Introduction : What is servlet and servlet container

• Servlet: A Servlet is a Java class that extends the capabilities of a server by handling HTTP requests and generating responses.
• Servlet Container: Lifecycle of a Servlet is managed by Servlet Container
• Responsibilities of a Servlet Container:
• Manages Servlet Lifecycle (loading, initialization, execution, and destruction)
• Handles HTTP Requests & Responses
• Manages Multithreading (each request is handled in a separate thread)
• Provides Security (authentication, authorization, HTTPS support)
• Manages Session Handling (cookies, session tracking)
• Handles Deployment & Configuration (web.xml or annotations)
• Popular Servlet Containers: Tomcat, Jetty, GlassFish

How a Servlet Container Manages Requests

Request Handling Flow:

• Client sends a request (e.g., http://example.com/login).
• Servlet Container receives the request and checks the servlet mapping.
• Container creates a new thread to handle the request.
• Servlet’s service() method is called, which delegates the request to doGet() or doPost().
• Servlet generates a response, which is sent back to the client.

Multithreading in Servlet Containers

• Each request is handled in a separate thread.
• This improves performance and scalability.
• Servlets must be thread-safe (avoid shared mutable data).

Understand by code

Servlet:

@WebServlet("/hello") // URL Mapping
public class HelloServlet extends HttpServlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Logic - return response
    }
}
    

web.xml

<web-app xmlns="http://java.sun.com/xml/ns/javaee" version="3.0">
    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.example.HelloServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>
    

Problems of Servlets & Servlet Containers Solved by Spring Framework

1. Manual Object Creation & Dependency Management
Problem:
• Servlets require manual object creation and dependency management using new keyword.
• Managing dependencies in large applications leads to tight coupling and hard-to-maintain code.
Solution
• Spring’s Dependency Injection (DI) allows automatic injection of dependencies using @Autowired, making the application loosely coupled and easy to maintain.

---

2. Boilerplate Code in Servlets
Problem
• Servlets require a lot of boilerplate code (e.g., handling request parameters, writing responses, exception handling).
• Every servlet must extend HttpServlet and override doGet() or doPost().
Solution
• Spring MVC simplifies request handling with @RequestMapping and @RestController
• No need to override doGet() manually.
• Response is automatically handled.

---

3. Complex Request Handling & Mapping
Problem
• In Servlets, handling different HTTP methods (GET, POST, etc.) requires checking request types manually inside doGet() or doPost().
• URL mappings are defined in web.xml, making it difficult to manage in large applications.
Solution
• Spring MVC simplifies request mapping with @GetMapping, @PostMapping, etc.
• No need for web.xml or @WebServlet
z

---

4. Session & State Management Complexity
Problem
• Managing session attributes (HttpSession) is manual in servlets.
• Developers must explicitly store/retrieve session data.
Solution
• Spring provides built-in session management using @SessionAttributes.

@SessionAttributes("user")
public class UserController {
    @ModelAttribute("user")
    public User setUser() {
        return new User();
    }
}
                    

• Spring manages session automatically.No need for HttpSession boilerplate code.

---

5. Exception Handling
Problem
• In Servlets, handling exceptions requires try-catch blocks inside every method or web.xml error pages.
Solution
• Spring provides global exception handling using @ControllerAdvice.
• Centralized exception handling without modifying every servlet.

@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleException(Exception e) {
        return new ResponseEntity<>("Error: " + e.getMessage(), HttpStatus.INTERNAL_SERVER_ERROR);
    }
}
                        
                    

6. Hard-to-Test Servlets
7. No Built-in Security in Servlets

How Spring Uses Servlet Concepts

• Spring MVC (Model-View-Controller) is built on top of Servlets.
• The DispatcherServlet acts as a front controller, processing incoming requests.
• Spring Boot internally uses Tomcat (or Jetty) as an embedded Servlet Container.