A REST endpoint in Spring Boot is a method on a @RestController, mapped to an HTTP verb and path. @RestController = @Controller + @ResponseBody, so return values are serialized straight to the response body (JSON via Jackson) instead of being resolved to a view.
@RestController
@RequestMapping("/api/users") // base path for every method below
public class UserController {
private final UserService service;
public UserController(UserService service) { this.service = service; }
@GetMapping("/{id}") // GET /api/users/42
public UserDto get(@PathVariable Long id) { // bind {id} from the URL
return service.findById(id); // returned object -> JSON
}
@PostMapping // POST /api/users
@ResponseStatus(HttpStatus.CREATED) // 201 instead of 200
public UserDto create(@Valid @RequestBody UserDto body) { // JSON -> object
return service.create(body);
}
@GetMapping // GET /api/users?role=admin
public List<UserDto> list(@RequestParam(required = false) String role) {
return service.findByRole(role);
}
}
Key annotations: @PathVariable pulls a value out of the path, @RequestParam reads a query-string parameter, and @RequestBody deserializes the JSON payload. Adding @Valid triggers Bean Validation (@NotNull, @Email…) on the incoming DTO before your method runs.
@GetMapping("/{id}")
public ResponseEntity<UserDto> get(@PathVariable Long id) {
return service.find(id)
.map(ResponseEntity::ok) // 200 + body
.orElse(ResponseEntity.notFound().build()); // 404, no body
}
This is bread-and-butter backend work, so interviewers expect fluency. Strong answers mention using DTOs rather than exposing JPA entities, validating input with @Valid, and returning correct status codes with ResponseEntity — the difference between a toy endpoint and a real API.
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