I got a similar question with what was asked here and here but with a additional requirements like thread-safe, liberal license, maven support and require no configuration (support embedded mode for Java). Basically, the need is a disk-based implementation of Map/Set that can help to store several millions of records. There are many Sets thus they can not be held entirely in memory. Besides, these Sets will be used once and discarded. Berkeley looks good but I got issue with its license. I know a SQL DB like H2 can fulfill the need but just want to check if there is anything similar in no-sql world.
Note: I tried jdbm2 but decide to continue my search.