MapDB
Проект заброшен, но рабочий.
DB db = DBMaker.fileDB("file.db").make()
https://www.baeldung.com/mapdb
https://github.com/jankotek/mapdb/
mapdb-manual-20.pdf
GSON
Save JSON to file: https://www.baeldung.com/gson-save-file
Примеры кода:
gson.toJson(123.45, new FileWriter(filePath)); User user = new User(1, "Tom Smith", "American"); gson.toJson(user, new FileWriter(filePath));