Drools is a business rule management system (BRMS) and an enhanced Rules Engine implementation, ReteOO, based on Charles Forgy's Rete algorithm tailored for the Java language. More importantly, Drools provides for Declarative Programming and is flexible enough to match the semantics of your problem domain with Domain Specific Languages, graphical editing tools, web based tools and developer productivity tools.
M. Halvey, and M. Keane. WWW '07: Proceedings of the 16th international conference on World Wide Web, page 1313--1314. New York, NY, USA, ACM Press, (2007)