Marshalling a List of objects implementing a common interface, with JaxB

Question

I am trying to marshall a list of objects implementing a common interface. There are 3 classes and 1 interface involved:

Community class (has one me

Answer 1

OK, if you're prepared to change Person from an interface into an abstract base class, then you're golden. Here's the code:

public class Main {


  public static void main(String[] args) throws Exception {

    Community community = new Community();

    JAXBContext context = JAXBContext.newInstance(Community.class);
    Marshaller marshaller = context.createMarshaller();
    marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
    marshaller.marshal(community, System.out);

  }
}

@XmlRootElement(name = "community")
@XmlSeeAlso({Person.class})
public class Community {

 private List<Person> people;

 @XmlElementWrapper(name="people")
 @XmlElementRef()
 public List<Person> getPeople() {
  return people;
 }

 public Community() {
  people = new ArrayList<Person>();
  people.add(new Girl());
  people.add(new Boy());
  people.add(new Girl());
  people.add(new Boy());
 }
}

@XmlRootElement(name="boy")
public class Boy extends Person {

 public String getName() {
  return "John";
 }
}

@XmlRootElement(name="girl")
public class Girl extends Person {

 public String getName() {
  return "Jane";
 }
}

@XmlRootElement(name = "person")
@XmlSeeAlso({Girl.class,Boy.class})
public abstract class Person {

  @XmlElement(name="name")
 public abstract String getName();
}

The main trick was the use of @XmlElementRef in the List of Community. This identifies the type of the class through it's @XmlRootElement. You may also be interested in the @XmlSeeAlso which helps to organise context declarations.

And the output is

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<community>
    <people>
        <girl>
            <name>Jane</name>
        </girl>
        <boy>
            <name>John</name>
        </boy>
        <girl>
            <name>Jane</name>
        </girl>
        <boy>
            <name>John</name>
        </boy>
    </people>
</community>

Answer 2

For this scenario I would recommend the use of @XmlElements. @XmlElements is used to represent the XML schema concept of choice:

• http://bdoughan.blogspot.com/2010/10/jaxb-and-xsd-choice-xmlelements.html

Here is how it would look for your example:

@XmlElements({ 
    @XmlElement(name="girl", type=Girl.class),
    @XmlElement(name="boy", type=Boy.class)
})
@XmlElementWrapper
public List<Person> getPeople() {
    return people;
}

@XmlElementRef corresponds to the concept of substitution groups in XML schema. This is why the previous answer requires Person to be changed from an interface to a class.

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