JAXB does not marshall my List

Question

I have the two following classes. Is there any reason why JAXB doesn't marshall the answer list to XML? I have left out other properties that is marshalled correctly. And yes, the list of answers is populated, but the answers element is not shown.

public class Question {

    private List<Answer> answers;

    public List<Answer> getAnswers() {
        return answers;
    }

    public void setAnswers(final List<Answer> answers) {
        this.answers = answers;
    }
}

and the quiz object

@XmlRootElement
public class Quiz {

    private List<Question> questions;

    public Quiz() {
        questions = new ArrayList<Question>();
    }

    public List<Question> getQuestions() {
        return questions;
    }

    public void setQuestions(List<Question> questions) {
        this.questions = questions;
    }
}

Answer 1

You model works for me as is. See example below:

Demo

package forum13350129;

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

public class Demo {

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

        Question question1 = new Question();
        List<Answer> answers1 = new ArrayList();
        answers1.add(new Answer());
        answers1.add(new Answer());
        question1.setAnswers(answers1);

        Question question2 = new Question();
        List<Answer> answers2 = new ArrayList();
        answers2.add(new Answer());
        answers2.add(new Answer());
        question2.setAnswers(answers2);

        Quiz quiz = new Quiz();
        quiz.getQuestions().add(question1);
        quiz.getQuestions().add(question2);

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

}

Output

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<quiz>
    <questions>
        <answers/>
        <answers/>
    </questions>
    <questions>
        <answers/>
        <answers/>
    </questions>
</quiz>

---

UPDATE #1

What if you try to get it as JSON as I do?

This depends. JSON-binding is not part of the JAXB (JSR-222) specification. This means your environment is doing one of the following:

1. Using a JAXB impl with a library like Jettison to produce JSON (see: http://blog.bdoughan.com/2011/04/jaxb-and-json-via-jettison.html)
2. Using a JSON-binding library (such as Jackson that has some JAXB annotation support)
3. Using a JAXB impl such as EclipseLink MOXy that offers JSON binding (see: http://blog.bdoughan.com/2011/08/json-binding-with-eclipselink-moxy.html).

Using EclipseLink JAXB (MOXy) as follows:

    Marshaller marshaller = jc.createMarshaller();
    marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
    marshaller.setProperty(MarshallerProperties.MEDIA_TYPE, "application/json");
    marshaller.marshal(quiz, System.out);

Would produce the following JSON:

{
   "quiz" : {
      "questions" : [ {
         "answers" : [ {
         }, {
         } ]
      }, {
         "answers" : [ {
         }, {
         } ]
      } ]
   }
}

---

UPDATE #2

I have fixed the issue with lazy loaded relationships. I am using Hibernate.initialize in the repository. I am trying to return this quiz object from the resource that I have. I am using JAX-RS.

I have seen people have problems marshalling Hibernate models to XML. I believe the issue was due to proxy objects that Hibernate uses. In the link below I suggested using an XmlAdapter to convert between the proxy objects and the real ones and it was helpful.

• (moxy) jaxb marshaling and hibernate proxy objects