Understanding MVC (Model-View-Controller)

MVC is a widely adopted design pattern in software development that promotes separation of concerns by dividing application responsibilities into three interconnected components. This ensures modularity, maintainability, and scalability in applications, particularly in web development.

MVC Components

1. Model:
   • Represents the data and business logic of the application.
   • Includes:
     • Java Beans/POJOs: Plain Old Java Objects that encapsulate data (e.g., User, Product entities).
     • Service Layer: Handles business rules and logic (e.g., UserService for processing user-related operations).
     • DAO Layer: Interacts with the database (e.g., UserDAO for CRUD operations using Hibernate).
   • Responsibility: Manages data persistence, retrieval, and business logic, independent of the UI or user interaction.
2. View:
   • Represents the user interface (UI) or presentation layer.
   • Technologies: JSP, Thymeleaf, Velocity, Angular, React, Vue, Next.js, etc.
   • Responsibility:
     • Displays data from the Model to the user.
     • Handles presentation logic (e.g., rendering HTML, formatting data).
     • Processes user input (e.g., form submissions) and sends it to the Controller.
   • Example: list.jsp, update_form.jsp, add_user.jsp in the web_mvc_hibernate_template project.
3. Controller:
   • Acts as an intermediary between the Model and View.
   • Typically implemented as a Servlet (e.g., in Spring MVC) or a Filter (e.g., in Struts 2).
   • Responsibility:
     • Handles user requests (e.g., HTTP GET/POST).
     • Manages navigation (routing) by directing requests to appropriate methods.
     • Interacts with the Model to process data and updates the View accordingly.
   • Example: FrontController servlet in the web_mvc_hibernate_template project.

Front Controller Pattern

• Definition: An additional design pattern used alongside MVC to provide a centralized entry point for handling all client requests.
• Role: Acts as a gatekeeper or dispatcher that intercepts every incoming request and routes it to the appropriate Controller logic.
• Implementation:
  • In the web_mvc_hibernate_template project, the FrontController servlet (annotated with @WebServlet("/*")) intercepts all requests.
  • It examines the request path (e.g., /, /update_form, /delete) and delegates to specific methods (e.g., listUsers, showUpdateForm).
• Benefits:
  • Centralizes request handling, reducing redundancy.
  • Simplifies navigation logic.
  • Ensures consistent preprocessing (e.g., authentication, logging) for all requests.

MVC Flow in web_mvc_hibernate_template

1. Client Request:
   • A user sends an HTTP request (e.g., http://localhost:8080/web_mvc_hibernate/ or http://localhost:8080/web_mvc_hibernate/update_form?id=3).
2. Front Controller:
   • The FrontController servlet intercepts the request (via /* URL pattern).
   • Based on the request path, it routes to methods like listUsers, showUpdateForm, deleteUser, etc.
3. Controller Logic:
   • The Controller interacts with the Model (e.g., UserDAO) to perform CRUD operations.
   • Example: For /, it calls userDAO.getAllUsers() to fetch user data.
4. Model:
   • The DAO layer uses Hibernate to interact with the database (e.g., retrieve users, update records).
   • Returns data to the Controller.
5. View:
   • The Controller sets data in the request scope (e.g., req.setAttribute("user_list", users)).
   • Forwards the request to a JSP (e.g., /WEB-INF/views/list.jsp) for rendering.
6. Response:
   • The JSP renders the HTML response (e.g., a table of users) using JSTL and EL.
   • For POST operations (e.g., /insert, /update), the Controller redirects to / to refresh the view.

Diagram Explanation