Jess (programming language)

Jess is a rule engine for the Java platform, written in Java and developed by Ernest Friedman-Hill of Sandia National Laboratories. It is a superset of the CLIPS language and was first written in late 1995. Instead of a procedural program whose loop runs only once, Jess follows a declarative style in which a set of rules is applied continuously to a set of facts through pattern matching, using the Rete algorithm to decide which rules fire. Rules can change the set of facts or run arbitrary Java code, which made Jess a common expert system shell and later a tool for intelligent agent systems that need a similar ability. Its licensing is unusual for the field: it is free for education and government use but proprietary software needing a license for commercial use, whereas CLIPS, the basis of Jess, is free and open source.