JAXB - How to set the xsi:type of a XML element based on its value?

Question

I have to generate a xml element that can have as value any "primitive type" (xsd:string, xsd:boolean, etc). Examples:

<field xsi:type="xsd:string" name="aString">String Value</field>
<field xsi:type="xsd:date" name="aDate">2011-10-21</field>
...

So, I tried two implementations:

public class Field {
    @XmlAttribute
    private String name;

    @XmlValue
    Object value;
}

and ...

public class Field<T> {
    @XmlAttribute
    private String name;

    @XmlValue
    T value;
}

I'm testing this with:

Marshaller marshaller = JAXBContext.newInstance(Field.class).createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
marshaller.setProperty("com.sun.xml.bind.xmlDeclaration", Boolean.FALSE);

Field field = new Field();
field.name = "name";
field.value = "value";

ByteArrayOutputStream stream = new ByteArrayOutputStream();
marshaller.marshal(field, new PrintWriter(stream));
System.out.println(stream);

But I'm getting this NullPointerException when I try to instantiate the JAXBContext.

java.lang.NullPointerException
at com.sun.xml.bind.v2.runtime.reflect.TransducedAccessor.get(TransducedAccessor.java:165)
at com.sun.xml.bind.v2.runtime.property.ValueProperty.<init>(ValueProperty.java:77)
at com.sun.xml.bind.v2.runtime.property.PropertyFactory.create(PropertyFactory.java:106)
at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.<init>(ClassBeanInfoImpl.java:179)
at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:515)
at com.sun.xml.bind.v2.runtime.JAXBContextImpl.<init>(JAXBContextImpl.java:330)
at 

The idea is allow schema validation for the field element (It is define in a schema must its type will be set in each instance). So, even is this a Bug (or not) ... how JAXB will put the correct xsi:type to this field instance? I'm missing a concept here?

I know that maybe the problem is the usage of the @XmlValue because of this restrictions (from the javadoc):

• At most one field or property can be annotated with the @XmlValue annotation.
• @XmlValue can be used with the following annotations: XmlList. However this is redundant since XmlList maps a type to a simple schema type that derives by list just as XmlValue would.
• If the type of the field or property is a collection type, then the collection item type must map to a simple schema type.
• If the type of the field or property is not a collection type, then the type must map to a XML Schema simple type.

... because an Object or a generic T is not necessarily a XML Schema simple type, this approach seems not to be the correct one ...

Thanks in advance ...

Answer 1

I have confirmed the issue you are seeing in both the reference and EclipseLink JAXB (MOXy) implementations of JAXB. The problem you are seeing is due to use of @XmlValue . If the value property was mapped as an @XmlElement you would see the xsi:type attribute appear as expected.

I have entered the following bug to track this issue in EclipseLink JAXB (MOXy):

• https://bugs.eclipse.org/361689

Depending upon what your domain model looks like you may be interested in the @XmlPath extension from EclipseLink JAXB (MOXy):

• http://blog.bdoughan.com/2011/03/map-to-element-based-on-attribute-value.html

UPDATE

This issue has now been fixed in the EclipseLink 2.3.3 and EclipseLink 2.4.0. The fix is available in these streams starting March 17, 2012 and can be obtained from:

• http://www.eclipse.org/eclipselink/downloads/nightly.php