WebSphere Application Server - IBM Redbooks

7.1.3 Servlets in an enterprise application
An enterprise application contains many components. It is important to specify
the role of each of the components to understand how to use them in an
application. Earlier, in 2.3, “Determine the application-flow architecture” on
page 26, we discussed the architecture models for application design.
Depending on the architecture you choose, it is important to understand the roles
that servlets should play.
The role of servlets in a Model 1 architecture
You will generally see Model 1 architecture used in the more simple,
straightforward applications. The bulk of the application is consumed with
presenting data to the user. JSPs are well suited for this and simpler to use than
servlets, so it follows that you will see more JSP use in a Model 1 type application
than you will servlets. There may be situations when servlets are more
appropriate due to the complexity of a task but you will probably not make
extensive use of servlets in a Model 1 application.
Of course, you have to remember that eventually a JSP becomes a servlet at
execution time.
The role of servlets in a Model 2 architecture
If you remember back to the discussion of Model 2 architecture, the application
components were divided into three categories: Model, view, and controller.
Servlets in the controller layer
The role of a servlet is to handle a request from a client and to generate the
output. To do these tasks, a servlet will communicate with other components
such as EJBs for the business logic and JSPs for the presentation of the output.
Servlets are the link between the client request and the model. They must
interface with the components performing the business logic of the application.
Typically, a servlet will have to transform a client request in an object that is
understandable for the model. It also determines which components are required
to perform the request and call these components.
Once the business logic is performed, the servlet will have to interface with the
presentation layer, providing an object used to generate the output. So the
servlet also manages the link between the model and the presentation and may
decide to delegate the output to an HTML page.
220 IBM WebSphere Application Server - Express V5.0.2 Developer Handbook