1. A method for handling messages within a data processing system, said method comprising: the data processing system generating a message types data structure for filtering messages communicated between a message source within the data processing system and a message destination within the data processing system, wherein the message types data structure indicates at least one allowable message type that can be communicated with one or more program modules within a group of program modules in the data processing system; the data processing system repetitively monitoring for a change in a status of a program module within the group of program modules between an active status in which the program module is being executed by the data processing system and an inactive status in which the program module is not being executed by the data processing system; in response to detecting the change in the status of the program module by the monitoring modifying the message types data structure, wherein the modifying includes adding an allowable message type if a program module changed status from inactive to active and removing an allowable message type if the program module changed status from active to inactive; and selectively communicating a message with one or more program modules among said group of program modules based on whether an item of identifying information within said message meets a filtering criterion specified within said allowable message types data structure.