JAXB: Unmarshal to subclass based on element value

Question

I am trying to parse an XML file using JAXB which contains a list of items. The class of the items depends on the value of an element in the XML.

This is a legacy system and I can't easily change the input format.

For example, given the following XML and class definitions:

<root>
    <type>a</type>
    <item>
       <a>a1</a>
    </item>
    <item>
       <a>a2</a>
    </item>
</root>

@XmlRootElement(name = "root")
public class Root {
    @XmlElement
    String type;

    @XmlElement(name="item")
    List<Item> items;
}

public class Item {}
public class ItemA extends Item {
    @XmlElement
    String a;
}
public class ItemB extends Item {
    @XmlElement
    String b;
}

As it works now, the items list contains two Item objects.

I need the items list in the resulting Root object to contain two ItemA objects, one with a="a1" and the other with a="a2".

If the type element is "b", I need the items list to contain ItemB objects.

There will only be one type element specified in a single XML file.

I have seen several solutions using attribute values, but none using element values.

Answer 1

Following the tip of Blaise Doughan you could create an XmlAdapter. Unfortunately Adapters are not available on the root level so you would have to add a bit extra code during un-/marshalling.

Root/Item/ItemA/ItemB are plain POJOs without any annotations here.

The Adapter with the adapted types:

public class RootAdapter extends XmlAdapter<AdaptedRoot, Root>
{
    @Override
    public Root unmarshal( AdaptedRoot v ) throws Exception
    {
        Root root = new Root();
        root.type = v.type;
        for ( AdaptedItem adaptedItem : v.items )
        {
            if ( v.type.equals( "a" ) )
            {
                ItemA a = new ItemA();
                a.a = adaptedItem.a;
                root.items.add( a );
            }
            if ( v.type.equals( "b" ) )
            {
                ItemB b = new ItemB();
                b.b = adaptedItem.b;
                root.items.add( b );
            }
        }
        return root;
    }

    @Override
    public AdaptedRoot marshal( Root v ) throws Exception
    {
        AdaptedRoot adapted = new AdaptedRoot();
        adapted.type = v.type;
        for ( Item item : v.items )
        {
            AdaptedItem adaptedItem = new AdaptedItem();
            if ( v.type.equals( "a" ) )
            {
                adaptedItem.a = ((ItemA) item).a;
            }
            if ( v.type.equals( "b" ) )
            {
                adaptedItem.b = ((ItemB) item).b;
            }
            adapted.items.add( adaptedItem );
        }
        return adapted;
    }

    @XmlRootElement( name = "root" )
    public static class AdaptedRoot
    {
        @XmlElement
        String            type;
        @XmlElement( name = "item" )
        List<AdaptedItem> items = new ArrayList<>();
    }

    public static class AdaptedItem
    {
        @XmlElement
        String a;
        @XmlElement
        String b;
    }
}

Un-/marshalling could be done like this:

public static void main( String[] args ) throws Exception
{
    String rawRootA = "<root><type>a</type><item><a>a1</a></item><item><a>a2</a></item></root>";
    String rawRootB = "<root><type>b</type><item><b>b1</b></item><item><b>b2</b></item></root>";

    Root rootA = unmarshal( rawRootA );
    for ( Item item : rootA.items )
    {
        System.out.println( item.getClass().getSimpleName() );
    }
    print( rootA );

    Root rootB = unmarshal( rawRootB );
    for ( Item item : rootB.items )
    {
        System.out.println( item.getClass().getSimpleName() );
    }
    print( rootB );
}

public static Root unmarshal( String xml ) throws Exception
{
    JAXBContext context = JAXBContext.newInstance( AdaptedRoot.class );
    Unmarshaller unmarshaller = context.createUnmarshaller();
    XmlAdapter<AdaptedRoot, Root> adapter = new RootAdapter();

    AdaptedRoot adapted = (AdaptedRoot) unmarshaller.unmarshal( new StringReader( xml ) );
    return adapter.unmarshal( adapted );
}

public static void print( Root root ) throws Exception
{
    JAXBContext context = JAXBContext.newInstance( AdaptedRoot.class );
    Marshaller marshaller = context.createMarshaller();
    marshaller.setProperty( Marshaller.JAXB_FORMATTED_OUTPUT, true );
    XmlAdapter<AdaptedRoot, Root> adapter = new RootAdapter();

    AdaptedRoot adaptedRoot = adapter.marshal( root );
    marshaller.marshal( adaptedRoot, System.out );
}

with the expected output:

ItemA
ItemA
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root>
    <type>a</type>
    <item>
        <a>a1</a>
    </item>
    <item>
        <a>a2</a>
    </item>
</root>
ItemB
ItemB
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root>
    <type>b</type>
    <item>
        <b>b1</b>
    </item>
    <item>
        <b>b2</b>
    </item>
</root>