Learn about how to use db4o with JPOX (a free and fully compliant implementation of the JDO1, JDO2 specifications). Queries can be expresed using either JDOQL, SQL, or JPQL. Marcel Wirth is actively working on JDOQL support for db4o.
http://www.jpox.org/docs/1_2/developer/db4o.html
db4o thanks the JPOX team and Marcel Wirth for their continuous support!
You can find more information about JPOX here: http://www.jpox.org/