Patterns: Implementing Self-Service in an SOA Environment

Chapter 6. Technology options 115

is usually represented to the View and the Controller with a set of JavaBean

components.

򐂰 The View is a visual representation of the model. Multiple views can exist

simultaneously for the same model, and each view is responsible for making

sure that it is presenting the most current data by either subscribing to state

change events or by making periodic queries to the model. With J2EE, the

view is generally implemented using JavaServer Pages (JSP).

򐂰 The Controller decouples the visual presentation from the underlying

business data and logic by handling user interactions and controlling access

to the model. It processes the incoming HTTP requests and invokes the

appropriate business or UI logic. Using J2EE, the controller is often

implemented as a servlet.

Given its wide spread acceptance, there are several design frameworks

supporting the implementation of the Model-View-Controller pattern. These

include the Java Server Faces (JSF) Framework, Apache Jakarta Project’s

Struts, and IBM Global Services’s EAD4J.

6.3.1 Java servlets

Servlets are Java-based software components that can respond to HTTP

requests with dynamically generated HTML. Servlets are more efficient than CGI

for Web request processing because they do not create a new process for each

request.

Servlets run within a Web container as defined by the J2EE model and, therefore

have access to the rich set of Java-based APIs and services. In this model, the

HTTP request is invoked by a client such as a Web browser using the servlet

URL. Parameters associated with the request are passed into the servlet through

the HttpServletRequest, which maintains the data in the form of name/value

pairs. Servlets maintain state across multiple requests by accessing the current

HttpSession object, which is unique per client and remains available throughout

the life of the client session.

Acting as an MVC Controller component, a servlet delegates the requested tasks

to beans that coordinate the execution of business logic. The results of the tasks

are then forwarded to a View component, such as a JSP, to produce formatted

output.

One of the attractions of using servlets is that the API is a very accessible one for

a Java programmer to master. The specification of the J2EE 1.4 platform

requires Servlet API 2.4 for support of packaging and installation of Web

applications.