SpringMVC Checkbox 对List 数据绑定

http://stackoverflow.com/questions/8700339/spring-mvc-usage-of-formcheckbox-to-bind-data

I know there have been already questions around this topic, but I have not figured out how to solve the following issue:

I have a user/roles relationship and I want to list all available roles in a JSP as checkbox items, where the users assigned checkboxes are selected. However, the matching items are not checked (here I am using Spring 3.1).

Extract from the User object:

private Set<RoleEntity> roles = new HashSet<RoleEntity>();

Extract from the Spring Controller (adding user object and role list to Model):

UserEntity userEntity = userEntityService.findById(UserEntity.class, new Long(id));model.addAttribute("userAttribute", userEntity);List<RoleEntity> roleList = roleEntityService.findAll();model.addAttribute("roleList", roleList);

Extract from the JSP:

<form:form modelAttribute="userAttribute" method="POST" action="${saveUrl}">...    <table align="center">
        <tr>
            <td>ID</td>
            <td>Role Name</td>
        </tr>
        <c:forEach items="${roleList}" var="role" varStatus="status">
            <tr>
                <td><form:checkbox path="roles" value="${role}" label="${role.id}" /></td>
                <td><c:out value="${role.name}" /></td>
            </tr>
        </c:forEach>
    </table>...</form:form>

The Spring MVC documentation says this: When the bound value is of type array or java.util.Collection, the input(checkbox) is marked as 'checked' if the configured setValue(Object) value is present in the bound Collection.

Isn't that the case here? What am I missing here?

My guess is you are missing the implementation for the equals and hashcode methods on the RoleEntity class.

When the bound value is of type array or java.util.Collection, the input(checkbox) is marked as 'checked' if the configured setValue(Object) value is present in the bound Collection.

This is correct, but to check for presence in a HashSet you need equals and hashcodeimplemented correctly.

Just as a quick test to see if that's the problem, replace this line:

model.addAttribute("roleList", roleList);

with this line:

model.addAttribute("roleList", userEntity.getRoles());

Do you get all your checkboxes checked? If yes, then you didn't provide your own equals and hashcode and the default ones are used (those inherited from Object).

The default equals compares identity which means that a variable holds the same instance as another variable. Equality means two different object contain the same state or have the same meaning, so to speak.

Using model.addAttribute("roleList", userEntity.getRoles()) triggers the default equalsmethod to return true because the list and values you check for presence in the list are identical (two identical objects are always equal).

But in your case you use userEntityService.findById for one and roleEntityService.findAll for the other which means different objects. At this point you have to use a proper equality test as opposed to identity.

Do you have equals/hashcode implemented?

Based on your code, here is an example that works:

Controller:

import java.util.Arrays;import java.util.Collections;import java.util.HashSet;import java.util.List;import org.springframework.stereotype.Controller;import org.springframework.ui.Model;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.RequestMethod;@Controllerpublic class SomeController {
    @RequestMapping(value = "/something", method = { RequestMethod.GET, RequestMethod.POST })
    public String handle(Model model) {

        UserEntity userEntity = new UserEntity();
        userEntity.setRoles(new HashSet<RoleEntity>());
        Collections.addAll(userEntity.getRoles(), 
                                new RoleEntity(1, "one"), 
                                new RoleEntity(3, "three"));
        model.addAttribute("userAttribute", userEntity);        

        List<RoleEntity> roleList = Arrays.asList(
                                        new RoleEntity(1, "one"), 
                                        new RoleEntity(2, "two"), 
                                        new RoleEntity(3, "three")
                                    );
        model.addAttribute("roleList", roleList);

        return "view";
    }}

User class:

import java.util.HashSet;import java.util.Set;public class UserEntity {
    private Set<RoleEntity> roles = new HashSet<RoleEntity>();

    public Set<RoleEntity> getRoles() {
        return roles;
    }
    public void setRoles(Set<RoleEntity> roles) {
        this.roles = roles;
    }}

Roles class (notice the equals and hashcode methods; if you remove them, the example no longer works):

public class RoleEntity {
    private long id;
    private String name;

    @Override
    public int hashCode() {
        return new Long(id).hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (! (obj instanceof RoleEntity)) {
            return false;
        }
        return this.id == ((RoleEntity)obj).getId();
    }

    public RoleEntity(long id, String name) {
        this.id = id;
        this.name = name;
    }

    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;
    }}

View:

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%><%@ taglib prefix="form" uri="http://www.springframework.org/tags/form" %><%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %><form:form modelAttribute="userAttribute" method="POST" action="/something">
    <table align="center">
        <tr>
            <td>ID</td>
            <td>Role Name</td>
        </tr>
        <c:forEach items="${roleList}" var="role">
            <tr>
                <td><form:checkbox path="roles" value="${role}" label="${role.id}" /></td>
                <td><c:out value="${role.name}" /></td>
            </tr>
        </c:forEach>
    </table></form:form>