Bind object with data using thymleaf and select-object form

Im starting to learn java and spring. I have problem with binding data using thymeleaf. My entity looks that i have User witch contains ClassRoom (List of entity) in relationship many to one. I want to prepare site to register user. User should have capability to select classRoom. In first step I bind html site with data from database (classroom table), but I have problem with return selected files. My user entity looks:

package Start.Model;


@Entity
@Table
@Getter
@Setter
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    public User() {
    }

    public User(String name, String password) {
        this.name = name;
        this.password = password;
    }

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private List<ClassRoom> classRoom;

    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name = "user_activity", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "activity_id"))
    private List<Activity> activity;

The ClassRoom entity looks:

package Start.Model;

@Entity
@Table
@Data
public class ClassRoom {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id", referencedColumnName = "id")
    @JsonBackReference
    private User user;
}

Dtos looks:

package Start.DTOs;
@Data
public class UserDto {
    private String name;
    private String password;
    private AddressDto addressDto;
    private PhoneDto phoneDto;
    private List<ClassRoomDto> classRoomDtoList = new ArrayList<>();
    private List<ActivityDto> activityDtoList = new ArrayList<>();
}

package Start.DTOs;

import lombok.*;

@Data
public class ClassRoomDto {
    private String name;

}

Controller looks:

package Start.Controller;

@Controller
public class WebController {

    @GetMapping("/registry")
    public String main(Model model) {
        model.addAttribute("userDto", new UserDto());
        model.addAttribute("classRoomListInput", classRoomService.getAllClassRoom());
        model.addAttribute("activityListInput", activityService.getActivityList());
        return "index";
    }

    @PostMapping("/save")
    public String save(@ModelAttribute("userDto") UserDto userDto, Model model) {
        UserDto userDto1 = (UserDto) model.getAttribute("userDto");
        User user = userService.prepareUserDtoToSave(userDto1);
        userRepo.save(user);
        return "display_user";
    }
}

And registry page looks:

<div class="container my-3">
    <div class="row">
        <div class="col-md-8 mx-auto">
            <h2>Employee Registration</h2>
            <form th:action="@{/save}" th:object="${userDto}" method="post">
                <div class="form-row">
                    </div>
                    <div class="col-md-6 form-group">
                        <label for="name_label">Name</label>
                        <input type="text" class="form-control" th:field="*{name}" id="name_label">
                    </div>
                <div class="col-md-6 form-group">
                    <label for="password_label">Password</label>
                    <input type="text" class="form-control" th:field="*{password}" id="password_label">
                </div>

                <div class="col-md-6 form-group">
                    <label for="phone_label">Phone</label>
                    <input type="text" class="form-control" th:field="*{phoneDto.number}" id="phone_label">
                </div>
                <div class="col-md-6 form-group">
                    <label for="address_label">Address</label>
                    <input type="text" class="form-control" th:field="*{addressDto.street}" id="address_label">
                    <label for="house_number_label">House Number</label>
                    <input type="text" class="form-control" th:field="*{addressDto.houseNumberDto.number}" id="house_number_label">
                </div>

                <div class="col-md-6 form-group">
                    <label for="classRoom-label">Choose ClassRooms</label>
                    <select class="form-control select picker" id="classRoom-label" multiple="multiple">
                        <option th:each="classRoom, iterstat : ${classRoomListInput}"
                                th:value="${classRoom.name}"
                                th:filed="*{classRoomDtoList}"
                                th:text="${classRoom.name}">
                        </option>
                    </select>
                </div>
                <div class="col-md-6 form-group">
                    <label for="activity-label">Choose Activity</label>
                    <select class="form-control select picker" id="activity-label" multiple="multiple" th:filed="*{activityDtoList}">
                        <option type="checkbox" th:each="activity : ${activityListInput}"
                                th:value="${activity.getId()}"
                                th:text="${activity.getName()}">
                        </option>
                    </select>
                </div>
                <button type="submit" class="btn btn-primary btn-block mt-3">Save</button>
            </form>
        </div>
    </div>
</div>

As you can see register should return userDto with filed classRoomDtoList. But in ma case classRoomDtoList is always null or 0 (if i initialize empty ArrayList). I create my project step by step. When i use entity to send data to registration and save page it works, but when i change entity to DTO I stuck on it. Please help me and show what mistake i made. Whole "project" is on https://github.com/oizo64/0001

Thank you for any help.