#199 rule: Mutable servlet field

[Hamlet D'Arcy]

MSF: Mutable servlet field (MSF_MUTABLE_SERVLET_FIELD)

A web server generally only creates one instance of servlet or jsp class (i.e., treats the class as a Singleton), and will have multiple threads invoke methods on that instance to service multiple simultaneous requests. Thus, having a mutable instance field generally creates race conditions.