How bind checkboxes with a Model object List

i have a jsp to register new users with yours respective roles. In the form, i receive a roleList in checkbox using c:forEach.

Everything is going well, BUT...

• If i try to save User with the first checkbox checked = OK, it is saved well
• If i try to save User with the first and second checkbox checked = Ok again, everything going well
• BUT if i try to save the user with the second or third role, jumping one element in the checkbox sequence, it throws a Exception like: Exception Save the transient instance before flushing...

So, what i am understanding here, is that the list that is passed in the bind to my List cannot have blank/null values. Even if i have 20 roles, i can save Users, with many roles, but if i jump anyone, it throws the exception.

How can i resolve that issue?? How can i pass the roles that i want without follow any order?

@Entity
public class User implements UserDetails {
@Id
private String login;
@NotBlank
private String password;
@NotBlank
private String name;
@ManyToMany(fetch=FetchType.EAGER)
private List<Role> roles = new ArrayList<>();


@Entity
public class Role implements GrantedAuthority {
@Id
private String name;


@Controller
@Transactional
@RequestMapping("/register")
@Scope(WebApplicationContext.SCOPE_REQUEST)
public class UserController {

@Autowired
UserDao userDao;
@Autowired
RoleDao roleDao;

@RequestMapping("userForm")
public ModelAndView userForm(User user) {
    ModelAndView modelAndView = new ModelAndView("user/userForm");
    modelAndView.addObject("roleList", roleDao.list());
    return modelAndView;
}

@RequestMapping (value="saveUser", method=RequestMethod.POST, name="saveUser")
public ModelAndView saveUser(@Valid User user, BindingResult bindingResult, RedirectAttributes redirectAttributes) {

    if (bindingResult.hasErrors()){
        return userForm(user);
    }
    userDao.save(user);

    redirectAttributes.addFlashAttribute("success", "User successfully registered");
    return new ModelAndView("redirect:/register/userForm");
}
}


<form:form action="${spring:mvcUrl('saveUser').build()}" method="post" commandName="user">

    <div>
        <label for="name">User name</label>
        <form:input path="name"/>
        <form:errors path="name"/>
    </div>

    <div>
        <label for="login">Login</label>
        <form:input path="login"/>
        <form:errors path="login"/>
    </div>

    <div>
        <label for="password">Password</label>
        <form:input path="password"/>
        <form:errors path="password"/>
    </div>

    <div>
        <c:forEach items="${roleList}" var="role" varStatus="status">
            <div>
                <label for="role_${role}">${role}</label>
                <input type="checkbox" value="${role}" name= "roles[${status.index}]" id="role_${role}"/>
            </div>
        </c:forEach>
    </div>

    <div>
        <input type="submit" value="Save">
    </div>
</form:form>

Thanks in advance