Jsp : Model 2 architecture – MVC

The Model 2 architecture for designing JSP pages is in reality, Model View Controller (MVC) applied to web applications. Hence the two terms can be used interchangeably in the web world. MVC originated in SmallTalk and has since made its way into Java community. Model 2 architecure and its derivatives are the cornerstones for all serious and industrial strength web applications designed
in the real world. Hence it is essential for you understand this paradigm thoroughly. Figure 1.2 shows the Model 2 (MVC) 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 Controller Servlet then by itself or through a controller helper communicates with 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.

JSP : Model 1 architecture

Model 1 architecture is the easiest way of developing JSP based web applications. It cannot get any easier. In Model 1, the browser directly accesses JSP pages. In other words, user requests are handled directly by the JSP.
Let us illustrate the operation of Model 1 architecture with an example. Consider a HTML page with a hyperlink to a JSP. When user clicks on the hyperlink, the JSP is directly invoked. This is shown in Figure below. The servlet container parses the JSP and executes the resulting Java servlet. The JSP contains embedded code and tags to access the Model JavaBeans. The Model JavaBeans contains attributes for holding the HTTP request parameters from the query string. In addition it contains logic to connect to the middle tier or directly to the database using JDBC to get the additional data needed to display the page. The JSP is then rendered as HTML using the data in the Model JavaBeans and other Helper classes and tags. 


Problems with Model 1 Architecture

Model 1 architecture is easy. There is some separation between content (Model JavaBeans) and presentation (JSP). This separation is good enough for  smaller applications. Larger applications have a lot of presentation logic. In Model 1 architecture, the presentation logic usually leads to a significant amount of Java code embedded in the JSP in the form of scriptlets. This is ugly and  maintenance nightmare even for experienced Java developers. In large applications, JSPs are developed and maintained by page authors. The intermingled scriptlets and markup results in unclear definition of roles and is very problematic.
Application control is decentralized in Model 1 architecture since the next page to be displayed is determined by the logic embedded in the current page. Decentralized navigation control can cause headaches. All this leads us to Model 2 architecture of designing JSP pages.