The present invention discloses a method, computer program product, and system for dynamically and programmatically modifying the semantics and/or logic of class files as they are being loaded for execution. The present invention permits a user to write a control file specifying in a programmatic manner the changes to be applied to class files and the conditions for carrying out the changes. As the class files are loaded, they are analyzed for the desired conditions and if the conditions are found, the control file is applied to them to effect the appropriate changes according to the user's control file.