We use H2 Database (JVM only) at work for that. It solves the problem really well of a bunch of stuff that needs access to a single file DB, while still being able to embed the DB engine.
I think it's a pretty different use case to what the parent wanted
Exact same story for its HyperSQL and Derby friends: http://hsqldb.org/doc/2.0/guide/running-chapt.html#rgc_hsqld... https://db.apache.org/derby/#What+is+Apache+Derby%3F
Redis is considered an in memory store as well, even though it can persist it's data on disk too. And the capability isn't what makes it in-memory either, as you can store a SQLite db only in-memory too, and nobody would consider it to be an in-memory database. You could technically do the same for PostgreSQL, though that's not officially supported and needs a little effort on the users part.
I guess my phrasing was poor if you misunderstood me there, but I do stand by what I said: while H2 is a great project in Java-Land, it's very different to what I understood the parents goal to be.