JAXB @XmlAdapter for arbitrary XML

Question

I have a org.w3c.dom.Element that I'm returning from my XmlAdapter for a custom @XmlElement and I'd like to include it as part of a JAXB object as arbitrary XML (I'm aware I'll have to hand-craft the XSD). However, JAXB complains with

org.w3c.dom.Element is an interface, and JAXB can't handle interfaces.

Apparently the w3c XML types are not supported as Java types, which is a shame. But further than this, I get the same error when I use javax.xml.transform.Result which is apparently supported.

How can I include arbitrary XML elements as elements in JAXB?

Note: as per https://forums.oracle.com/thread/1668210 I've also tried

MessageFactory factory = MessageFactory.newInstance();
message = factory.createMessage();          
SOAPElement element = message.getSOAPBody().addDocument(doc);

but that is also giving the same error.

@MattR I thought that's more for the XSD mapping: you still need to hand a supported object type to JAXB for it to convert it into XML. — fommil
I've used it to unmarshal XML elements I didn't specifically model, then marshal it back to XML, so I'm guessing you could also create your own List<Element> annotated with @XmlAnyElement and it would marshal it... — MattR
@MattR that's probably because your beans met the rules described in the docs linked above (public constructor, getters/setters, all primitive types supported by JAXB). — fommil
So no part of your bean is valid? Perhaps I'm misunderstanding your requirement, can you show some code? — MattR

bdoughan bdoughan · Accepted Answer · 2013-08-20T17:45:15

TL;DR

You can have an XmlAdapter that converts you domain object to an instance of org.w3c.dom.Element as long as you specify the value type as Object (not Element).

Below is a full example.

XmlAdapter

A field/property of type java.lang.Object will keep unknown content as DOM nodes. You can leverage this in your use case by specifying the value type in your XmlAdapter as Object. You will need to ensure that the root element returned from the marshal method matches the field/property as defined by the @XmlElement annotation.

import javax.xml.bind.annotation.adapters.XmlAdapter;
import javax.xml.parsers.*;
import org.w3c.dom.*;

public class BarAdapter extends XmlAdapter<Object, Bar>{

    private DocumentBuilder documentBuilder;

    public BarAdapter() {
        try {
            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
            documentBuilder = dbf.newDocumentBuilder();
        } catch(Exception e) {
            // TODO - Handle Exception
        }
    }

    @Override
    public Bar unmarshal(Object v) throws Exception {
        Bar bar = new Bar();
        Element element = (Element) v;
        bar.value = element.getTextContent();
        return bar;
    }

    @Override
    public Object marshal(Bar v) throws Exception {
        Document document = documentBuilder.newDocument();
        Element root = document.createElement("bar");
        root.setTextContent(v.value);
        return root;
    }

}

Java Model

Foo

The @XmlJavaTypeAdapter annotation is used to reference the XmlAdapter.

import javax.xml.bind.annotation.*;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

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

    @XmlJavaTypeAdapter(BarAdapter.class)
    private Bar bar;

}

Bar

import javax.xml.bind.annotation.*;

@XmlAccessorType(XmlAccessType.FIELD)
public class Bar {

    String value;

}

Demo Code

Demo

Since there is a cost to creating the DocumentBuilderFactory we can leverage JAXB's ability to handle stateful instances of XmlAdapter by setting an instance on the Marshaller.

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

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/forum18272059/input.xml");
        Foo foo = (Foo) unmarshaller.unmarshal(xml);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setAdapter(new BarAdapter());
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(foo, System.out);
    }

}

input.xml/Output

<?xml version="1.0" encoding="UTF-8"?>
<foo>
    <bar>Hello World</bar>
</foo>