Using JAXB to marshall an XML node that can contain only one of multiple child node types (SharePoint Query)

问题

I'm trying to create JAXB annotated classes to generate XML based on the Microsoft SharePoint Query schema. I have an SpWhereClause class:

@XmlType(name="Where")
@XmlAccessorType(XmlAccessType.FIELD)
public class SpWhereClause {

}

But I'm not sure how to structure/annotate its properties. A <Where> element can have many different types of child elements (<Eq>, <BeginsWith>, <Contains> etc. Let's ignore <And> and <Or> for now), but not more than one. Eq and BeginsWith are each individually valid children of Where, but it can't be like this:

<Where>
    <Eq>...</Eq>
    <BeginsWith>...</BeginsWith>
</Where>

without nesting the <Eq> and <BeginsWith> in an <Or> element.

My first thought was to create an AbstractSpComparison class with the <FieldRef> and <Value> elements common to all comparisons:

@XmlAccessorType(XmlAccessType.FIELD)
public abstract class AbstractSpComparison {

    @XmlElement
    private SpFieldRef fieldRef;

    @XmlElement
    private SpValue value;

    ...
}

then have SpEqualsComparison extend it:

@XmlType(name="Eq")
public class SpEqualsComparison extends AbstractSpComparison {

}

However, using the abstract class in SpWhereClause leaves me unable to control the name of the child element. This:

@XmlElement
private AbstractSpComparison comparison;

Results in this:

<Where>
    <comparison>...</comparison>
</Where>

instead of this:

<WHERE>
    <Eq>...</Eq>
</WHERE>

Why isn't the "Eq" name from SpEqualsComparison being used to name the comparison element? What's the right way to handle a situation like that?

I considered just having every possible child element type as a property of SpWhereClause, and only set the one I need (with some validation logic somewhere), but that seems unnecessarily verbose.

If it matters, I'm using a Spring OXM Jaxb2Marshaller.

回答1:

There are a couple of different options you could do:

Option #1 - `@XmlElementRef`

If everything extends a common super class (i.e. AbstractSpComparison) then you could leverage the @XmlElementRef annotation. When you do this the child element will correspond to the value of the @XmlRootElement annotation on the class of the referenced object.

@XmlElementRef
private AbstractSpComparison comparison;

For More Information

I have written more about this approach on my blog:

http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-substitution.html

Option #2 - `@XmlElements`

If everything doesn't extend a common super class then you can leverage the @XmlElements annotation where you explicitly annotate which elements could appear and what class they correspond to:

@XmlElements({
    @XmlElement(name="Eq", type=Eq.class),
    @XmlElement(name="BeginsWith", type=BeginsWith.class)
})
private AbstractSpComparison comparison;

For More Information

I have written more about this approach on my blog: