@RestController in Spring Boot — Annotation Explained

Direct Answer

@RestController combines @Controller and @ResponseBody. It registers an MVC controller whose handler return values use response-body processing rather than view-name resolution. A compatible HttpMessageConverter writes the response; JSON via Jackson is common, but the annotation also supports text, XML, bytes, resources, and other configured representations.

Code Example

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

    private final UserRepository userRepository;

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

    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return userRepository.findById(id)
            .orElseThrow(() -> new ResourceNotFoundException("User not found")); // Jackson serializes User to JSON
    }

    @PostMapping
    public User createUser(@RequestBody User user) { // @RequestBody deserializes JSON to User
        return userRepository.save(user); // Return value becomes JSON automatically
    }
}

Common Mistakes & Pitfalls

• Redundant @ResponseBody: Freshers often add @ResponseBody to individual methods inside a @RestController class. This is unnecessary because @RestController already applies @ResponseBody to all methods. It's like wearing two jackets—the annotation is already there.
• Mixing @Controller and @RestController: Using @Controller for REST endpoints and manually adding @ResponseBody everywhere. This creates inconsistency in the codebase. Stick to @RestController for REST APIs to make intent clear.
• Forgetting @RequestMapping: Placing @RestController without a base @RequestMapping path means every method needs a full path, leading to duplication. Always define the base path at class level for cleaner code.