Java/JAXB: Unmarshall Xml to specific subclass based on an attribute

Question

Is it possible to use JAXB to unmarshall xml to a specific Java class based on an attribute of the xml?

Answer 1

JAXB is a spec, specific implementations will provide extension points to do things such as this. If you are using EclipseLink JAXB (MOXy) you could modify the Shape class as follows:

import javax.xml.bind.annotation.XmlAttribute;
import org.eclipse.persistence.oxm.annotations.XmlCustomizer;

@XmlCustomizer(ShapeCustomizer.class)
public abstract class Shape {

    int points;

    @XmlAttribute
    public int getPoints() {
        return points;
    }

    public void setPoints(int points) {
        this.points = points;
    }

}

Then using the MOXy @XMLCustomizer you could access the InheritancePolicy and change the class indicator field from "@xsi:type" to just "type":

import org.eclipse.persistence.config.DescriptorCustomizer;
import org.eclipse.persistence.descriptors.ClassDescriptor;

public class ShapeCustomizer implements DescriptorCustomizer {

    @Override
    public void customize(ClassDescriptor descriptor) throws Exception {
        descriptor.getInheritancePolicy().setClassIndicatorFieldName("@type");
    }
}

You will need to ensure that you have a jaxb.properties file in with you model classes (Shape, Square, etc) with the following entry specifying the EclipseLink MOXy JAXB implementation:

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

Below is the rest of the model classes:

Shapes

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Shapes {

    private List shape = new ArrayList();;

    public List getShape() {
        return shape;
    }

    public void setShape(List shape) {
        this.shape = shape;
    }

}

Square

import javax.xml.bind.annotation.XmlAttribute;

public class Square extends Shape {
    private String squareSpecificAttribute;

    @XmlAttribute(name="square-specific-attribute")
    public String getSquareSpecificAttribute() {
        return squareSpecificAttribute;
    }

    public void setSquareSpecificAttribute(String s) {
        this.squareSpecificAttribute = s;
    }

}

Triangle

import javax.xml.bind.annotation.XmlAttribute;

public class Triangle extends Shape {
    private String triangleSpecificAttribute;

    @XmlAttribute(name="triangle-specific-attribute")
    public String getTriangleSpecificAttribute() {
        return triangleSpecificAttribute;
    }

    public void setTriangleSpecificAttribute(String t) {
        this.triangleSpecificAttribute = t;
    }

}

Below is a demo program to check that everything works:

import java.io.StringReader;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jaxbContext = JAXBContext.newInstance(Shapes.class, Triangle.class, Square.class);

        StringReader xml = new StringReader("43");
        Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
        Shapes root = (Shapes) unmarshaller.unmarshal(xml);

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

I hope this helps.

For more information on EclipseLink MOXy see:

• http://www.eclipse.org/eclipselink/moxy.php

EDIT

In EclipseLink 2.2 we're making this easier to configure, check out the following article for more information:

• http://bdoughan.blogspot.com/2010/11/jaxb-and-inheritance-moxy-extension.html