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.