MapDB
MapDB provides concurrent TreeMap and HashMap backed by disk storage or off-heap-memory. It is a fast, scalable and easy to use embedded Java database engine. It is packed with features such as transactions, space efficient serialization, instance cache and transparent compression/encryption. It also has outstanding performance rivaled only by native embedded db engines.
http://www.mapdb.org/
jdbm2
Embedded Key Value Java database.
https://code.google.com/p/jdbm2/