Question

When unmarshalling using jaxb, I have in class A something along:

public class A {
    @XmlElements( { //
    @XmlElement(name = "g", type = A.class),
        @XmlElement(name = "x", type = X.class), 
        @XmlElement(name = "y", type = Y.class),        
    })
    List<XXX> children;
}

That is, I have a list, children, consisting of X:s and Y:s

Now for my question: I would like to subclass A, and I would like to
redefine the ‘XmlElements’ list and bind it to the same variable, ‘children’,
like:

public class B extends A {
    @XmlElements( { //
    @XmlElement(name = "g", type = B.class),
        @XmlElement(name = "x", type = X.class), 
        @XmlElement(name = "y", type = Y.class), 
        @XmlElement(name = "z", type = Z.class),        
    })
    List<XXX> children;
}

The issues with the above are twofold:

1. I create a new variable children, I would like to refer to the variable in class A.

2. I would like to avoid respecifying the ‘x’ and ‘y’ since they are already specified in ‘A’.

Is there some good pattern to achieve this?

Or some pointers/articles or other info for how to build something like this?

Answer 1

There’s no avoiding the re-declaration of the annotation, but you can move the annotation from the field on to the getter method, as long as you use the @XmlAccessorType annotation to tell JAXB to look for public getter methods rather than fields.

So you could override getChildren() in class B wit the new annotation set:

@XmlAccessorType(PROPERTY)
public class A {
    private List<XXX> children;

    @XmlElements( { //
    @XmlElement(name = "g", type = A.class),
        @XmlElement(name = "x", type = X.class), 
        @XmlElement(name = "y", type = Y.class),        
    })
    public List<XXX> getChildren() {
       return children;
    }

    public void setChildren(List<XXX> children) {
       this.children = children;
    }
}

@XmlAccessorType(PROPERTY)
public class B extends A {
    @XmlElements( { //
    @XmlElement(name = "g", type = B.class),
        @XmlElement(name = "x", type = X.class), 
        @XmlElement(name = "y", type = Y.class), 
        @XmlElement(name = "z", type = Z.class),        
    })
    public List<XXX> getChildren() {
       return super.getChildren();
    }
}

What I’m not sure about is how JAXB will handle the overriden getChildren() method. Hopefully it’ll take the annotations from B, but it’s possible it’ll get confused.

Try it and see.