement(lax = true)
    @XmlElementWrapper(name = "xmlData")
    Object wrappedMD;
}

Then MDWrap can contain an object of any type, and will unmarshal properly, as long as the class of which wrappedMD is an instance of is annotated with @XmlRootElement. The trick is to annotate wrappedMD as XmlAnyElement.