jsp - Multiple Select in Spring 3.0 MVC

Question

Ok so I've been trying to accomplish multiple selects in Spring MVC for a while and have had no luck.

Basically what I have is a Skill class:

public class Skill {
    private Long id;
    private String name;
    private String description;
    //Getters and Setters
}

And an Employee who has multiple Skills:

public class Employee {
    private Long id;
    private String firstname;
    private String lastname;
    private Set<Skill> skills;
    //Getters and Setters
}

All of these are mapped to Hibernate but that shouldn't be an issue.

Now I would like to be able to do in the JSP is to select Skills for an Employee from a <select multiple="true"> element.

I have tried this in the JSP with no luck:

<form:select multiple="true" path="skills">
    <form:options items="skillOptionList" itemValue="name" itemLabel="name"/>
<form:select>

Here is my Controller:

@Controller
@SessionAttributes
public class EmployeeController {
     @Autowired
     private EmployeeService service;

     @RequestMapping(value="/addEmployee", method = RequestMethod.POST)
     public String addSkill(@ModelAttribute("employee") Employee emp, BindingResult result, Map<String, Object> map) {

        employeeService.addEmployee(emp);

        return "redirect:/indexEmployee.html";
    }

    @RequestMapping("/indexEmployee")
    public String listEmployees(@RequestParam(required=false) Integer id, Map<String, Object> map) {

        Employee emp = (id == null ? new Employee() : employeeService.loadById(id));

        map.put("employee", emp);
        map.put("employeeList", employeeService.listEmployees());
        map.put("skillOptionList", skillService.listSkills());

        return "emp";
    }
}

But this does not seem to work. I get the following Exception:

SEVERE: Servlet.service() for servlet jsp threw exception
javax.servlet.jsp.JspException: Type [java.lang.String] is not valid for option items

I feel like it should be possible where we can have a form for a Model that has multiple select from a list of options provided. What is the best practice to have form:select and form:options in Spring 3.0 MVC?

Thanks!

Solution:

Ok so just in case anyone wonders what the solution is. In addition to user 01001111 fix:

<form:select multiple="true" path="skills">
    <form:options items="${skillOptionList}" itemValue="name" itemLabel="name"/>
<form:select>

We need to add a CustomCollectionEditor to the controller as follows:

@Controller
@SessionAttributes
public class EmployeeController {

    @Autowired
    private EmployeeeService employeeService;

    @Autowired
    private SkillService skillService;

    @InitBinder
    protected void initBinder(WebDataBinder binder) {
        binder.registerCustomEditor(Set.class, "skills", new CustomCollectionEditor(Set.class)
          {
            @Override
            protected Object convertElement(Object element)
            {
                Long id = null;

                if(element instanceof String && !((String)element).equals("")){
                    //From the JSP 'element' will be a String
                    try{
                        id = Long.parseLong((String) element);
                    }
                    catch (NumberFormatException e) {
                        System.out.println("Element was " + ((String) element));
                        e.printStackTrace();
                    }
                }
                else if(element instanceof Long) {
                    //From the database 'element' will be a Long
                    id = (Long) element;
                }

                return id != null ? employeeService.loadSkillById(id) : null;
            }
          });
    }
}

This allows Spring to add Sets of Skills between the JSP and Model.

Answer 1

You need to treat the items attribute as a variable, not just reference the variable name:

<form:select multiple="true" path="skills">
    <form:options items="${skillOptionList}" itemValue="name" itemLabel="name"/>
</form:select>

put ${skillOptionList} instead of skillOptionList