@Controller :

 @Controller annotation plays a crucial role in building web applications by helping manage HTTP requests and responses. It's a fundamental part of the Model-View-Controller (MVC) architectural pattern. Let's delve into its details:

Purpose: The @Controller annotation is used to mark a class as a controller component within a Spring-based web application. Controllers are responsible for handling incoming HTTP requests, processing the necessary logic, and determining the appropriate response to send back to the client.

Usage: To use @Controller, you annotate a class with it. Inside this class, you define methods that correspond to different URL paths or endpoints. These methods handle various HTTP operations such as GET, POST, PUT, DELETE, etc.


Example:

@Controller

@RequestMapping("/app") 
public class ProductController {
@GetMapping("/products") 
public String listProducts(Model model) { 
 List<Product> products = productService.getAllProducts();
 model.addAttribute("products", products);
return "product_list"; 
// Returns the name of a view template 
 } 
@GetMapping("/product/{id}") 
public String showProduct(@PathVariable Long id, Model model) { 
Product product = productService.getProductById(id); 
 model.addAttribute("product", product);
return "product_detail"; // Returns the name of a view template
    } 
// Other methods for handling form submissions, updating, deleting, etc.

}


In this example, the ProductController class is marked with @Controller. It has methods that handle requests for listing products and showing individual product details.

View Resolution: Controllers often work in tandem with view templates to generate HTML or other output to be sent back to the client. The return value of a controller method is often the name of a view template that should be used to render the response.

Model: Controllers interact with a Model object to pass data to the view templates. In the example above, the Model object is used to add the list of products or a specific product before rendering the view.

RequestMapping: The @RequestMapping annotation at the class level specifies a base URL path for all endpoints within the controller. In this case, all paths start with /app.

Advantages: @Controller separates concerns in web applications. It allows developers to focus on handling user interactions (requests and responses) without worrying about data retrieval or rendering the final view.

Integration: Controllers are typically used in conjunction with service classes that handle business logic and data access, as well as view templates (HTML, JSP, Thymeleaf, etc.) that generate the final response.


In summary, the @Controller annotation is a fundamental building block in the Spring MVC framework. It enables the creation of web applications by providing a structured way to handle HTTP requests, process data, and produce responses, ultimately following the Model-View-Controller design pattern.