JDBM is a transactional persistence engine for Java. It aims to be for Java
what GDBM is for other languages: a fast, simple persistence engine.
You can use it to store a mix of objects and BLOBs, and all updates are done
in a transactionally safe manner. JDBM also provides scalable data structures,
such as HTree and B+Tree, to support persistence of large object collections.
JDBM2 provides HashMap and TreeMap which are backed by disk storage.
It is very easy and fast way to persist your data.
JDBM2 also have minimal hardware requirements and is highly embeddable.

WWW: http://code.google.com/p/jdbm2/