1️⃣ Servlet and Servlet Container

🔹 What is a Servlet?

• A Servlet is a Java class that handles HTTP requests and generates HTTP responses.
• It acts as a bridge between client and server.
• Used mainly for building web applications.

Key Responsibilities:

• Accept client request
• Process business logic
• Generate response (HTML / JSON / XML)

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🔹 What is a Servlet Container?

• A Servlet Container manages the lifecycle of servlets.
• Examples:
  • Tomcat
  • Jetty
  • WebLogic

Responsibilities:

• Create servlet instances
• Manage lifecycle (init, service, destroy)
• Handle threading
• Map URLs to servlets using web.xml or annotations

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🔹 Request–Response Flow

1. Client sends HTTP request
2. Request reaches Servlet Container (Tomcat)
3. Container checks web.xml
4. Identifies mapped servlet
5. Invokes servlet
6. Servlet processes and returns response

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🔹 Servlet Example

public class DemoServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response) {
        String path = request.getPathInfo();
        if ("/firstendpoint".equals(path)) {
            // logic
        }
    }
}

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🔹 web.xml Mapping

<servlet>
    <servlet-name>DemoServlet</servlet-name>
    <servlet-class>DemoServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>DemoServlet</servlet-name>
    <url-pattern>/demo/*</url-pattern>
</servlet-mapping>

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✅ Quick Summary

• Servlet handles request/response
• Servlet Container manages lifecycle
• web.xml maps URLs to servlets
• Tight coupling and manual configuration

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❓ Interview / Exam Questions

1. What is a Servlet Container?
2. Explain servlet lifecycle.
3. Role of web.xml?
4. Difference between doGet() and doPost()?

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2️⃣ Problems with Traditional Servlets

🔸 Major Challenges

• ❌ Too much XML configuration
• ❌ Tight coupling
• ❌ Difficult unit testing
• ❌ Hard REST API management
• ❌ No built-in integration support

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✅ Quick Summary

• Servlets are low-level
• Not scalable for large apps
• Spring solves these issues

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❓ Interview / Exam Questions

1. Why are Servlets not preferred today?
2. What makes Servlets tightly coupled?
3. Why is testing difficult in Servlets?

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3️⃣ Spring Framework Introduction

🔹 What is Spring Framework?

• A lightweight Java framework
• Solves problems of Servlets
• Based on Inversion of Control (IoC)

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🔹 Core Features

• Dependency Injection
• Loose coupling
• Annotation-based configuration
• Spring MVC
• Easy integration

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✅ Quick Summary

• Spring simplifies Java enterprise apps
• Promotes clean architecture
• Enables easy testing

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❓ Interview / Exam Questions

1. What is Spring Framework?
2. Advantages of Spring?
3. Why Spring is lightweight?

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4️⃣ Dependency Injection (IoC)

🔹 Without Dependency Injection

public class Payment {
    User sender = new User(); // tight coupling
}

Problems:

• ❌ Tight coupling
• ❌ Cannot mock
• ❌ Hard to extend

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🔹 With Dependency Injection

@Component
public class Payment {

    @Autowired
    User sender;
}

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🔹 Key Annotations

• @Component → Bean creation
• @Autowired → Inject dependency

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🔹 Internal Working

• Spring scans classes
• Creates beans
• Resolves dependencies
• Injects at runtime

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🔹 Best Practices

• Prefer constructor injection
• Avoid field injection in production
• Use interfaces

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✅ Quick Summary

• DI removes tight coupling
• Improves testability
• Managed by Spring IoC container

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❓ Interview / Exam Questions

1. What is Dependency Injection?
2. Difference between IoC and DI?
3. Types of DI in Spring?

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5️⃣ Spring MVC & DispatcherServlet

🔹 What is DispatcherServlet?

• Front Controller in Spring MVC
• Handles all incoming requests

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🔹 Request Flow

1. Request hits DispatcherServlet
2. HandlerMapping identifies controller
3. Controller method invoked
4. Response returned

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🔹 Controller Example

@Controller
@RequestMapping("/payment")
public class PaymentController {

    @GetMapping("/details")
    public String getDetails() {
        return "details";
    }
}

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✅ Quick Summary

• DispatcherServlet is central controller
• Clean separation of concerns
• Supports REST APIs

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❓ Interview / Exam Questions

1. What is DispatcherServlet?
2. Role of HandlerMapping?
3. Difference between Controller & RestController?

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6️⃣ Spring Boot Introduction

🔹 What is Spring Boot?

• Built on top of Spring Framework
• Used to create production-ready apps quickly

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🔹 Problems Solved by Spring Boot

• Dependency management
• Configuration complexity
• Server setup

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🔹 Key Features

1. Auto Configuration
2. Embedded Server
3. Starter Dependencies
4. Convention over Configuration

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🔹 Main Class

@SpringBootApplication
public class SpringbootApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringbootApplication.class, args);
    }
}

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🔹 Embedded Server

• No WAR file
• No external Tomcat
• Just run main() method

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🔹 REST API Example

@RestController
@RequestMapping("/myapi")
public class MyController {

    @GetMapping("/firstapi")
    public String getData() {
        return "Hello from concept and coding";
    }
}

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✅ Quick Summary

• Spring Boot accelerates development
• Reduces configuration
• Embedded server support

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❓ Interview / Exam Questions

1. Difference between Spring & Spring Boot?
2. What is Auto Configuration?
3. What is Embedded Server?

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📌 FINAL CONCLUSION

• Servlets are foundation but limited
• Spring introduces IoC and MVC
• Spring Boot makes Spring production-ready
• Ideal for microservices & REST APIs

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