Do Domain Classes usually get JPA or JAXB Annotations or both?

Question

I have a Java enterprise application that provides a web service, has a domain layer, and a hibernate persistence layer. In this particular case, there is not a huge difference (at the moment) between the objects I send over the wire, the domain objects, and the persistence objects.

Currently the application uses DTO's on the persistence side and annotates the Domain classes with JAXB annotations. However, the more I read and think about it, the more this seems backwards! (Not to mention there is a lot of code to support the mindless back and forth between the DTO's and the Domain objects.) It seems like most architects suggest puting JPA annotations on the domain model and create DTO's for sending objects over the wire.

In my case, could I put both the JAXB and JPA (Hibernate) annotations on my domain classes?

The thought of keeping my web service facade, my domain, and my persistence all tightly bundled together seems easy to maintain, but does concern me as these may need to change in time. But would it be smarter to create a set of DTO classes for the web services side and skip the DTO's for the persistence side?

Answer 1

There's no functional reason for not annotating the same class with both JPA and JAXB annotations, I've done it myself on occasion. It does become a bit hard to read, though, and sometimes you want different class design trade-offs with JAXB and JPA. In my experience, these trade-offs usually mean you end up with two class models.

Answer 2

I agree that using the same model classes is the right approach. If you are concerned about annotation clutter, you could use a JAXB implementation (such as EclipseLink JAXB) that provides a mechanism for externalizing the metadata:

• http://wiki.eclipse.org/EclipseLink/Examples/MOXy/EclipseLink-OXM.XML

Also since you are using a JPA model EclipseLink JAXB (MOXy) has extensions for making this easier:

• http://bdoughan.blogspot.com/2010/07/jpa-entities-to-xml-bidirectional.html
• http://wiki.eclipse.org/EclipseLink/Examples/MOXy/JPA

Here is an example of using one model with JAXB & JPA to create a RESTful service:

• Part 1 - The database
• Part 2 - JPA entities
• Part 3 - Mapping entities to XML using JAXB
• Part 4 - The RESTful service
• Part 5 - The client

Answer 3

There is no problem in using both annotations on the same class. I even tend to encourage this, because thus you don't have to copy-paste when changes occur. In some cases, some properties differ in behaviour - for example an auto-generated ID might not be required to be marshalled. @XmlTransient and @Transient are then combined. It does become a bit hard to read, but not too hard, because it is obvious what all the annotations mean.

Answer 4

Anyone tempted to put atom link objects in your persisted domain because you have committed to defining your web service xml structure there? It seems strange to me to do this. Hateoas links seem like a good idea but the persisted domain and the service impl (not the web service) have no interest in atom links. Then again, using xml annotations and having jersey serialize my domain for me certainly is convenient. Another downside of this approach though is that it is to easy to impact your web service consumers at runtime with persistence domain "layer" refactoring.

Answer 5

I know this question is a bit old, but I thought I'd weigh in anyways since this is an issue that I've recently come across. My recommendation would be to leave your JAXB annotated classes alone, since any schema change will require re-generating these classes. Meaning you will have to re-enter any hibernate annotations, etc. manually. This may be a little out-dated solution, but I think it would be perfectly reasonable to create a hibernate mapping file (.hbm.xml) to house the mappings externally. This is a little more flexible, less cluttered, and just as useful in my opinion.