JFact is a Java port of the FaCT++ OWL DL reasoner. JFact's design matches FaCT++ design very closely, except for the low level details where Java offers different ways of doing things.
Therefore, if you are knowledgeable of FaCT++ internals you'll find that many of the JFact classes and methods look familiar.
However, JFact does not include some parts of FaCT++: the DIG interface and the command line support are absent.
JFact is a cross-platform utility capable of running on any operating system that comes with Java support (e.g. Mac OS X, Windows, Linux).