Siena is a persitence library for Java inspired on the Google App Engine datastore.

Siena is

• intrusive. You make a class persistent by extending a certain class. Relationships are defined also in an intrusive way.
• simple. Simplicity is the most important thing.
• limited. Due to the simplicity there are several unsupported things such as JOINs.
• extensible. There are two interfaces you can implement on your own way.