Difference Between @Controller and @RestController in Spring Boot

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🚀 Introduction: Understanding `@Controller` vs `@RestController`

Spring Boot provides two main annotations for handling web requests:

@Controller (Used for traditional MVC applications with views).
@RestController (Used for REST APIs returning JSON/XML responses).

✅ Key Differences:

1️⃣ Understanding `@Controller` in Spring Boot

📌 Use @Controller when building traditional web applications that return HTML views.

1. Example: Using `@Controller` for an MVC Web Application

@Controller
public class HomeController {

    @GetMapping("/")
    public String home(Model model) {
        model.addAttribute("message", "Welcome to Spring Boot!");
        return "home"; // Returns home.html (Thymeleaf/JSP/Freemarker)
    }
}

📌 How It Works:

The method returns a view name (home) instead of JSON.
Spring Boot resolves it to a template (home.html).
The Model object carries data to the view.

2️⃣ Understanding `@RestController` in Spring Boot

📌 Use @RestController when building RESTful APIs that return JSON responses.

1. Example: Using `@RestController` for a REST API

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

    @GetMapping("/users")
    public List<String> getUsers() {
        return List.of("Ramesh", "Suresh", "Mahesh");
    }
}

📌 How It Works:

The method returns JSON directly (["Ramesh", "Suresh", "Mahesh"]).
No need for @ResponseBody—Spring Boot automatically converts the return object to JSON.

3️⃣ How `@RestController` Internally Uses `@ResponseBody`

📌 @RestController = @Controller + @ResponseBody

Example: Using `@Controller` with `@ResponseBody` (Equivalent to `@RestController`)

@Controller
@RequestMapping("/api")
public class UserController {

    @GetMapping("/users")
    @ResponseBody // Manually specifying response type
    public List<String> getUsers() {
        return List.of("Ramesh", "Suresh", "Mahesh");
    }
}

✅ In @RestController, @ResponseBody is applied automatically, so you don’t need to add it manually.

4️⃣ Returning HTTP Responses in `@RestController`

📌 Use ResponseEntity<> in @RestController to customize HTTP responses.

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

    @GetMapping("/user/{id}")
    public ResponseEntity<String> getUser(@PathVariable int id) {
        if (id == 1) {
            return ResponseEntity.ok("User: Ramesh");
        } else {
            return ResponseEntity.status(HttpStatus.NOT_FOUND)
                    .body("User not found");
        }
    }
}

📌 How It Works:
✅ Returns 200 OK when the user exists.
✅ Returns 404 Not Found when the user does not exist.

5️⃣ When to Use `@Controller` vs `@RestController`?

🎯 Summary: Best Practices for `@Controller` and `@RestController`

✅ Use @Controller for MVC applications that return views (Thymeleaf, JSP).
✅ Use @RestController for REST APIs that return JSON responses.
✅ Use @ResponseBody if you need JSON responses inside @Controller.
✅ Use ResponseEntity<> in @RestController to customize HTTP responses.
✅ Follow REST API best practices for building scalable microservices.

🚀 Following these best practices ensures a clean and maintainable Spring Boot application!

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