JAXB - Composite pattern and @XmlValue

Question

Following up JAXB and Composite Pattern , I managed to map:

<precondition>
    <or>
        <and>
            <just><query>foo</query></just>
            <just><query>bar</query></just>
        </and>
        <just><query>baz</query></just>
    </or>
</precondition>

But I'd like to map:

<precondition>
    <or>
        <and>
            <query>foo</query>
            <query>bar</query>
        </and>
        <query>baz</query>
    </or>
</precondition>

My JAXB class hierarchy is as follows:

@XmlRootElement
@XmlSeeAlso({SimplePreconditionQuery.class, CompoundAndPreconditionQuery.class, CompoundOrPreconditionQuery.class})
public abstract class PreconditionQuery {
    // JAXB does not deal with interfaces by default >:(
}

With several kinds of queries:

@XmlSeeAlso(PreconditionQuery.class)
@XmlRootElement(name = "just")
public class SimplePreconditionQuery extends PreconditionQuery {

    private String query;

    @XmlElement(name = "query")
    public String getQuery() {
        return query;
    }

    public void setQuery(String query) {
        this.query = query;
    }
}

The compound ones (AND/OR) are very similar:

@XmlSeeAlso(PreconditionQuery.class)
@XmlRootElement(name = "and")
public class CompoundAndPreconditionQuery extends PreconditionQuery {

    private Collection<PreconditionQuery> preconditionQueries = newArrayList();

    @XmlElementRef(name = "query")
    public Collection<PreconditionQuery> getPreconditionQueries() {
        return preconditionQueries;
    }

    public void setPreconditionQueries(Collection<PreconditionQuery> preconditionQueries) {
        this.preconditionQueries = preconditionQueries;
    }
}

And the enclosing bean:

public class Precondition {

    private PreconditionQuery query;

    @XmlElementRef(required = true)
    public PreconditionQuery getQuery() {
        return query;
    }

    public void setQuery(PreconditionQuery query) {
        this.query = query;
    }
}

JAXB won't allow me to just map a @XmlValue on SimplePreconditionQuery . Why and what's the alternative?

Answer 1

The Problem

If you simply do:

import javax.xml.bind.annotation.*;

@XmlRootElement(name = "query")
public class SimplePreconditionQuery extends PreconditionQuery {

    private String query;

    @XmlValue
    public String getQuery() {
        return query;
    }

    public void setQuery(String query) {
        this.query = query;
    }

}

You will get the following exception because SimplePreconditionQuery subclasses something other than java.lang.Object .

Exception in thread "main" com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 1 counts of IllegalAnnotationExceptions
@XmlValue is not allowed on a class that derives another class.
    this problem is related to the following location:
        at public java.lang.String forum26714143.SimplePreconditionQuery.getQuery()
        at forum26714143.SimplePreconditionQuery
        at @javax.xml.bind.annotation.XmlSeeAlso(value=[class forum26714143.SimplePreconditionQuery, class forum26714143.CompoundAndPreconditionQuery, class forum26714143.CompoundOrPreconditionQuery])
        at public forum26714143.PreconditionQuery forum26714143.Precondition.getQuery()
        at forum26714143.Precondition

---

Removing PreconditionQuery from the inheritance hierarchy.

We can use the @XmlTransient annotation at the class level to remove the class from JAXB. All the properties from the super class will be treated as properties of the super class.

import javax.xml.bind.annotation.*;

@XmlSeeAlso({SimplePreconditionQuery.class, CompoundAndPreconditionQuery.class, CompoundOrPreconditionQuery.class})
@XmlTransient
public abstract class PreconditionQuery {
    // JAXB does not deal with interfaces by default >:(
}

Now we get the following exception:

Exception in thread "main" com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 3 counts of IllegalAnnotationExceptions
Invalid @XmlElementRef : Type "class forum26714143.PreconditionQuery" or any of its subclasses are not known to this context.
    this problem is related to the following location:
        at public forum26714143.PreconditionQuery forum26714143.Precondition.getQuery()
        at forum26714143.Precondition

---

Making JAXB Aware of the Subclasses

Now that we have removed PreconditionQuery from the set of classes that JAXB cares about, we now longer benefit from the @XmlSeeAlso annotation that it contains. As such we need to make our @XmlElementRef annotations more explicit:

Precondition

import javax.xml.bind.annotation.*;

@XmlRootElement
public class Precondition {

    private PreconditionQuery query;

    @XmlElementRefs({
        @XmlElementRef(name="and", type = CompoundAndPreconditionQuery.class),
        @XmlElementRef(name="or", type= CompoundOrPreconditionQuery.class),
        @XmlElementRef(name="query", type=SimplePreconditionQuery.class)
    })
    public PreconditionQuery getQuery() {
        return query;
    }

    public void setQuery(PreconditionQuery query) {
        this.query = query;
    }

}

CompoundAndPreconditionQuery

You will also need to do this for CompoundAndPreconditionQuery and CompoundAndPreconditionQuery .

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

@XmlRootElement(name = "and")
public class CompoundAndPreconditionQuery extends PreconditionQuery {

    private Collection<PreconditionQuery> preconditionQueries = new ArrayList();

    @XmlElementRefs({
            @XmlElementRef(name="and", type = CompoundAndPreconditionQuery.class),
            @XmlElementRef(name="or", type= CompoundOrPreconditionQuery.class),
            @XmlElementRef(name="query", type=SimplePreconditionQuery.class)
    })
    public Collection<PreconditionQuery> getPreconditionQueries() {
        return preconditionQueries;
    }

    public void setPreconditionQueries(Collection<PreconditionQuery> preconditionQueries) {
        this.preconditionQueries = preconditionQueries;
    }

}

---

Handling Interfaces

// JAXB does not deal with interfaces by default >:(

Now that JAXB is no longer aware of the Precondition query class, you could make it an interface if you wanted to.

public interface PreconditionQuery {

}