method and apparatus for extending functionality of a first set of classes and methods in an application hosted by a computing arrangement . a mapping is established that maps original class names in the first set to corresponding substitute class names of classes in a second set . 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 , the original class names are replaced with corresponding substitute class names in the class file . classes referenced by the substitute class names are then instantiated in lieu of classes referenced by the original class names .