JAXB: Unmarshal to subclass based on element value

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.