Jess

Jess is a rule engine and scripting environment written entirely in Sun's JavaTM language by Ernest Friedman-Hill at Sandia National Laboratories in Livermore, CA. Jess was originally inspired by the CLIPS expert system shell, but has grown into a complete, distinct, dynamic environment of its own. Using Jess, you can build Java software that has the capacity to 'reason' using knowledge you supply in the form of declarative rules. Jess is small, light, and one of the fastest rule engines available.