java embedded library on-disk key-value database

Since MapDB is a possible solution for your problem, Chronicle Map is also worth consideration. It's an embeddable Java key-value store, optionally persistent, offering a very similar programming model to MapDB: it also via the vanilla java.util.Map interface and transparent serialization of keys and values.

The major difference is that according to third-party benchmarks, Chronicle Map is times faster than MapDB.

Regarding stability, no bugs were reported about the Chronicle Map data storage for months now, while it is in active use in many projects.

Disclaimer: I'm the developer of Chronicle Map.

http://www.mapdb.org/

Also take a look at this question: Alternative to BerkeleyDB?