H2 (h2database.com) is an open-source pure-Java SQL database. It’s fast,  can be encrypted, can be used in embedded mode, and has other nice features. The only thing I didn’t like when looking for an embedded database for my next Java project was that file extensions are fixed. It’s always “.h2.db” for the page file, “.lock.db” for lock file and so on. What I wanted was the distinct extension that my application would use.