Develop your projects using an open source object database engine.
db4o (database for objects) is the open source object database, native to Java. Free software, available under the GPL and under a commercial license. Persist any object with one line of code. Other features are object-oriented replication, Native Queries, automatic schema recognition, 350K footprint. Ideal for embedded use, e.g., in software running on mobile or medical devices, in packaged software, and for real-time systems.