{
   "firstName": @s,
   "lastName": @s,
   "birthDay": @s
}

{
   "id": @d,
   "firstName": @s,
   "lastName": @s
}

class UserDataDto{
   private Long id;
   private String firstName;
   private String lastName;
   private LocalDate birthDay;
   private String email;
   private String phoneNumber;
}

UserDataDto userData = userRepo.findById(1);

{
   "firstName": "Иван",
   "lastName": "Иванов",
   "birthDay": "2000-01-01"
}

{
   "id": 1,
   "firstName": "Иван",
   "lastName": "Иванов"
}

public class App {
    private final String MY_FIELDS = "firstName,lastName,phone";

    public static void main(String[] args) {
        App app = new App();
        try {
            app.run();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

    private void run() throws NoSuchFieldException, IllegalAccessException {
        List<String> scopes = Arrays.stream(MY_FIELDS.split(",")).collect(Collectors.toList());
        Field[] classFields = UserDataDto.class.getDeclaredFields();

        UserDataDto oldUserDataDto = new UserDataDto("1", "Иван", "Иванов", "+123456789");
        UserDataDto newUserDataDto = new UserDataDto();

        for (Field f : classFields) {
            if (scopes.contains(f.getName())) {
                Field newField = newUserDataDto.getClass()
                        .getDeclaredField(f.getName());
                newField.setAccessible(true);

                newField.set(newUserDataDto, newField.get(oldUserDataDto));
            }
        }

        System.out.println(newUserDataDto);

        try {
            ObjectMapper mapper = new ObjectMapper();
            String json = mapper.writeValueAsString(newUserDataDto);
            System.out.println(json);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
    }

    @Setter
    @Getter
    @ToString
    @AllArgsConstructor
    @NoArgsConstructor
    @JsonInclude(JsonInclude.Include.NON_NULL)
    class UserDataDto {
        private String id;
        private String firstName;
        private String lastName;
        private String phone;

    }
}

public class App {
    private final String MY_JSON = "{\"firstName\":\"\",\"lastName\":\"\",\"phone\":\"\"}";

    public static void main(String[] args) {
        App app = new App();
        try {
            app.runService();
        } catch (JsonProcessingException | NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }


    private void runService() throws JsonProcessingException, NoSuchFieldException, IllegalAccessException {
        UserDataDto u = new UserDataDto("1", "fff", "lll", "ppp");
        Map<String, Object> retMap = new Gson().fromJson(
                MY_JSON, new TypeToken<HashMap<String, Object>>() {
                }.getType()
        );

        Field[] classFields = UserDataDto.class.getDeclaredFields();
        for (Field f : classFields) {
            Field newField = UserDataDto.class
                    .getDeclaredField(f.getName());
            newField.setAccessible(true);

            if (retMap.containsKey(f.getName())) {
                retMap.put(f.getName(), newField.get(u));
            }
        }
        retMap.values().removeIf(Objects::isNull);
        retMap.values().remove("");
        System.out.println(retMap);
    }

    @Setter
    @Getter
    @ToString
    @AllArgsConstructor
    @NoArgsConstructor
    @JsonInclude(JsonInclude.Include.NON_NULL)
    static class UserDataDto {
        private String id;
        private String firstName;
        private String lastName;
        private String phone;
    }
}