Model 1 and Model 2 (MVC) Architecture

There are two types of programming models:
1. Model 1 Architecture
2. Model 2 (MVC) Architecture

Model 1 Architecture

Servlet and JSP are the main technologies to develop the web applications.

Servlet was considered superior to CGI. Servlet technology doesn't create process, rather it creates thread to handle request. The advantage of creating thread over process is that it doesn't allocate separate memory area. Thus many subsequent requests can be easily handled by servlet.

Problem in Servlet technology Servlet needs to recompile if any designing code is modified. It doesn't provide separation of concern. Presentation and Business logic are mixed up.

JSP overcomes almost all the problems of Servlet. It provides better separation of concern, now presentation and business logic can be easily separated. You don't need to redeploy the application if JSP page is modified. JSP provides support to develop web application using JavaBean, custom tags and JSTL so that we can put the business logic separate from our JSP that will be easier to test and debug.

As you can see in the above figure, there is picture which show the flow of the model1 architecture.
1. Browser sends request for the JSP page
2. JSP accesses Java Bean and invokes business logic
3. Java Bean connects to the database and get/save data
4. Response is sent to the browser which is generated by JSP

Advantages

1. Easy and Quick to develop web application

Disadvantages

1. Navigation control is decentralized since every page contains the logic to determine the next page. If JSP page name is changed that is referred by other pages, we need to change it in all the pages that leads to the maintenance problem.
2. Time consuming You need to spend more time to develop custom tags in JSP. So that we don't need to use scriptlet tag.
3. Hard to extend It is better for small applications but not for large applications.

Model 2 (MVC) Architecture

Model 2 is based on MVC design pattern. The MVC design consists of three modules model, view and controller.
Model - The model represents the state(data) and business logic of the application.
View - The view module is responsible to display the data i.e. it represents the presentation.
Controller - The controller acts as an interface between view and model. It intercepts all the requests i.e. receives input and commands to Model / View to change accordingly.

Advantages

1. Navigation control is centralized. Now only controller contains the logic to determine the next page.
2. Easy to maintain .
3. Easy to extend.
4. Easy to test.
5. Better separation of concerns.

Disadvantages

1. We need to write the controller code self. If we change the controller code, we need to recompile the class and redeploy the application.

Solution of Model 2 Architecture: Configurable MVC Components

It uses the declarative approach for defining view components, request mapping etc. It resolves the problem of Model 2 architecture. The Struts framework provides the configurable MVC support. In struts 2, we define all the action classes and view components in struts.xml file.