I am announcing a new open-source project I am working on: BIOB
It is still very early days, but there is already a fully functional local file system implementation and an in-memory implementation for testing. See https://github.com/wimdeblauwe/biob for full details and a usage example on how to use the library in your own project.
It does not depend on Spring or Spring Boot, but I have used it in that context so far because that is what I use for all of my projects.
Feel free to try it out and contact me if you have questions, remarks or ideas for the project!