Wednesday, February 20, 2008

MVC+dispatcher, in a purist design

See other posts on this blog about spring, struts ...

In a purist MVC design, elements below get less and less purist as we move down the list, so I usually spend more time on the first few layers to keep them well-behaving, well-defined and well-understood.

* model -- the model in ModelAndView is just a Map, allowing the use of multiple objects keyed by name. In general, "model" means domain model in the form of beans. These classes should not import or depend on controller or view classes. I believe a lot of business logic can go into the models, because it is the behavior of the domain models.

What if some low-level logic is shared between a model and a controller (or something below)? We risk tight coupling if a model imports a class, which in turn imports the controller. Solution: make that utility independent of models or controllers.

* view -- should have as little logic as possible. Ideally, views can change in response to controller, so the biz logic lies outside the view -- examples include hiding input fields.

View classes need to import model classes, but seldom controller classes. Since controllers control views, it's a sign of tight coupling if views start to import controller classes. You end up with a entangled architecture -- poor separation of concern.

* front dispatcher -- usually required. Struts and spring.

* sub-controllers -- for each operation or action, such as login, CRUD.

* helper classes but excluding well-defined beans -- the miscellaneous. Nothing purist about these. MVC pattern won't help you much here, but other patterns might.

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