Tuesday, September 25, 2012

Ruminating on Hibernate Entities and DTO

Today we had a lengthy debate on the hot topic of Hibernate entities vs DTO to be used across layers of a n-tiered application. Summarizing the key concepts that were clarified during our brainstorming :)

Q. Is is possible to use Hibernate entities in SOA style web services?

Ans. Yes. It is possible, but with a few important caveats and serious disadvantages.

If you are using Hibernate annotations to decorate your entities, then it would mandate that your service clients also have the necessary Hibernate/JPA jar files. This can be a big issue if you have non-Java clients (e.g. a .NET webservice client). If you use a mapping file (*.xml), then you are good to go as then there are no dependencies on Hibernate jars. But any change in your data model will affect your entities and this will result in changes to all your webservice consumers. So a lot of tight coupling :(

Also you have to understand that Hibernate has used Javaassist to create new dynamic proxies of your Hibernate entities. So the entities that you are referencing are actually that of the generated dynamic proxy - that contains a reference to the actual entity object. So U would need to deproxy the entity and then use it - either using dozer or a similar mapping library.

Person cleanPerson = mapper.map(person, Hibernate.getClass(person));

Note: In the above code, Hibernate.getClass() returns the actual entity class of the proxy object. 

If you are using lazy loading (which most applications do), then you might encounter the famous "LazyInitializationException". This occurs when you detact the Hibernate entity and the serialization process would trigger the lazy loading for a property. To understand this exception, you need to understand the concept of sessions and transactions as given here.
If you do not have lazy loading and do a eager load of all your entities, then it would work - but this is only a short term solution that may work only for small applications.


Q. Ok. I understand the pitfalls. So if I go with DTO, will my problems be solved?

Ans. Yes, DTO is the best option, but you may encounter a few issues again.

Many developers encounter the "LazyInitializationException" when they try to use libraries such as a Dozer to copy properties. This happens because Dozer uses reflection behind the scenes to copy properties and again attempts to access uninitialized lazy collections. There are 2 ways to resolve this problem. Use a custom field mapper as shown here or use the latest version of Dozer that has a HibernateProxyResolver class to get the real entity class behind the Javaassist proxy. This is explained in the proxy-handling section of Dozer site.