using ResponseEntity on Spring WebFlux

UserController.java

@RestController
public class UserController {

    private UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/users/{userId}")
    public Mono<ResponseEntity<User>> get(@PathVariable Long userId) {
        Mono<User> userMono = userService.getById(userId);
        return userMono.map((user) -> {
            if (user.isAdult()) {
                return ResponseEntity.ok().header("X-User-Adult", "true").build();
            }
            return ResponseEntity.status(HttpStatus.FORBIDDEN).build();
        });
    }
}