The rule S2226 finds mutable instance fields of Servlets and generally complains about them.
Fields should be ignored by this rule, if they’re only written to in the init() method.
I think, it is common practice to parse servlet parameters and collect information depending on them from within the Servlet’s init() method. It would be a pain, if the parameters would have to be parsed every time again every time you use them. At least I see no other way but to use mutable fields, because I cannot initialize them in the constructor.