Model 2 Architecture - MVC

Another important buzzword in our initial statement was MVC. MVC stands for Model View Controller. MVC originated in SmallTalk and has since made its way into Java community. The Model 2 JSP architecture is actually MVC applied to web applications and hence the two terms can be used interchangeably in the web world. Figure 1.2 shows the Model2 (MVC) architecture.

Figure 1.2: Model 2 Architecture.

The main difference between Model 1 and Model 2 is that in Model 2, a controller handles the user request instead of another JSP. The controller is implemented as a Servlet. The following steps are executed when the user submits the request.

1. The Controller Servlet handles the user ‚ s request. (This means the hyperlink in the JSP should point to the controller servlet).

2. The Controller Servlet then instantiates appropriate JavaBeans based on the request parameters (and optionally also based on session attributes).

3. The JavaBeans talks to the middle tier or directly to the database to fetch the required data.

4. The Controller sets the resultant JavaBeans (either same or a new one) in one of the following contexts ‚ request, session or application.

5. The controller then dispatches the request to the next view based on the request URL.

6. The View uses the resultant JavaBeans from Step 4 to display data.

Note that there is no presentation logic in the JSP. The sole function of the JSP in Model 2 architecture is to display the data from the JavaBeans set in the request, session or application scopes.

Advantages of Model 2 Architecture

Since there is no presentation logic in JSP, there are no scriptlets. This means lesser nightmares. [Note that although Model 2 is directed towards elimination of scriptlets, it does not architecturally prevent you from adding scriptlets. This has led to widespread misuse of Model 2 architecture.]

With MVC you can have as many controller servlets in your web application. In fact you can have one Controller Servlet per module. However there are several advantages of having a single controller servlet for the entire web application. In a typical web application, there are several tasks that you want to do for every incoming request. For instance, you have to check if the user requesting an operation is authorized to do so. You also want to log the user ‚ s entry and exit from the web application for every request. You might like to centralize the logic for dispatching requests to other views. The list goes on. If you have several controller servlets, chances are that you have to duplicate the logic for all the above tasks in all those places. A single controller servlet for the web application lets you centralize all the tasks in a single place. Elegant code and easier to maintain.

Web applications based on Model 2 architecture are easier to maintain and extend since the views do not refer to each other and there is no presentation logic in the views. It also allows you to clearly define the roles and responsibilities in large projects thus allowing better coordination among team members .

Controller gone bad ‚ Fat Controller

If MVC is all that great, why do we need Struts after all? The answer lies in the difficulties associated in applying bare bone MVC to real world complexities. In medium to large applications, centralized control and processing logic in the servlet ‚ the greatest plus of MVC is also its weakness. Consider a mediocre application with 15 JSPs. Assume that each page has five hyperlinks (or five form submissions). The total number of user requests to be handled in the application is 75. Since we are using MVC framework, a centralized controller servlet handles every user request. For each type of incoming request there is ‚“ if ‚½ block in the doGet method of the controller Servlet to process the request and dispatch to the next view. For this mediocre application of ours, the controller Servlet has 75 if blocks. Even if you assume that each if block delegates the request handling to helper classes it is still no good. You can only imagine how bad it gets for a complex enterprise web application. So, we have a problem at hand. The Controller Servlet that started out as the greatest thing next to sliced bread has gone bad. It has put on a lot of weight to become a Fat Controller .