I'm developing a closed source Android application. I was using db4o to persist data on the device and I found it very easy and straightforward to use. Unfortunately I didn't get informed about it's license which is GPL until now. As far as I understood it, by using db4o in my app I would have to make it open source, which I can't do. What would be the easiest, most painless replacement for db4o under a less restictive license (Apache)?