Sleepycat Software has released the beta version of their Berkeley DB, Java Edition.
Berkeley DB JE is a high performance, transactional storage engine written entirely in Java. Like the highly successful Berkeley DB product, Berkeley DB JE executes in the address space of the application, without the overhead of client/server communication. It stores data in the application's native format, so no runtime data translation is required. Source
The source code is included with the (very small) download.