JAXB removing unnecessary nested XML tag

Question

Currently I have following output from my program:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<container>
    <elements>
        <property name="e1">
            <foo name="Alex" status="Married"/>
        </property>
        <property name="e2">
            <foo name="Chris" status="Married with 2 children"/>
        </property>
    </elements>
</container>

As you can see, having both <container> and <elements> tags is useless. I'd like to remove <elements>, so the output would look like:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<container>
    <property name="e1">
        <foo name="Alex" status="Married"/>
    </property>
    <property name="e2">
        <foo name="Chris" status="Married with 2 children"/>
    </property>
</container>

Code that's generating first output is listed below:

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Container {

    @XmlElement
    @XmlJavaTypeAdapter(MyAdapter.class)
    private Map<String, Foo> elements = new HashMap<String, Foo>();

    public Container() {
        this.elements = new HashMap<String, Foo>();
    }

    public Map<String, Foo> getElements() {
        return elements;
    }

    @XmlAccessorOrder(XmlAccessOrder.ALPHABETICAL)
    @XmlRootElement(name = "foo")
    static class Foo {
        @XmlAttribute
        public String name;

        @XmlAttribute
        public String status;

        public Foo(String name, String status) {
            this.name = name;
            this.status = status;
        }

        public Foo() {
        }
    }

    public static void main(String[] args) throws JAXBException {
        final JAXBContext context = JAXBContext.newInstance(Container.class);
        final Marshaller m = context.createMarshaller();
        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        final Container c = new Container();
        final Map<String, Foo> elementsMap = c.getElements();
        elementsMap.put("e1", new Foo("Alex", "Married"));
        elementsMap.put("e2", new Foo("Chris", "Married with 2 children"));

        m.marshal(c, System.out);
    }
}

And MyAdapter class, based on JAXB @XmlAdapter: Map -> List adapter? (marshall only) :

public class MyAdapter extends XmlAdapter<MyAdapter.AdaptedFoo, Map<String, Foo>> {

    static class AdaptedFoo {
        public List<Property> property = new ArrayList<>();
    }

    public static class Property {
        @XmlAttribute
        public String name;

        @XmlElementRef(type = Foo.class)
        public Foo value;
    }

    @Override
    public Map<String, Foo> unmarshal(AdaptedFoo v) throws Exception {
        return null;
    }

    @Override
    public AdaptedFoo marshal(Map<String, Foo> map) throws Exception {
        if (null == map) {
            return null;
        }
        AdaptedFoo adaptedFoo = new AdaptedFoo();
        for (Entry<String, Foo> entry : map.entrySet()) {
            Property property = new Property();
            property.name = entry.getKey();
            property.value = entry.getValue();
            adaptedFoo.property.add(property);
        }
        return adaptedFoo;
    }
}

How can I remove <elements> tag from my output?

---

edit: I've found 'dirty' way to do it - setting @XmlRootElement(name = "") for Container class. But is there any 'elegant' way?

Answer 1

The XML element <elements> is required to associate the enclosed element seqence with the property Map<?,?> elements. You can't drop it: how would an unmarshaller know where the <property> elements belong *on the level of the element <container>.

Having a List<Property> is different since JAXB handles repeated elements x "hardwired" as a List<?> x, so there's no need for a wrapper.

Since you write Java classes with annotations, you could use this by adding (to Container) another "virtual" field and modify some annotations:

@XmlAccessorType(XmlAccessType.PROPERTY)
public class Container {
// ...
@XmlTransient    
public Map<String,Foo> getElements(){
    return elements;
}
private List<Property> property;
@XMLElement
public List<Property> getProperty(){
    List<Property>  props = elements.entrySet().stream()
                            .map( e -> new Property( e.getKey(), e.getValue() )
                            .collect( Collectors.toList() );
    return props;
}

Older Javas might do

   List<Property> props = new ArrayList<>();
   for( Map.Entry<String,Foo> e: elements.entrySet() ){
       props.add( new Property( e.getKey(), e.getValue() ) );
   }
   return props;

This marshals like this:

<container>
    <property name="aaa">
       <foo name="aaaname" status="aaastatus"/>
    </property>
    <property name="bbb">
       <foo name="bbbname" status="bbbstatus"/>
    </property>
</container>

It doesn't unmarshal!