Request → Response Lifecycle

Spring Boot applications follow a layered and modular architecture, which ensures that every part of your application has a clear responsibility. The Request → Response lifecycle shows how a client request flows through these layers until a response is returned.

Understanding this helps you debug issues faster, write better code, and design scalable applications.

This lifecycle can be divided into 8 main/core steps, which cover the essential flow from when a client sends a request to when the response is sent back:

Step 1: Client Sends HTTP Request

Everything starts when a client (browser, mobile app, or API consumer) sends an HTTP request to the Spring Boot application.

Example Request:

GET /users/123 HTTP/1.1
Host: localhost:8080

• The client does not need to know anything about your internal layers.
• This request triggers the Spring Boot application to handle it from start to finish.

Step 2: Embedded Server Receives Request

Spring Boot comes with embedded servers like Tomcat (default), Jetty, or Undertow.

Responsibilities of the Embedded Server:

• Listens on a port (default 8080).
• Receives incoming HTTP requests.
• Passes requests to Spring Boot's DispatcherServlet (the central controller).

➡ Why Embedded Servers Matter:

You don’t need to install or configure an external server manually. Your app is self-contained and production-ready.

Step 3: DispatcherServlet (Front Controller)

The DispatcherServlet is the heart of Spring MVC. It manages how requests are routed in the application.

Key Tasks:

1. Routing: Determines which Controller should handle the request using HandlerMapping.
2. Interceptors & Filters: Pre-processing, security checks, logging.
3. Response Handling: Sends the processed response back to the client.

Internal Flow:

DispatcherServlet → HandlerMapping → Controller → View/Response

Step 4: Controller Layer (Presentation Layer)

Controllers are annotated with @RestController or @Controller.

Responsibilities:

• Accept request parameters (@RequestParam, @PathVariable).
• Parse request body (@RequestBody).
• Handle validation (@Valid, @Validated).
• Manage exceptions (@ExceptionHandler, @ControllerAdvice).

Example:

@RestController
@RequestMapping("/users")
public class UserController {

    private final UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        User user = userService.getUserById(id);
        return ResponseEntity.ok(user);
    }
}

➡ Key Point: Controllers are only responsible for handling requests and sending responses, not business logic.

Step 5: Service Layer (Business Layer)

The Service Layer (@Service) contains all business logic.

Responsibilities:

• Processes data received from controllers.
• Applies application rules and validations.
• Manages transactions (@Transactional).
• Calls repository layer to fetch or save data.

Example:

@Service
public class UserService {

    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public User getUserById(Long id) {
        return userRepository.findById(id)
                 .orElseThrow(() -> new ResourceNotFoundException("User not found"));
    }
}

Step 6: Repository Layer (Data Access Layer)

Repositories (@Repository) interact with the database.

Responsibilities:

• CRUD operations
• JPA/Hibernate interactions
• Query execution
• Automatic transaction management

Example:

public interface UserRepository extends JpaRepository<User, Long> {}

Spring Boot auto-configures repositories using Spring Data JPA, so you don’t need boilerplate SQL or JDBC code.

Step 7: Database Layer (Persistence Layer)

The database layer is the actual storage system.

• Could be SQL (MySQL, PostgreSQL, Oracle), NoSQL (MongoDB, Redis), or in-memory DB (H2).
• Spring Boot automatically sets up:
  • DataSource
  • Connection Pool
  • JPA/Hibernate
  • Entity scanning

➡ Benefit: Minimal manual configuration, fully managed by Spring Boot.

Step 8: Response Handling

After the service and repository layers finish processing:

1. Service layer returns data to the Controller.
2. Controller sends a response using ResponseEntity or JSON object.
3. DispatcherServlet finalizes the response.
4. Response is sent back to the client.

Response Example:

{
  "id": 123,
  "name": "John Doe",
  "email": "[email protected]"
}

Extra / Optional Steps

Step 9: Filter Chain (Optional Pre-processing)

• If configured, the request passes through Servlet Filters before reaching the DispatcherServlet.
• Filters can handle cross-cutting concerns like:
  • Authentication / Authorization
  • Logging
  • Request modification
  • Rate-limiting

Step 10: Handler Interceptors (Optional)

• Interceptors execute before and after controller execution.
• Useful for:
  • Security checks
  • Logging
  • Modifying requests/responses before/after controller

Step 11: View Resolution (for MVC Applications)

• If the controller returns a view name (Thymeleaf, JSP, etc.), the DispatcherServlet resolves it using ViewResolver into a concrete view.
• Skipped for REST APIs.

Step 12: Message Conversion (for REST APIs)

• Controller return objects are converted to JSON, XML, or other formats using HttpMessageConverters.
• Ensures client receives data in the correct format.

Step 13: Response Generation

• DispatcherServlet packages the processed data or resolved view into an HttpServletResponse object.
• Optional interceptors or filters may modify the response before sending.

Step 14: Embedded Server Sends Response

• The embedded server sends the HTTP response back to the client.
• Includes HTTP status code, headers, and body (JSON/XML/View).

Step 15: Key Supporting Features

Spring Boot simplifies the lifecycle with:

• Auto-Configuration: Automatically sets up Web MVC, JPA, Security, Logging.
• Starters: Pre-packaged dependencies for common features.
• ApplicationContext: Manages beans, dependencies, and lifecycle events.
• Embedded Server: No external server required.
• Logging & Actuator: Automatic logging and monitoring endpoints for production readiness.

Lifecycle Flowchart

Client → Embedded Server → Filter Chain → DispatcherServlet → HandlerMapping → Handler Interceptors → Controller → Service → Repository → Database → View Resolution / Message Conversion → Response Generation → Embedded Server → Client

➡ This flow ensures modularity, scalability, and clean separation of concerns, making Spring Boot ideal for modern web applications and microservices.

Conclusion

The Spring Boot Request → Response lifecycle is well-structured and highly modular. Each component has a clear responsibility, which allows for:

• Rapid development
• Easier debugging
• Automatic configuration
• Scalable and maintainable applications