How to validate data in service layer in spring boot

Controller:

@RestController
@RequestMapping(path = "api/v1/users")
public class UserController {
    @Autowired
    UserService userService;

    @PostMapping("/register")
    public ResponseEntity<Map<String, String>> registerUser(@RequestBody Map<String, Object> userMap){
        Map<String, String> map = new HashMap<>();

        try {
            String first_name = (String) userMap.get("first_name");
            String last_name = (String) userMap.get("last_name");
            Users user = userService.registerUser(first_name, last_name);

            map.put("message:","User registered successfully");
        }catch(Exception e) {
            map.put("message:",e.getMessage());
            return new ResponseEntity<>(map, HttpStatus.UNAUTHORIZED);
        }

        return new ResponseEntity<>(map, HttpStatus.OK);
    }

}

Service Layer:

@Service
@Transactional
public class UserServicesImpl implements UserService{
    @Autowired
    UserRepository userRepository;

    @Override
    public Users registerUser(String first_name, String last_name) throws EtAuthException {

        String username = first_name+99;
        Integer userId = userRepository.create(first_namea, last_name);
        return userRepository.findById(userId);
    }
}

Repository:

@Repository
public class UserRepositoryImpl implements UserRepository {

    private final UserRepositoryBasic userRepositoryBasic;

    public UserRepositoryImpl(UserRepositoryBasic userRepositoryBasic) {
        this.userRepositoryBasic = userRepositoryBasic;
    }


    @Override
    public Integer create(String first_name, String last_name) throws EtAuthException {
        try{
            
            Users insertData = userRepositoryBasic.save(new Users(first_name, last_name));
            return insertData.getId();
        }catch (Exception e){
            throw new EtAuthException(e.getMessage());
        }
    }
}

Model/Entity:

@Entity
@Table(name="users")
public class Users {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", updatable = false, nullable = false)
    private Integer id;

    @NotBlank(message="First name can not be empty")
    @Column(nullable = false, length = 40)
    private String first_name;

    @NotBlank(message="Last name can not be empty")
    @Column(nullable = false, length = 40)
    private String last_name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getFirst_name() {
        return first_name;
    }

    public void setFirst_name(String first_name) {
        this.first_name = first_name;
    }

    public String getLast_name() {
        return last_name;
    }

    public void setLast_name(String last_name) {
        this.last_name = last_name;
    }

    public Users(String first_name, String last_name) {
        this.first_name = first_name;
        this.last_name = last_name;
    }

    public Users() {
    }
}

The response I am getting is:

I want to do the validation at Service Layer but unable to do so. Validation works for my code implementation but the problem is that the validation message is shown with the package and class names which I do not want to show. I am trying to get only the validation error message if validation fail.

I tried adding @Valid annotation but unable to get the response I am looking for.

I am looking for a response below if validation fails.

"message": ["First name can not be empty","Last name can not be empty"]

Can anyone help me out with this problem. Thank you in advance.