There have been some implementation efforts using the LMDB key-value store, which is a high-performance native key-value store with similar characteristics as BerkeleyDB, but remains free and open-source. It is a drop in replacement as it has all the necessary features: sorted values, transactions etc.

The project is here:

https://github.com/LMDB/lmdb

And Java wrappers are here:

https://github.com/lmdbjava/lmdbjava