This class is used to customize the way the evaluator resolves variable references. For example, instances of this class can implement their own variable lookup mechanisms, or introduce the notion of "implicit variables" which override any other variables. An instance of this class should be passed to the evaluator's constructor.

Whenever the evaluator is invoked, it is passed a "context" Object from the application. For example, in a JSP environment, the "context" is a PageContext. That context object is eventually passed to this class, so that this class has a context in which to resolve variables.