I'm glad to announce that the world has got yet another (tiny!) open source project, Apache JaxMas. JaxMas is basically a very, very incomplete JAXR implementation.
Why another JAXR implementation? The world has already got plenty of them, for example Apache Scout. My job is mainly about CentraSite, my employers implementation (there's more to it than a simple JAXR implementation, of course). But they all share a common problem: It is basically impossible to use them for unit testing and non-trivial, to use them within integration tests.
The problem is that they all require a running server. And, what's more, it is difficult to start from a predefined set. That's where JaxMas comes into play. JaxMas is running on HSQLDB. The embedded mode of HSQLDB is obviously ideal for unit tests. JaxMas is based on the assumption of an initial schema, that is created by the unit test. The schema is basically a set of SQL commands, but a macro processor allows to create taxononmies and other registry objects relatively easily.
Btw, you might ask me why I didn't choose Apache Derby as the engine. Well, I did initially, but DERBY-338 was blocking me. This issue prevents that two foreign key references exist between the same tables with different ON DELETE clauses. However, I need that feature for deleting even complex registry objects with a simple DELETE statement.