I'd recommend Hibernate (or, more general, OR-mapping) like Matt, but there is also a RDBMS at the backend and I'm not so sure about what you mean by
...without using a relational DB?...
It also would be interesting to know more about the application, because OR-mapping is not always a good idea (development performance vs. runtime performance).
Edit: I shortly learned about terracotta and there is a good stackoverflow discussion here about replacing DBs with that tool. Still experimental, but worth reading.
