JAXB: How should I marshal complex nested data structures?

I’ve solved the problem without XmlAdapter’s.

I’ve written JAXB-annotated objects for Map, Map.Entry and Collection.
The main idea is inside the method xmlizeNestedStructure(…):

Take a look at the code:

public final class Adapters {

private Adapters() {
}

public static Class<?>[] getXmlClasses() {
    return new Class<?>[]{
                XMap.class, XEntry.class, XCollection.class, XCount.class
            };
}

public static Object xmlizeNestedStructure(Object input) {
    if (input instanceof Map<?, ?>) {
        return xmlizeNestedMap((Map<?, ?>) input);
    }
    if (input instanceof Collection<?>) {
        return xmlizeNestedCollection((Collection<?>) input);
    }

    return input; // non-special object, return as is
}

public static XMap<?, ?> xmlizeNestedMap(Map<?, ?> input) {
    XMap<Object, Object> ret = new XMap<Object, Object>();

    for (Map.Entry<?, ?> e : input.entrySet()) {
        ret.add(xmlizeNestedStructure(e.getKey()),
                xmlizeNestedStructure(e.getValue()));
    }

    return ret;
}

public static XCollection<?> xmlizeNestedCollection(Collection<?> input) {
    XCollection<Object> ret = new XCollection<Object>();

    for (Object entry : input) {
        ret.add(xmlizeNestedStructure(entry));
    }

    return ret;
}

@XmlType
@XmlRootElement
public final static class XMap<K, V> {

    @XmlElementWrapper(name = "map")
    @XmlElement(name = "entry")
    private List<XEntry<K, V>> list = new LinkedList<XEntry<K, V>>();

    public XMap() {
    }

    public void add(K key, V value) {
        list.add(new XEntry<K, V>(key, value));
    }

}

@XmlType
@XmlRootElement
public final static class XEntry<K, V> {

    @XmlElement
    private K key;

    @XmlElement
    private V value;

    private XEntry() {
    }

    public XEntry(K key, V value) {
        this.key = key;
        this.value = value;
    }

}

@XmlType
@XmlRootElement
public final static class XCollection<V> {

    @XmlElementWrapper(name = "list")
    @XmlElement(name = "entry")
    private List<V> list = new LinkedList<V>();

    public XCollection() {
    }

    public void add(V obj) {
        list.add(obj);
    }

}

}

It works!

Let’s look at a demo output:

<xMap>
    <map>
        <entry>
            <key xsi:type="xCount" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                <count>1</count>
                <content xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a</content>
            </key>
            <value xsi:type="xCollection" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                <list>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a1</entry>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a2</entry>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a3</entry>
                </list>
            </value>
        </entry>
        <entry>
            <key xsi:type="xCount" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                <count>2</count>
                <content xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b</content>
            </key>
            <value xsi:type="xCollection" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                <list>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b1</entry>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b3</entry>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b2</entry>
                </list>
            </value>
        </entry>
        <entry>
            <key xsi:type="xCount" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                <count>3</count>
                <content xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c</content>
            </key>
            <value xsi:type="xCollection" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                <list>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c1</entry>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c2</entry>
                    <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c3</entry>
                </list>
            </value>
        </entry>
    </map>
</xMap>

Sorry, the demo output uses also a data structure called “count”
which is not mentioned in the Adapter’s source code.

BTW: does anyone know how to remove all these annoying
and (in my case) unnecessary xsi:type attributes?