1. A computer-implemented method for extending functionality of a first set of classes and methods in an application hosted by a computing arrangement, comprising: establishing a mapping of original class names of classes in the first set to corresponding substitute class names of classes in a second set, wherein the classes in the second set change the functionality of the classes in the first set; in response to loading a class file of a class in the first set, replacing in the class file original class names with corresponding substitute class names; and instantiating classes referenced by the substitute class names in lieu of classes referenced by the original class names.