Answerer: a design pattern for dynamical conditional execution

We propose a new design pattern named Answerer whose purpose is to eliminate the effort of recompiling a program due to changes of run-time environments. Most programs have to behave differently to accommodate with various run-time environments. Usually the behaviors of such programs have to be specified in conditioned blocks. When the run-time environment of a program changes, these blocks need to be modified and recompiled. The Answerer pattern applies the reflection architectural pattern to take care of conditional execution. The reflection architectural pattern provides a mechanism for changing structure and behavior dynamically. Based on this idea, the Answerer pattern provides: (1) a model of conditional execution and specification rules, (2) a mechanism to structure a model dynamically from information acquired at run-time, (3) a mechanism to create and execute a conditional operation from a model created from (2). We developed the Answerer pattern with the following two steps. Firstly, we found that it is necessary for structural items of (1) to implement the following information in order to implement the Answerer design pattern based on the above model and mechanisms: (a) information about environmental factors that should be observed; (b) information about behavior that should be executed according to the factors. Secondly, we created the Answerer design pattern that implements (2) and (3). We explain how we applied and implemented the Answerer pattern with an example of Java applet of DVD authoring tool.