The exposed model was implemented using exPOJOs JPOX/JDO wrapper (a Hibernate wrapper is also available) as their transparent persistence solution. JPOX was chosen because it provides virtually 100% transparent persistence at the domain model level and has a significant performance edge due to its ability to perform build time byte code enhancement instead of relying on much slower run time reflection to do its job. The domain model objects are all pure POJOs without any need for annotations or the need to add extra methods to enable the underlying ORM to function correctly.
All domain models are designed and maintained using the light weight, high productivity, Javelin Modeler/Coder which automatically generates all necessary JPOX meta data files based on the information available in Javelin's UML style class diagrams (Hibernate meta data generation is also supported in Javelin)
If you want to try out persisting your POJOs without the massive learning curve or XML configuration nightmare required by Spring et al then you should give exPOJO a try.