"db4o is an excellent open source object database for Java & .NET platform by Carl Rosenberger's team. I highly recommend it for rapid prototyping and RAD. It transparently handles object storage and retrieval"
"In db4o you can create an complex objects with other objects as its member and db4o will save them all like a champ with a single set() (now store()) method"
Angsuman Chakraborty's blog post