java嵌入式库磁盘键值数据库

What I think I'm looking for is a no-SQL, library-embedded, on disk (ie not in-memory) database, thats accessible from java (and preferably runs inside my instance of the JVM). That's not really much of a database, and I'm tempted to roll-my-own. Basically I'm looking for the "should we keep this in memory or put it on disk" portion of a database. I'm looking for is a no-SQL, libra