Need super fast persistence? Prevayler is the free-software Prevalence layer for Java!

Dump your RDBMS and reachitect your system if size of your data can fit into a memory


Performance

Queries with Prevayler are more than 9000 times faster than querying Oracle through JDBC.
Queries with Prevayler are more than 3000 times faster than querying MySQL through JDBC.

http://www.prevayler.org/wiki.jsp

For versioning xstream can be used to serialize as XML

http://xstream.codehaus.org/