Multiple Selects mit Spring MVC

@Entity
public class Task implements Serializable {
	@Id
	@GeneratedValue
	private long id;
 
	@ManyToMany
	private Set<Staff> staffs = new HashSet<Staff>(0);
 
	//...
}

@NamedQuery(name = "staff.activeStaff", query = "select s from Staff s where s.disabled = false")
@Entity
@DiscriminatorValue("STAFF")
public class Staff extends Person {
 
	//...
}

@NamedQuery(name = "person.findByUsername", query = "from Person p where p.login.username = :username")
@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.STRING)
public abstract class Person implements Serializable {
 
	@Id
	@GeneratedValue
	private long id;
 
	//...
}

@Controller
@SessionAttributes("project")
public class ProjectController {
	@RequestMapping(method = RequestMethod.GET, value = "/project/createTask.action")
	public void createTaskView(@ModelAttribute Task task, Model model) {
		List<Staff> activeStaff = timetrackingService.getActiveStaff();
		model.addAttribute("activeStaff", activeStaff);
	}
 
	//...

<td><label for="staffs">Chose Staff: </label></td>
<td><form:select path="staffs" multiple="true" items="${activeStaff}" itemLabel="fullName" itemValue="id"/></td>
<td><form:errors path="staffs" /></td>

org.springframework.web.bind.ServletRequestBindingException: Errors binding onto object 'task'; nested exception is org.springframework.validation.BindException: org.springframework.validation.BeanPropertyBindingResult: 1 errors
Field error in object 'task' on field 'staffs': rejected value [3]; codes [typeMismatch.task.staffs,typeMismatch.staffs,typeMismatch.java.util.Set,typeMismatch]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [task.staffs,staffs]; arguments []; default message [staffs]]; default message [Failed to convert property value of type [java.lang.String] to required type [java.util.Set] for property 'staffs'; nested exception is java.lang.IllegalArgumentException: Cannot convert value of type [java.lang.String] to required type [de.codecentric.timetracking.model.Staff] for property 'staffs[0]': no matching editors or conversion strategy found]

<select id="staffs" multiple="multiple" name="staffs">
<option value="">firstname lastname</option>
<option value="">aaaa aaaa</option>
<option value="">firstname lastName</option>
<option value="">firstname aaaa</option>
<option value="">firstname lastname</option>
</select>
<input type="hidden" value="1" name="_staffs"/>

public abstract class Person implements Serializable {
	//...
	public String getIdAsString() {
		return new Long(id).toString();
	}
}

<form:select path="staffs" itemValue="idAsString" multiple="true" items="${activeStaff}" itemLabel="fullName"/>

public class ProjectController {
 
private Map<String, Staff> staffCache;
 
@RequestMapping(method = RequestMethod.GET, value = "/project/createTask.action")
public void createTaskView(@ModelAttribute Task task, Model model) {
	List<Staff> activeStaff = timetrackingService.getActiveStaff();
	staffCache = new HashMap<String, Staff>();
	for (Staff staff : activeStaff) {
		staffCache.put(staff.getIdAsString(), staff);
	}
	model.addAttribute("activeStaff", activeStaff);
}
 
@InitBinder
protected void initBinder(WebDataBinder binder) throws Exception {
	binder.registerCustomEditor(Set.class, "staffs", new CustomCollectionEditor(Set.class) {
		protected Object convertElement(Object element) {
			if (element instanceof Staff) {
				System.out.println("Converting from Staff to Staff: " + element);
				return element;
			}
			if (element instanceof String) {
				Staff staff = staffCache.get(element);
				System.out.println("Looking up staff for id " + element + ": " + staff);
				return staff;
			}
			System.out.println("Don't know what to do with: " + element);
			return null;
		}
	});
}

Looking up staff for id 1: Staff(firstname lastname)
Looking up staff for id 1: Staff(firstname lastname)
Converting from Staff to Staff: Staff(firstname lastname)
Looking up staff for id 2: Staff(aaaa aaaa)
Looking up staff for id 2: Staff(aaaa aaaa)
Converting from Staff to Staff: Staff(aaaa aaaa)
Looking up staff for id 3: Staff(firstname lastName)
Looking up staff for id 3: Staff(firstname lastName)
Converting from Staff to Staff: Staff(firstname lastName)
Looking up staff for id 4: Staff(firstname aaaa)
Looking up staff for id 4: Staff(firstname aaaa)
Converting from Staff to Staff: Staff(firstname aaaa)
Looking up staff for id 5: Staff(firstname lastname)
Looking up staff for id 5: Staff(firstname lastname)
Converting from Staff to Staff: Staff(firstname lastname)

Looking up staff for id 3: Staff(firstname lastName)