Return error messages on POST

[markvr]

With the following code:

 @RequestMapping(value = "adminUsers", method = RequestMethod.GET)
    protected ModelAndView listAdminUsers() throws Exception {
        Person person = new Person();
        ModelAndView mav = new ModelAndView("adminUsersList");
        mav.addObject("adminUsersList", adminService.findAllAdminUsers());
        mav.addObject("person", person);
        return mav;
    }

    // Create a new item
    @RequestMapping(value="adminUsers", method = RequestMethod.POST)
    public ModelAndView listAdminUsers(Person newPerson, HttpServletResponse response)  throws Exception {
        Person person = personService.findPerson(newPerson.getUsername());
        if (person == null) {
            ModelAndView mav = new ModelAndView("adminUsersList");  //This is ugly, it's copy & pasted from above
            mav.addObject("adminUsersList", adminService.findAllAdminUsers()); //Ugly...
            // How to add error message though?
            return mav;
        } else {
            Roles roles = new Roles();
            roles.setPersonCode(person.getPersonCode());
            roles.setRoleType("ADMN");
            rolesMapper.insert(roles);
            return new ModelAndView("redirect:/admin/adminUsers.html");
        }
    }

<form:form method="post" action="${action}" commandName="person" >
    <form:label path="username">Add new administrator:</form:label>
    <form:input path="username" size="20"/>
    <form:errors path="username" />
    <input type="submit" value="Submit Changes"/>
</form:form>

        <table>
            <c:forEach var="user" items="${adminUsersList}">
                <tr>
                    <td>
                        ${user.title}&nbsp;<c:choose><c:when test="${not empty user.knownAs}">${user.knownAs}</c:when><c:otherwise>&nbsp;${user.forename}</c:otherwise></c:choose>&nbsp;${user.surname}
                    </td>
                    <td>
                        ${user.emailAddress}
                    </td>
                </tr>
            </c:forEach>
        </table>

If there is an error, (e.g. if (person == null) ) what is the best way to redisplay the form with:

• The input box filled in
• An error message
• The contents of adminService.findAllAdminUsers()

I've marked //ugly where I've just copied and pasted code. Although it's only 2 lines here, it could be a lot more complex and I don't like the feel of it. I also can't see how to bind an error message so it gets displayed in the <form:errors path="username" /> tag.

Thanks!

[markvr]

I worked it out:

    // Create a new item
    @RequestMapping(value="adminUsers", method = RequestMethod.POST)
    public ModelAndView listAdminUsers(Person newPerson,  BindingResult result, HttpServletResponse response)  throws Exception {
        Person person = personService.findPerson(newPerson.getUsername());
        if (person == null) {
            result.rejectValue("username","username.notvalid","Username doesn't exist");
            ModelAndView mav = new ModelAndView("adminUsersList");  //This is ugly, it's copy & pasted from above
            mav.addObject("adminUsersList", adminService.findAllAdminUsers()); //Ugly...
            return mav;
        } else {
            Roles roles = new Roles();
            roles.setPersonCode(person.getPersonCode());
            roles.setRoleType("ADMN");
            rolesMapper.insert(roles);
            return new ModelAndView("redirect:/admin/adminUsers.html");
        }
    }