Using JAXB to pass subclass instances as superclass

Question

What I have is a set of Java classes (close to 25) representing message types. They all inherit from a Message class which I'd like to be abstract. Each message type adds a few additional fields to the set provided by the Message superclass.

I'm implementing some RESTful web services using RESTeasy and would like to have methods like this:

public Response persist(Message msg) {
    EntityTransaction tx = em.getTransaction();
    tx.begin();
    try {
        em.persist(msg);
    } catch (Exception e) {
        e.printStackTrace();
    }
    tx.commit();
    em.close();
    return Response.created(URI.create("/message/" + msg.getId())).build();
}

instead of having 25 separate persist methods, each tailored to a particular message type.

Currently, I've annotated my Message class like this:

@MappedSuperclass
@XmlRootElement(name = "message")
public abstract class Message implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    Integer id;
    @Embedded
    Header header;
    @Embedded
    SubHeader subHeader;

My subclass then looks like this:

@Entity
@XmlRootElement(name="regmessage")
@XmlAccessorType(XmlAccessType.FIELD)
public class REGMessage extends Message {

    @XmlElement(required = true)
    int statusUpdateRate;
    @XmlElement(required = true)
    int networkRegistrationFlag;

This creates a schema which looks like it should work, but all that's seen on the server side during a persist operation is a Message object (the subtype is completely lost, or at least it isn't marshalled back into its proper subtype). On the client side, to invoke the method I do this:

REGMessage msg = new REGMessage();
// populate its fields
Response r = client.createMessage(msg);

Is what I'm attempting possible? What JAXB magic do I need to use to make the translations happen the way they should -- ie, to treat everything in Java as if it's a Message to keep the number of methods down yet still preserve all the subtype-specific information?

---

Thanks to Blaise's blog pointers, this now looks like it's on the road to working fully. Here's what I've got, and it does work:

//JAXB annotations
@XmlRootElement(name="message")
@XmlAccessorType(XmlAccessType.FIELD)
@XmlSeeAlso(REGMessage.class)
//JPA annotations
@MappedSuperclass
public class Message {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @XmlAttribute
    private Integer id;

    private JICDHeader header;
    private int subheader;

    @XmlAnyElement
    @Transient
    private Object body;

One of the problems I encountered this morning was a cryptic error from Hibernate about the number of columns being mismatched. Once I realized that "body" was being mapped into the table, I marked it transient and voila!

@XmlRootElement(name="regmessage")
@XmlAccessorType(XmlAccessType.FIELD)
@Entity
public class REGMessage extends Message {

    private int field1;
    private int field2;

The only table generated from this code now is the regmessage table. On the RESTeasy side:

@Path("/messages")
public class MessageResource implements IMessageResource {

    private EntityManagerFactory emf;
    private EntityManager em;

    Logger logger = LoggerFactory.getLogger(MessageResource.class);

    public MessageResource() {
        try {
            emf = Persistence.createEntityManagerFactory("shepherd");
            em = emf.createEntityManager();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    @POST
    @Consumes("application/xml")
    public Response saveMessage(Message msg) {

        System.out.println(msg.toString());

        logger.info("starting saveMessage");
        EntityTransaction tx = em.getTransaction();
        tx.begin();

        try {
            em.persist(msg);
        } catch (Exception e) {
            e.printStackTrace();
        }

        tx.commit();
        em.close();
        logger.info("ending saveMessage");

        return Response.created(URI.create("/message/" + msg.getId())).build();
    }
}

This implements an interface:

@Path("/messages")
public interface IMessageResource {

    @GET
    @Produces("application/xml")
    @Path("{id}")
    public Message getMessage(@PathParam("id") int id);

    @POST
    @Consumes("application/xml")
    public Response saveMessage(Message msg) throws URISyntaxException;

}

Marshalling & unmarshalling work as expected, and persistence is to the subclass's table (and there is no superclass table at all).

I did see Blaise's note about JTA, which I may attempt to bring into this mix after I finish fleshing the Message & REGMessage classes back out fully.

Solution

Have you tried adding the following to your message class? The @XmlSeeAlso annotation will let the JAXBContext know about the subclasses.

import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlSeeAlso;

@XmlRootElement
@XmlSeeAlso(RegMessage.class)
public abstract class Message {

    Integer id;

}

Alternate Strategy:

Here is a link to a strategy I have helped people use:

• http://bdoughan.blogspot.com/2010/08/using-xmlanyelement-to-build-generic.html

Essentially you have one message object, and multiple individual message payloads. The relationship between the message and payload is handled through a @XmlAnyElement annotation.

Note on Transaction Handling

I noticed that you are handling your own transactions. Have you considered implementing your JAX-RS service as a session bean and leverage JTA for your transaction handling? For an example see:

• http://bdoughan.blogspot.com/2010/08/creating-restful-web-service-part-45.html

Comments

[Bret]

That looks perfect -- I'll give it a go. Thanks!

[Bret]

How important is it to have the java members in the superclass be defined as attributes? Mine are complex types and I'm going to try to leave them as such.

[Bret]

It appears to be rather important. Just converting this pretty simply and re-annotating my existing classes results in objects that, upon unmarshalling, aren't getting the body part. Those fields are all 0 or null and my type-specific fields are also getting lost.

[bdoughan]

The approach from my blog involves having a generic Message object and specialized payloads. The message object is always an instance of the same class, and the payload is a different object outside the Message hierarchy identified by the root element.

[Bret]

I also noticed that in your original example, the classes don't extend the Message class, where mine do. I'm now getting an error: "org.hibernate.MappingException: property mapping has wrong number of columns: com.techma.shepherd.messages.REGMessage.body type: object" which I presume comes from the fact that my REGMessage class has inherited the body attribute from Message. Digging back into my books again.

[bdoughan]

Did you see my update about @XmlSee also, it may help your original setup.

[Bret]

I did. What I'm planning to do now is start a new project and build this up from a basic class structure skeleton to get the XML working first. Then add the rest of the data members I need, and finally add the JPA annotations. Not that you'll be having sleepless nights, but I'll post back here with a final solution after I work it out. And yes, I did see the @XmlSeeAlso -- it had an effect but it didn't solve the whole problem. Time to clear the board & start over.

[user3157855]

One problem with @XmlSeeAlso is that the specified subclass has to be on the classpath during Java classes generation, a pretty huge problem if you want to split things into different projects/jars