JAXB marshalling purely from interfaces

Question

I have a complex hierarchy of Java interfaces that I\'d like to marshal (and not necessarily unmarshal) with JAXB. These interfaces represent objects that will be returned from

Answer 1

Short answer: use @XmlElement(type = Object.class) on your interface field.

Details below:

I have found 2 ways in which you could make JAXB serialize your interfaces:

1. @XmlAnyElement
2. @XmlElement(type = Object.class)

1.@XmlAnyElement

Simply annotate your interface type field with @XmlAnyElement and JAXB will serialize the interface from it's concrete type. Don't forget to annotate the concrete types with @XmlRootElement and to add the concrete types to the JAXBContext. Full example follows:

public class InterfaceSerializer {

    @XmlRootElement
    public static class Pojo {
        Pojo() {
            field1 = new PojoFieldImpl1();
            field2 = new PojoFieldImpl2();
            field3 = new PojoFieldImpl1();
        }

        @XmlAnyElement
        public IPojoField field1;
        @XmlAnyElement
        public IPojoField field2;
        @XmlAnyElement
        public IPojoField field3;

        @Override
        public String toString() {
            return "field1 = " + field1 + "\nfield2 = " + field2 + "\nfield3 = " + field3;
        }
    }

    public static interface IPojoField {

    }

    @XmlRootElement
    public static class PojoFieldImpl1 implements IPojoField {

        PojoFieldImpl1() {
            value = "PojoFieldImpl1 value";
        }

        public String value;

        @Override
        public String toString() {
            return value;
        }
    }

    @XmlRootElement
    public static class PojoFieldImpl2  implements IPojoField {

        PojoFieldImpl2() {
            value = "PojoFieldImpl2 value1";
            value2 = "PojoFieldImpl2 value2";
        }

        public String value;
        public String value2;

        @Override
        public String toString() {
            return value + " " + value2;
        }
    }

    public static void main(String []args) throws JAXBException {
        Pojo pojo = new Pojo();
        JAXBContext jaxbContext = JAXBContext.newInstance(Pojo.class, PojoFieldImpl1.class, PojoFieldImpl2.class);
        Marshaller marshaller = jaxbContext.createMarshaller();

        marshaller.marshal(pojo, new File("interfaceSerializer.xml"));
    }
}

Output XML:

    
        PojoFieldImpl1 value
    
    
        PojoFieldImpl2 value1
        PojoFieldImpl2 value2
    
    
        PojoFieldImpl1 value
    

The downsides of this method:

• you can't distinguish from XML each individual field from your pojo (same implementations will be written with the same tag)
• you don't have any type information to Unmarshall your XML (if you would wish to do so)

These downsides are fixed in the second solution:

2.@XmlElement(type = Object.class)

I've stumbled upon this in mikesir87's blog post. Simply replace the @XmlAnyElement annotations from above with @XmlElement(type = Object.class) You should have something like this in the Pojo class from above:

@XmlElement(type = Object.class)
public IPojoField field1;
@XmlElement(type = Object.class)
public IPojoField field2;
@XmlElement(type = Object.class)
public IPojoField field3;

Re-running our example, the resulting XML:

    
        PojoFieldImpl1 value
    
    
        PojoFieldImpl2 value1
        PojoFieldImpl2 value2
    
    
        PojoFieldImpl1 value
    

This can also be deserialized:

Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
Pojo unmarshalledPojo = (Pojo) unmarshaller.unmarshal(new File("interfaceSerializer.xml"));  
System.out.println(unmarshalledPojo);

Resulting output:

field1 = PojoFieldImpl1 value
field2 = PojoFieldImpl2 value1 PojoFieldImpl2 value2
field3 = PojoFieldImpl1 value

Probably a "hackish" solution, but it gets the job done.