What does people use for modeling taxonomies in Java?

I want something that supports generic DAGs (including not necessarily tree-shaped ones) and can be stored in a relational database. Google and SO give results from back when Psy was a person people cared about.

Nice to haves: merging of taxonomies (I want to provide templates people can mix and match, and combine with their personalized taxonomies), efficient membership decision, can be easily used in a web-application.