JanusGraph supports Oracle Berkeley DB which is embedded into Java and works either with a filesystem (by default) or in-memory (if configured so).
That's might be something what you are looking for. For BDB to work all you need to do is set 2 required parameters:
storage.directory = /path/to/directory/where/you/want/to/store/data/
storage.backend = berkeleyje
Thus, all your data will be stored in files on your local filesystem (or distributed if used with something like nfs, glusterfs, etc.).
That said, berkeleyje has some limitations. I would recommend testing your usecase strongly before considering using it in production as it's quite easy to corrupt your files with BDB.
You may also want to use configuration `storage.berkeleyje.lock-mode: LockMode.READ_UNCOMMITTED` as I remember I had some transaction problems without it but don't remember those problems anymore.
Hope it helps.