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JAXB marshalling fields whose type is a generic type parameter

Answer 1

+1 A:

Firstly: Your XmlAdapter is wrong. The generic types are the other way around.

Then you seem to have to put the @XmlJavaTypeAdapter on CustomRootElement.

Furthermore the JAXBContext needs to be told about all classes involved. Either create a jaxb.index or ObjectFactory and create the context by giving the package name to the newInstance method or list all the classes.

The complete code (slightly modified as I use a main() and not a JUnit test method):

public static class CustomType {
}

public static class CustomTypeAdapter extends
        XmlAdapter<String, CustomType> {

    @Override
    public String marshal(CustomType v) throws Exception {
        return "CustomType";
    }

    @Override
    public CustomType unmarshal(String v) throws Exception {
        return new CustomType();
    }

}

public static class RootElement<V> {
    public V value;
}

@XmlJavaTypeAdapter(CustomTypeAdapter.class)
@XmlRootElement(name = "root")
public static class CustomRootElement extends RootElement<CustomType> {
    public CustomRootElement() {
        value = new CustomType();
    }
}

public static void main(String[] args) throws Exception {
    JAXBContext context = JAXBContext.newInstance(CustomRootElement.class,
            CustomType.class, RootElement.class);
    StringWriter w = new StringWriter();
    CustomRootElement cre = new CustomRootElement();
    cre.value = new CustomType();

    Marshaller marshaller = context.createMarshaller();
    marshaller.setProperty("jaxb.formatted.output", Boolean.TRUE);
    marshaller.marshal(cre, w);

    System.err.println(w.toString());

    // just to see whether unmarshalling works too
    CustomRootElement c = (CustomRootElement) context.createUnmarshaller()
            .unmarshal(new StringReader(w.toString()));
    System.err.println(c.value);
}

Now the result of marshalling a CustomRootElement is not what you expect in your test (and neither is it what I expected) but you can unmarshall it and get what you marshalled previously. So (un)marshalling both work but the XML doesn't look as nice:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root>
    <value xsi:type="customType" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/&gt;
</root>

I also put a field in CustomType and it worked as well. So if you don't need the nice XML this solution should suffice. I hope I didn't forget any changes I made. If I did, just leave a comment and I'll edit accordingly.

musiKk 2010-06-29 11:07:01

You're right about the order of the XmlAdapter's types, I intended for them to be the other way around. This was a transcription error when I wrote up the fake example for my question. I've fixed this in my question.However none of the rest of your suggestions result in working code for me.The placement of the @XmlJavaTypeAdapter annotation is arbitrary; in my actual code, it is at the package level in package-info.class.I've tried adding the other classes involved to the JAXBContext.newInstance() call in case JAXB can't find them itself, but that also makes no difference to the problem.

DanC 2010-06-29 12:17:46

If you have a working version of the test in my question, can you please put it in a pastebin and link to it here, so we can see what you changed in order to get it working?

DanC 2010-06-29 12:18:37

I edited the post. Don't need to bring external dependencies like pastebin into this. I hope this works for you because I used JAXB as it is distributed with the JDK.

musiKk 2010-06-30 07:55:26

Thanks for updating your answer. Unfortunately you've left out the @XmlValue annotation on the field of RootElement. When I add that back in (since I need the field's value to become the content of the XML element) I see the original exception I pasted. I suppose this means either I am misusing @XmlValue or there is a bug with Sun's JAXB connected to @XmlValue somehow.

DanC 2010-07-04 08:52:56

Well, to be honest I never used @XmlValue and after reading the documentation I don't really see why you would need it here. If it really is necessary, I don't think I can be of further assistance, sorry.

musiKk 2010-07-04 09:09:39

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