JAXB How to marshal an element both optional or nillable

Question

I'm trying to marshal an element which can be null and in some cases should be not written in the final XML at all, in some other cases I should specify that it is nillable.

Consider the following example

<root>
    <element>
        <sub1>Whatever1</sub1>
        <sub2 xsi:nil="true"/>
    </element>
    <element>
        <sub1>Whatever2</sub1>
        <sub2>Not empty</sub2>
    </element>
    <element>
        <sub1>Whatever3</sub1>
    </element>
</root>

The element "sub2" is the one I'm having trouble with.

From my understanding I can easily achieve the first 2 cases with the Element class declared as follows:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "element", propOrder = {
        "sub1",
        "sub2",
})
public class Element {

    @XmlElement(required = true)
    public String sub1;
    @XmlElement(nillable = true)
    public String sub2;
    //...
}

To obtain the last two instead, I would do:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "element", propOrder = {
        "sub1",
        "sub2",
})
public class Element {

    @XmlElement(required = true)
    public String sub1;
    @XmlElement(required = false) //I could omit it entirely
    public String sub2;
    //...
}

Is there a way to obtain both?

I'm forced to Java 6 / JAXB 2.1.10.

Thank you.

Answer 1

Yes, it is possible to have elements which are both not required and nillable. There are a few things you need to do to make this work:

1. Make the field a JAXBElement<String> instead of String.
2. Create an ObjectFactory class with a factory method.
3. Put @XmlElementDecl on the factory method in the ObjectFactory and @XmlElementRef on the field in class Element.

Example:

Make the field a JAXBElement<String> with an @XmlElementRef annotation:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "element", propOrder = {"sub1", "sub2"})
@XmlRootElement
public class Element {

    @XmlElement(required = true)
    public String sub1;

    @XmlElementRef(name = "sub2", required = false)
    public JAXBElement<String> sub2;

    // ...
}

Create an ObjectFactory class with a factory method, with an @XmlElementDecl annotation. Note that the namespace and name of the @XmlElementRef and @XmlElementDecl annotation are the same:

@XmlRegistry
public class ObjectFactory {

    @XmlElementDecl(name = "sub2")
    public JAXBElement<String> createSub2(String value) {
        return new JAXBElement<>(new QName(null, "sub2"),
                                 String.class, Element.class, value);
    }
}

Marshalling example 1: Have a value for sub2:

ObjectFactory objectFactory = new ObjectFactory();

Element element = new Element();
element.setSub1("one");
element.setSub2(objectFactory.createSub2("two"));

JAXBContext context = JAXBContext.newInstance(ObjectFactory.class, Element.class);

Marshaller marshaller = context.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(element, System.out);

Output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<element>
    <sub1>one</sub1>
    <sub2>two</sub2>
</element>

Marshalling example 2: No element at all in the XML:

Element element = new Element();
element.setSub1("one");
// Leave the field sub2 set to null

Output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<element>
    <sub1>one</sub1>
</element>

Marshalling example 3: Create an element with the value null which will appear as an XML element which is set to nil:

Element element = new Element();
element.setSub1("one");
element.setSub2(objectFactory.createSub2(null));

Output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<element>
    <sub1>one</sub1>
    <sub2 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</element>