It's Jena's own rules engine.
http://jena.apache.org/documentation/inference/index.html#rules
Question
I have designed some rules and am wondering what the reasoner is is returned by the Jena's GenericRuleReasonerFactory (as shown in the following code)? Is it Racer, Pellet, or HermiT?
Reasoner reasoner = GenericRuleReasonerFactory.theInstance().create(configuration);
Solution
It's Jena's own rules engine.
http://jena.apache.org/documentation/inference/index.html#rules