Controller Layer (Web & REST Controllers)

About

Spring Boot provides a set of annotations for defining controllers in web applications, handling HTTP requests, and configuring response behaviors. These annotations are used to build RESTful APIs or traditional MVC applications.

1. Core Controller Annotations

These annotations define the controller layer in a Spring Boot application.

@Controller

• Marks a class as a Spring MVC controller.

• Used in Spring MVC applications that return HTML views.

@Controller
public class MyController {
    @GetMapping("/home")
    public String home() {
        return "home";  // Returns a view name
    }
}

@RestController

• Specialized version of @Controller that combines @Controller and @ResponseBody.

• Used in RESTful APIs where the response is JSON or XML instead of HTML.

@RestController = @Controller + @ResponseBody

@RestController
public class MyRestController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello, World!";  // Returns raw data, not a view
    }
}

2. Request Mapping Annotations

These annotations map HTTP requests to controller methods.

@RequestMapping

• Maps an HTTP request to a method or class.

• Supports various attributes like value, method, produces, consumes.

• Class-level @RequestMapping prefixes all method mappings.

Example (Class-level and Method-level Mapping):

@RestController
@RequestMapping("/api")
public class MyApiController {
    
    @RequestMapping(value = "/users", method = RequestMethod.GET)
    public List<String> getUsers() {
        return List.of("Alice", "Bob");
    }
}

Shortcuts for @RequestMapping

Spring Boot provides specific annotations for each HTTP method:

Annotation	Purpose
@GetMapping	Handles HTTP GET requests
@PostMapping	Handles HTTP POST requests
@PutMapping	Handles HTTP PUT requests
@DeleteMapping	Handles HTTP DELETE requests
@PatchMapping	Handles HTTP PATCH requests

@GetMapping

• Maps HTTP GET requests.

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

@PostMapping

• Maps HTTP POST requests (used for creating resources).

@PostMapping("/users")
public String createUser(@RequestBody String user) {
    return "User created: " + user;
}

@RequestBody is used to extract JSON request payload.

@PutMapping

• Maps HTTP PUT requests (used for updating resources).

@PutMapping("/users/{id}")
public String updateUser(@PathVariable Long id, @RequestBody String user) {
    return "User " + id + " updated with: " + user;
}

@DeleteMapping

• Maps HTTP DELETE requests.

Example:

@DeleteMapping("/users/{id}")
public String deleteUser(@PathVariable Long id) {
    return "User " + id + " deleted";
}

@PatchMapping

• Maps HTTP PATCH requests (used for partial updates).

Example:

@PatchMapping("/users/{id}")
public String updateUserPartially(@PathVariable Long id, @RequestBody String user) {
    return "User " + id + " partially updated with: " + user;
}