Spring MVC: How to test whether param exists when there's no value?

Question

I want to display an error message with my custom login.jsp form. When there's an error, the url is ../loginForm?error without any value assigned to error. (This seems to be the behavior of Spring Security.) If there's no error, the url is simply ../loginForm (without the parameter). In the controller I can capture the parameter with @RequestParam, but how do I check whether or not error is passed? In other words, how can I test a parameter alone without a value?

Here's the controller code I have now:

@RequestMapping("/loginForm")
public String showLoginForm(@RequestParam(value="error", defaultValue="false") 
                                    boolean error,
                                Model model)
{
    if (error == true) 
    {
        model.addAttribute("loginError", "Invalid username and password.");
    }

    return "/user/loginForm";
}

...and here's the JSP snippet:

 <c:if test="${not empty loginError}">
  <tr>
   <td><c:out value="${loginError}" /></td>
  </tr>
 </c:if>

At this point I'm not including the Security configuration I have set up, since everything else seems to be working and I want to keep this focused on the issue at hand.

Thanks in advance for any suggestions!

Answer 1

Ok, I figured it out (while taking a break). The @RequestParam only works when there's actually a parameter available for mapping. If no such parameter is passed in, it's useless. So instead, I checked the Map provided by ServletRequest:

@RequestMapping("/loginForm")
public String showLoginForm(ServletRequest request, Model model)
{
    Map<String, String[]> paramMap = request.getParameterMap();

    if (paramMap.containsKey("error")) 
    { 
        model.addAttribute("loginError", "Invalid username and password.");
    }

    return "/user/loginForm";
}

It works fine now.

Answer 2

There is another way to do that. Just create one more method where @RequestMapping will check presence of "error" parameter, add required attribute and return view. Both methods could exist together.

@RequestMapping(value = "/loginForm", params = {"error"})
public String loginError(Model model)
{
    model.addAttribute("loginError", "Invalid username and password.");
    return "/user/loginForm";
}

@RequestMapping(value = "/loginForm")
public String login()
{
    return "/user/loginForm";
}