SpringMVC 3 Convert <form:checkboxes ..> return String[] to Entity Object

I have an Entity Campaign that has a OneToOne relationship with CampaignCities cities. In turn, CampaignCities contains a Set cities;

The campaign entity

    @Entity 
    @javax.persistence.Table(uniqueConstraints={@UniqueConstraint(columnNames={"name","company_id"}), @UniqueConstraint(columnNames={"id"})})
    public class Campaign implements Serializable {
        /**
         * 
         */
        private static final long serialVersionUID = 1L;

        private Long id;

        @NotEmpty
        @Size(min=1, max=100)
        private String name;

        private Date startDate;

        private Date endDate;

        @Valid
        private Deal deal;

        @Valid
        private Company company;

        @OneToOne
        private CampaignCities cities = new CampaignCities();

The CampaignCities entity

@Entity
public class CampaignCities {
    private long id;
    private Set<City> cities = new HashSet<City>();

    @Id
    @javax.persistence.GeneratedValue(strategy=GenerationType.AUTO)
    public long getId() {
        return id;
    }

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

    @OneToMany
    public Set<City> getCities() {
        return cities;
    }

    public void setCities(Set<City> cities) {
        this.cities = cities;
    }
}

The City entity:

@Entity
public class City implements Serializable {
    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    private String name;
    private Long id;

    @javax.persistence.Id
    @javax.persistence.GeneratedValue(strategy=GenerationType.AUTO)
    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

My NewCampaignController

@SessionAttributes(value={"campaign", "campaignCities"})
@Controller
public class NewCampaignController {
    //private static final Logger logger = LoggerFactory.getLogger(NewDealController.class);

    @Autowired
    private CampaignManager campaignManager;
    @Autowired
    private CityManager cityManager;
    @Autowired
    SimpleDateFormat dateFormat;
    @Autowired
    CustomDateEditor dateEditor;

    @RequestMapping(value = "campaign/new", method = RequestMethod.GET)
    public String showForm(Map<String, Object> model) {
        //List<Campaign> campaigns = campaignManager.getCampaigns();
        Campaign campaignForm = new Campaign();
        CampaignCities cities = new CampaignCities();
        cities.setCities(new HashSet<City>(cityManager.getCity()));
        //campaignForm.setCities(cities);
        model.put("campaignCities", cities);
        model.put("campaign", campaignForm);
        return "campaign/new";
    }

    @RequestMapping(value = "campaign/new", method = RequestMethod.POST)
    public String processForm(@Valid Campaign campaignForm, BindingResult result, Map<String,Object> model) {
        new CampaignValidator().validate(campaignForm, result);
        if (result.hasErrors()) {
            return "campaign/new";
        }
        this.campaignManager.saveCampaign(campaignForm);
        model.put("campaign", campaignForm);
        model.put("campaigns", this.campaignManager.getCampaigns());
        return "campaign/added";
    }

I have been able to get campaign to render in a form and I've rendered the list of cities successfully using:

<form:checkboxes items="${campaignCities.cities}" path="cities" itemLabel="name" itemValue="id" delimiter="<br/>" />

However when i submit the form, I get the following validation error.

Field error in object 'campaign' on field 'cities': rejected value [2,1]; codes [typeMismatch.campaign.cities,typeMismatch.cities,typeMismatch.com.groupdealclone.app.domain.CampaignCities,typeMismatch]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [campaign.cities,cities]; arguments []; default message [cities]]; default message [Failed to convert property value of type 'java.lang.String[]' to required 开发者_Python百科type 'com.groupdealclone.app.domain.CampaignCities' for property 'cities'; nested exception is java.lang.IllegalStateException: Cannot convert value of type [java.lang.String[]] to required type [com.groupdealclone.app.domain.CampaignCities] for property 'cities': no matching editors or conversion strategy found]

I've tried to figure out a way to handle this in SpringMVC 3 but I've been stuck for over a day with no success. I simply want a List or Set or Cities that where checked on the form to be submitted to the controller and added to the Campaign. How do I get around the conversion problem where I can convert the String[] returned to a List or Set of Cities.

The project I'm working on is a public GitHub project, you can download the source and set it up using Maven if you like the project is called Group-Deal-Clone

---

After what is almost 2 days, the answer was simpler than I expected. Thanks to this thread I was guided to the answer.

In my NewCampaignController I did:

@InitBinder
public void initBinder(WebDataBinder binder) {
    dateFormat.setLenient(false);
    binder.registerCustomEditor(Date.class, dateEditor);

    binder.registerCustomEditor(CampaignCities.class, "cities", new PropertyEditorSupport() {
        @Override
        public void setAsText(String text) {
            String [] ids = text.split(",");
            CampaignCities cities = null;
            for(String id:ids){
                if(cities == null)
                    cities = new CampaignCities();
                City city = cityManager.getCity(new Long(id));
                if(city != null)
                    cities.getCities().add(city);

            }
            if(cities != null){
                cities.setId(null);
                setValue(cities);
            }
        }
    });