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

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";
}