Drools - Java Rules Engine

Drools is a Rules Engine implementation based on Charles Forgy's Rete algorithm tailored for the Java language. Adapting Rete to an object-oriented interface allows for more natural expression of business rules with regards to business objects. Drools is written in Java, but able to run on Java and .Net.

Drools is designed to allow pluggeable language implementations. Currently rules can be written in Java, Python and Groovy. More importantly, Drools provides for Declarative Programming and is flexible enough to match the semantics of your problem domain with Domain Specific Languages (DSL) via XML using a Schema defined for your problem domain. DSLs consist of XML elements and attributes that represent the problem domain.