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UBC Theses and Dissertations

Essential software structure through implicit context Walker, Robert James


Software reuse and evolution are problematic. Modules tend to express a great deal of knowledge about the external modules, large-scale structure, and behaviour of the systems in which they reside. As a result, the reusability of individual modules is decreased as they are too dependent on other modules. Likewise, the evolvability of systems is decreased as they are too dependent on the details of individual modules. However, we must have some dependences between our modules, or they would not be able to operate together. Not all dependences are equally bad, then. Each module can be seen to have a minimal core, an essential structure, beyond which it makes no sense to reduce. This essential structure describes the basic responsibilities of that module, and its expectations of the responsibilities of external modules. The thesis of this dissertation is that expressing the essential structure of our software modules, through the use of implicit context, makes those modules easier to reuse and the systems containing those modules easier to evolve. Implicit context revolves around the notion that we must abandon the need for all modules to be defined with respect to an absolute frame of reference. Instead, modules can be defined relative to a reference frame of convenience. In order for modules to work together, the inconsistencies in their independent world views must be reconciled when they attempt to communicate. Implicit context provides the novel mechanism of contextual dispatch to perform this reconciliation. When any communication passes into or out of a module, that communication may be rerouted, altered, replaced, or discarded altogether, as the situation dictates. This translation process sometimes requires access to information about the communication history of the system; objects previously passed can be retrieved, or the appropriate recipient of a message can be determined. The dissertation describes a prototype tool that supports the application of implicit context to Java source code. The thesis is validated by applying this tool in two case studies: comparing the evolution of an implicit context-based implementation of an FTP server to an object-oriented implementation; and reusing the Outline View of the Eclipse integrated development environment in a different application, even though the Outline View was not designed for such reuse.

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