Axion is an Java embedded database – similar to HypersonicSQL or Mckoi – but possibly not as mature or as well known.

I was planning to use it for Classifier4J demos because of the license of Mckoi and
because I think the “log the SQL” architecture of HypersonicSQL is weird, but I've run into an interesting performance issue. Basically insert performance for persistent tables is atrocious. In some basic tests I averaged 40 inserts per
second over roughly 4500 rows (JDK 1.4.1, Athlon 2000+ 512M RAM). The most concerning thing was that as more rows were inserted performance got worse – when I insert 10000 rows I only get 30 rows per second.