JAXB/MOXy: Do not call XmlElementWrapper setter when element missing?

Question

I have a list setter on a class that's annotated with both @XmlElementWrapper(name = "foos") and @XmlElement(name = "foo").

When I unmarshall XML that has no <foos></foos> or <foo/> elements, the setter is called and passed an empty list. Is there a way to get the following?:

• When there is no <foos/>, do not call the setter. Or if the setter must be called, pass null.
• When <foos/> is present but empty, pass an empty list to the setter.
• When <foos> has one or more child <foo/> elements, pass a populated list.

Answer 1

You could use an XmlAdapter for this use case:

input1.xml

When there is no , do not call the setter. Or if the setter must be called, pass null.

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <child/>
</root>

input2.xml

When is present but empty, pass an empty list to the setter.

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <child>
        <foos/>
    </child>
</root>

input3.xml

When has one or more child elements, pass a populated list.

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <child>
        <foos>
             <foo>Hello World</foo>
       </foos>
   </child>
</root>

Root

import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement
public class Root {

    private Child child;

    @XmlJavaTypeAdapter(ChildAdapter.class)
    public Child getChild() {
        return child;
    }

    public void setChild(Child child) {
        this.child = child;
    }

}

Child

import java.util.List;

public class Child {

    private List<String> strings;

    public List<String> getStrings() {
        return strings;
    }

    public void setStrings(List<String> strings) {
        System.out.println("setStrings");
        this.strings = strings;
    }

}

ChildAdapter

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.adapters.XmlAdapter;

public class ChildAdapter extends XmlAdapter<ChildAdapter.AdaptedChild, Child> {

    public static class AdaptedChild {
        public Foos foos;
    }

    public static class Foos {
        public List<String> foo;
    }

    @Override
    public Child unmarshal(AdaptedChild adaptedChild) throws Exception {
        Child child = new Child();
        Foos foos = adaptedChild.foos;
        if(null != foos) {
            List<String> foo = foos.foo;
            if(null == foo) {
                child.setStrings(new ArrayList<String>());
            } else {
                child.setStrings(foos.foo);
            }
        }
        return child;
    }

    @Override
    public AdaptedChild marshal(Child child) throws Exception {
        AdaptedChild adaptedChild = new AdaptedChild();
        List<String> strings = child.getStrings();
        if(null != strings) {
            Foos foos = new Foos();
            foos.foo = strings;
            adaptedChild.foos = foos;
        }
        return adaptedChild;
    }

}

Demo

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Root.class);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        Object o;

        o = unmarshaller.unmarshal(new File("input1.xml"));
        marshaller.marshal(o, System.out);

        o = unmarshaller.unmarshal(new File("input2.xml"));
        marshaller.marshal(o, System.out);

        o = unmarshaller.unmarshal(new File("input3.xml"));
        marshaller.marshal(o, System.out);
    }

}

Output

<?xml version="1.0" encoding="UTF-8"?>
<root>
   <child/>
</root>
setStrings
<?xml version="1.0" encoding="UTF-8"?>
<root>
   <child>
      <foos/>
   </child>
</root>
setStrings
<?xml version="1.0" encoding="UTF-8"?>
<root>
   <child>
      <foos>
         <foo>Hello World</foo>
      </foos>
   </child>
</root>