Is it possible to programmatically configure JAXB?

Question

Say I have two JavaBeans Person and Address.

If I create a list of Person objects, I'd like to marshal to something like this:

<persons>
  <person>...</person>
</persons>

It's possible to use the technique described here: Using JAXB to unmarshal/marshal a List<String>

By annotating JaxbList with @XmlRootElement(name = "persons") and @XmlElement(name = "person"), then it's possible to marshal to the XML above.

But, it'd be nice to be able to reuse the same JaxbList<T> class to also marshal a list of Address objects. And in reality, I will have many other types of beans. I can go with something like:

<list>
   <item xsi:type="person" xmlns:xsi="http://www.w2.org/2001/XmlSchema-instance"></item>
</list>

But, ideally, it'd be nice to have it replace "list" with the plural version of class name and "item" with the class name.

So, is it possible to programmatically configure the JaxbContext or something during runtime and essentially set the value of the name inside @XmlRootElement and @XmlElement?

Or any other way to get this working without having to write a separate implementation of JaxbList for every bean type? Maybe XmlJavaTypeAdapter can achieve this sort of thing?

Update @Blaise Doughan's solution accepted below works great. For my use case, I needed to go straight from Java object to XML, here's what worked (note this is not my full implementation, it's sort of just pseudo code for demonstration):

    //JAXBContext is thread safe and so create it in constructor or 
    //setter or wherever:
    ... 
    JAXBContext jc = JAXBContext.newInstance(Wrapper.class, clazz);
    ... 

    public String marshal(List<T> things, Class clazz) {

      //configure JAXB and marshaller     
      Marshaller m = jc.createMarshaller();
      m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

      //Create wrapper based on generic list of objects
      Wrapper<T> wrapper = new Wrapper<T>(things);
      JAXBElement<Wrapper> wrapperJAXBElement = new JAXBElement<Wrapper>(new QName(clazz.getSimpleName().toLowerCase()+"s"), Wrapper.class, wrapper);

      StringWriter result = new StringWriter();
      //marshal!
      m.marshal(wrapperJAXBElement, result);

      return result.toString();

    }

Chemosynthesis

In your update since a JAXBContext is thread safe you will see better before if you are able to cache it instead of creating a new one for each marshal operation. – Plenary

Yes, good point, the real implementation I'm using it's cached, I abbreviated for the answer. But probably should have take the time to add that eh? I'll update it now. – Chemosynthesis

You probably want to change the toLowerCase() in that code snippet into your favorite implementation of decapitalize(). – Scarify

Answer 1

You could create a generic Wrapper object like the following:

Wrapper

You could create a generic wrapper class with a List property annotated with @XmlAnyElement(lax=true). The type of the object used to populate this list will be based on its root element (see: http://blog.bdoughan.com/2010/08/using-xmlanyelement-to-build-generic.html).

package forum13272288;

import java.util.*;
import javax.xml.bind.annotation.XmlAnyElement;

public class Wrapper<T> {

    private List<T> items = new ArrayList<T>();

    @XmlAnyElement(lax=true)
    public List<T> getItems() {
        return items;
    }

}

Address

You will need to annotate the possible contents of the list with @XmlRootElement.

package forum13272288;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Address {

}

Person

package forum13272288;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Person {

}

Demo

The demo code below demonstrates how to use the Wrapper class. Since the root element can be different you will need to specify that you want to unmarshal to the wrapper class. Alternatively you could leverage the @XmlElementDecl annotation to associate multiple root elements with the wrapper class (see: http://blog.bdoughan.com/2012/07/jaxb-and-root-elements.html).

package forum13272288;

import javax.xml.bind.*;
import javax.xml.transform.stream.StreamSource;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Wrapper.class, Person.class, Address.class);
        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        StreamSource personsXML = new StreamSource("src/forum13272288/persons.xml");
        JAXBElement<Wrapper> wrapper1 = unmarshaller.unmarshal(personsXML, Wrapper.class);
        marshaller.marshal(wrapper1, System.out);

        StreamSource addressesXML = new StreamSource("src/forum13272288/addresses.xml");
        JAXBElement<Wrapper> wrapper2 = unmarshaller.unmarshal(addressesXML, Wrapper.class);
        marshaller.marshal(wrapper2, System.out);
    }

}

Output

Below is the output from running the demo code. The files persons.xml and addresses.xml look just like there corresponding output.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<persons>
    <person/>
    <person/>
</persons>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<addresses>
    <address/>
    <address/>
</addresses>

For More Information

• http://blog.bdoughan.com/2012/11/creating-generic-list-wrapper-in-jaxb.html

Plenary

Awesome, thanks much, works great. I'll have to read up more about XmlAnyElement to wrap my brain around it. In my case, I need to start from marshal (and so instead of using unmarshal to create a JAXBElement<Wrapper>, I was able to manually create it). I'll update the question with how I did that in case it helps others. – Chemosynthesis

I have a very similar question here – Seven

yes another question is, how do i marshal JAXBElement into Response? – Seven