java ORM for a read only DB

You can check out JIRM (yes, that is a shameless plug) which is an ORM focused on using immutable objects.

It sits right on top of Spring JDBC so, if you're using Spring, it's very low risk to try it.

It also has really nice SQL placeholder templates that do not require Spring.

Another project you should checkout is jOOQ.

I think you should consider MyBatis (IBatis 3) ORM library. It fits the requirements from light-weight work to some heavy batch executions. So it'll fit your current needs very well and won't through you out of luck when you require some heavy-lifting work from your underlying ORM library.

If you are looking for a lightweight ORM framework, look no further than OrmLite. It allows you to avoid most of the overhead of setting up more complicated ORM frameworks like Hibernate. The downside to this framework is that it does not support standard JPA annotations so it would not be easy to swap this ORM out for a different one later on.

I would suggest trying out OpenJPA which is a JPA standard compliant ORM framework maintained by the Apache foundation. It is one of Hibernate major competitors. It does have a bit more overhead to setup but it has the advantage of allowing you to swap it out in the future for a different JPA compliant framework (like Hibernate) if it no longer suits your needs.

Both of these frameworks are Open Source as well which is always a good reason for using them.