JAXB and inheritance in collection

Question

how can I map (via JAXB in java 1.6) Collection to XML and from XML, where

class mapping{
    @XmlElementWrapper(name="list")
    @XmlElement(name="item")
    Collection<A> list;
}

abstract class A{
}

class B extends A{
    public String onlyB;
}

class C extends A{
    public String onlyC;
}

a would like to see XML like this:

<something> (doesnt matter, I'm using it in another structure)
<list>
<item xsi:type="b"><onlyB>b</onlyB></item>
<item xsi:type="c"><onlyC>c</onlyC></item>
</list>
</something>

its working if I have

class mapping{        
    @XmlElement(name="item")
    A item;
}

I already tried xmlelementref, but with no success

and I dont want to use @XmlElements({@XmlElement ...}) because other project which are using this can add derived class from A

Answer 1

Your mapping appears to be correct. You need to ensure that the B and C classes are included when you create the JAXBContext. One way to accomplish this is to use @XmlSeeAlso.

@XmlSeeAlso(B.class, C.class)
abstract class A{
}

Below is an example of using xsi:type to represent inheritance in the domain model with JAXB:

• http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-xsitype.html

@XmlElementRef is used when you want to represent inheritance using the XML schema concept of substitution groups:

• http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-substitution.html

XmlElements corresponds to the choice structure in XML schema:

• http://blog.bdoughan.com/2010/10/jaxb-and-xsd-choice-xmlelements.html
• http://blog.bdoughan.com/2011/04/xml-schema-to-java-xsd-choice.html

---

FULL EXAMPLE

Below is a complete example:

Mapping

package forum7672121;

import java.util.Collection;

import javax.xml.bind.annotation.*;

@XmlRootElement(name="something")
@XmlAccessorType(XmlAccessType.FIELD)
class Mapping{
    @XmlElementWrapper(name="list")
    @XmlElement(name="item")
    Collection<A> list;
}

A

package forum7672121;

import javax.xml.bind.annotation.XmlSeeAlso;

@XmlSeeAlso({B.class, C.class})
abstract class A{
}

B

package forum7672121;

class B extends A{
    public String onlyB;
}

C

package forum7672121;

class C extends A{
    public String onlyC;
}

Demo

package forum7672121;

import java.io.File;
import javax.xml.bind.*;

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/forum7672121/input.xml");
        Mapping mapping = (Mapping) unmarshaller.unmarshal(xml);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(mapping, System.out);
    }

}

input.xml/Output

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<something>
    <list>
        <item xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="b">
            <onlyB>b</onlyB>
        </item>
        <item xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="c">
            <onlyC>c</onlyC>
        </item>
    </list>
</something>