Question

I currently have this environment on my project:

public abstract class Foo {

   private List<Thing> things;

   public List<Thing> getThings() { return this.things; }
}

public abstract class Bar extends Foo {


   @XmlElements({@XmlElement(name = "first", type = First.class)})
   public List<Thing> getThings() { return super.getThings(); }

}

public class Bobar extends Bar {

   @XmlElements({@XmlElement(name = "second", type = Second.class)})
   public List<Thing> getThings() { return super.getThings(); }

}

For the following XML document

<bobar>
   <first>blablabla</first>
   <second>blublublu</second>
</bobar>

When I do

context = JAXBContext.newInstance("the.package.structure");
unmarshaller = context.createUnmarshaller();
Bar object = (Bar) unmarshaller.unmarshal("path-to-xml-document");

The Bar object only has one element in the collection, not 2. The First element is completly lost, when I try to do object.getThings(), its size is 1 and the only object inside the collection is an instance of Second. Can someone help me how can I achieve to get both objects in the collection? And if that’s not possible, how can I achieve something similar to this?

The reason I’m doing this is that (in my project logic) every Bobars things collection has a First in its collection, but not every Bar has a Second in its collection, and Foo is a generic class.

Edit:

When I change the order in my XML document, the output is different.

<bobar>
   <second>blablabla</second>
   <first>blublublu</first>
</bobar>

In this scenario, I get only an instance of First in the collection, and Second is lost. And changing the scenario more, I get interesting results:

public abstract class Foo {

   private List<Thing> things;

   public List<Thing> getThings() { return this.things; }
}

public abstract class Bar extends Foo {


   @XmlElements({@XmlElement(name = "first", type = First.class), @XmlElement(name = "third, type = Third.class)})
   public List<Thing> getThings() { return super.getThings(); }

}

public class Bobar extends Bar {

   @XmlElements({@XmlElement(name = "second", type = Second.class)})
   public List<Thing> getThings() { return super.getThings(); }

}

If I do

<bobar>
   <third>bliblibli</third>
   <second>blablabla</second>
   <first>blublublu</first>
</bobar>

In theory, I think this shouldn’t be validated against the XML Schema generated by that, as the order here is not correct. But besides that, in such scenario, I get Second and First, the Third is lost.

Answer 1

It is not possible to annotate a property on a super type, and have a sub try incrementally add to that mapping. Below is a way you could support all the use cases that you are after. One thing to be cautious of is that all levels in the object hierarchy would support the same set of elements. You would need to use a external means of validation to restrict the desired values.

If Thing is a class not an interface, and First and Second extend Thing then you may be interested in using @XmlElementRef instead of @XmlElements (see: http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-substitution.html). It will offer you more flexibility, at the cost of some validation (hard to restrict the set of valid values).

Bar

We will annotate the Bar with @XmlTransient so that the JAXB implementation doesn’t process it.

package forum11698160;

import java.util.List;

import javax.xml.bind.annotation.XmlTransient;

@XmlTransient
public abstract class Bar extends Foo {

    public List<Thing> getThings() {
        return super.getThings();
    }

}

Bobar

@XmlElementRef corresponds to the concept of substitution groups in XML schema. The values matching the property will be based on @XmlRootElement declarations.

package forum11698160;

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

@XmlRootElement
public class Bobar extends Bar {

    @XmlElementRef
    public List<Thing> getThings() {
        return super.getThings();
    }

}

Thing

As JAXB implementations can not use reflection to find all the subclasses of a type, we can use the @XmlSeeAlso annotation to help out. If you don’t use this annotation then you will need to include all the subtypes when bootstrapping the JAXBContext.

package forum11698160;

import javax.xml.bind.annotation.XmlSeeAlso;

@XmlSeeAlso({First.class, Second.class})
public class Thing {

}

First

We will need to annotate First with @XmlRootElement:

package forum11698160;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class First extends Thing {

}

Second

Second will also need to be annotated with @XmlRootElement:

package forum11698160;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Second extends Thing {

}

Demo

package forum11698160;

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

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/forum11698160/input.xml");
        Bobar bobar =  (Bobar) unmarshaller.unmarshal(xml);

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

}

input.xml/Output

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<bobar>
    <first/>
    <second/>
</bobar>

---

OTHER FILES

Below are the other files you need to run this example:

Foo

package forum11698160;

import java.util.*;

public abstract class Foo {

    private List<Thing> things = new ArrayList<Thing>();

    public List<Thing> getThings() {
        return this.things;
    }

}