Provided is a JIT compiler that changes the status of run-time determined software routines between being activated and deactivated in an application program. Conditional statements which are used at run-time to determine whether such a routine has been activated or deactivated are presented as discrete conditions that always evaluate to a known result by using an invariant flag in the compiled bytecode. The JIT compiler optimises an application program by excluding discrete condition checks. Consequently, if the branch of the discrete condition that will never execute includes a run-time determined software routine, then the routine will be excluded from the JIT compilation and will not form part of the JIT compiled native machine code. A run-time class or code replacement facility is used as a means of activating or deactivating a run-time determined software routine at run-time by changing the value of the invariant flag of said routine.