Question

I’m having issues with downcasting in JAXB with classes that are self-referential.

My setup:

@XmlRootElement
class IdentifiableObject {
  @XmlID
  @XmlAttribute
  String id;

  @XmlAttribute
  String name;
}

@XmlRootElement
class Node extends IdentifiableObject {
  @XmlElement
  @XmlJavaAdapter(SimpleAdapterThatJustDowncastsToIdentifiableObject.class)
  Node parent;

  @XmlElement
  String aField;
}

I have done this with plenty of other objects, and it works fine. But when I’m using a class that refers to itself, it doesn’t work.

Is there something I can do to fix this? I know using XmlID/XmlIDREF kinda solves the problem, but I really want more than just a simple ref (I want id and name from identifiable)

To clarify, this is what I get:

<nodes>
    <node id="49ad1cb6-f6fe-47f9-a544-4a1c6337c4a5" name="Node 1">
        <aField>This is Node 1</aField>
    </node>
    <node id="0a1d1895-49e1-4079-abc1-749c304cc5a2" name="Node 2">
        <parent xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="node" id="49ad1cb6-f6fe-47f9-a544-4a1c6337c4a5" name="Node 1">
            <aField>This is Node 1</aField>
        </parent>
        <aField>This is Node 2</aField>
    </node>
</nodes>

And this is what I want:

<nodes>
    <node id="49ad1cb6-f6fe-47f9-a544-4a1c6337c4a5" name="Node 1">
        <aField>This is Node 1</aField>
    </node>
    <node id="0a1d1895-49e1-4079-abc1-749c304cc5a2" name="Node 2">
        <parent id="49ad1cb6-f6fe-47f9-a544-4a1c6337c4a5" name="Node 1"/>
        <aField>This is Node 2</aField>
    </node>
</nodes>

UPDATE: just a note here, schemagen actually does the correct thing. So it might be a bug in the JAXB RI.

Regards,
Morten

Answer 1

You do not require an XmlAdapter for your use case. You can solve the issue by marking the IdentifiableObject class with @XmlTransient:

Nodes

package forum8257098;

import java.util.List;

import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Nodes {

    @XmlElement(name="node")
    private List<Node> nodes;

}

Node

package forum8257098;

import javax.xml.bind.annotation.*;

@XmlRootElement
public class Node extends IdentifiableObject {

    @XmlElement
    private Node parent;

    @XmlElement
    private String aField;

}

IdentifiableObject

package forum8257098;

import javax.xml.bind.annotation.*;

@XmlTransient
public class IdentifiableObject {

    @XmlID
    @XmlAttribute
    private String id;

    @XmlAttribute
    private String name;

}

Demo

package forum8257098;

import java.io.File;

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 jc = JAXBContext.newInstance(Nodes.class);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/forum8257098/input.xml");
        Nodes nodes = (Nodes) unmarshaller.unmarshal(xml);

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

}

Input/Output

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<nodes>
    <node name="Node 1" id="49ad1cb6-f6fe-47f9-a544-4a1c6337c4a5">
        <aField>This is Node 1</aField>
    </node>
    <node name="Node 2" id="0a1d1895-49e1-4079-abc1-749c304cc5a2">
        <parent name="Node 1" id="49ad1cb6-f6fe-47f9-a544-4a1c6337c4a5"/>
        <aField>This is Node 2</aField>
    </node>
</nodes>

For More Information

• http://blog.bdoughan.com/2011/06/ignoring-inheritance-with-xmltransient.html