UML Weekend Crash Course™ - To Parent Directory

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Sunday Afternoon
Controller: The code that reacts to user requests, modifies the data, and controls
the flow of the application
This basic structure is shown in Figure 28-3. MVC may be applied to almost any application
you develop, whether it is Web based or not. For a non-Web application, your application may
have a set of classes for your data model that coordinate database access and data manipulation,
another set of classes for your GUI views, and a third set of classes for your controlling
event-handling code. One of the most important advantages of MVC is that it allows you to
more easily change one aspect of the system without affecting the other aspects, exemplifying
loose coupling. For example, you may want to offer different interfaces for the traditional
wired Web devices and the wireless devices without changing or duplicating the data model. If
you have intermixed your database access with your GUI code, then changing the GUI without
changing the data model will be much more difficult. Because the programmer who is good at
writing data model code is likely not the same programmer who is good at writing GUI view
code, another advantage of MVC is that it simplifies the independent development of these
components.
Model
Controller
Data
Classes
Business
Services
Application
Flow Control /
Event Handlers
View
GUI
Classes
Figure 28-3 UML Component diagram, Model View Controller
In Web development, the view is the Web content such as HTML. The model is the business
logic for data manipulation and database access and is usually written in a programming
language like Java. The controller is the code for verifying the data coming in from
the HTTP request, interacting with the data model, and selecting the next view (Web page)
to be sent to the user. Just as in non-Web application development, separating these three
aspects will greatly simplify your maintenance and improve both cohesion and coupling. In
the remaining portion of this session, you separate the model from the view. In Session 29,
you separate out the controller.
JavaBeans
The JSP in Session 27 contained an unusually small amount of Java code because it had
extremely little dynamic content, but most JSPs will have to use a lot more Java code for
querying and updating databases, calculating data, and other operations. The result is that
you have a lot of Java code inside your Web content. With this usage, a JSP is really just a