Catching NullPointerException

[Sanjay Anand]

Hi,
I have a curious problem. More academic at this point than anything else.
I am writing a login servlet.
We are required by our client to not allow the user / customer to see any exceptions at all. Users need simply be redirected to a 'temporary difficulties' html page.
So, in addition to other checks, we've put in a method-wide try - catch in the doPost method that logs the error and redirects the user in case something untoward occurs.
Now, in updating and tweaking the code, I made an error in logic that was caught only in runtime. I was trying to call methods for an object before instantiation. No problem. It is now fixed.
But the question is, why was the runtime NullPointerException not caught? Why did it spit up to the browser level immediately? I even tried catching the NPE explicitly, to no effect.
Any thoughts on this?
I can post code snippets if anyone thinks them relevant.
Sanjay

[L Goundalkar]

Please post the code snippet.

[Jessica Sant]

As a workaround -- you could define an <error-page> in your web.xml so that every 500 Internal Servlet Error or every NullPointerException; it automatically sends your client to a "Temporary troubles" page.

From the Servlet 2.3 spec:
<!--The error-page element contains a mapping between an error code or exception type to the path of a resource in the web application -->
<!ELEMENT error-page ((error-code | exception-type), location) >
<!--The error-code contains an HTTP error code, ex: 404 -->
<!ELEMENT error-code (#PCDATA) >
<!--The exception-type contains a fully qualified class name of a Java exception type. -->
<!ELEMENT exception-type (#PCDATA) >
<!--The location element contains the location of the resource in the web application. -->
<!ELEMENT location (#PCDATA)>

[ May 31, 2002: Message edited by: Jessica Sant ]

[Sanjay Anand]

Thanks Jessica for a good suggestion.
It does not solve my academic question and it is also not usable because my servlet container, Macromedia's JRun, does not support the specs required for the error page functionality.
And, "L" (a name not in keeping with the JavaRanch custom by the way). Here's a code snippet. I hope it formats correctly and is readable.

  
  public void doPost (HttpServletRequest request, HttpServletResponse response) throws
       ServletException {
      String uid            = "";
      String referringpage  = "";
      String userpassword    = "";
      String http_head      = "";
      User user              = null;

  try { // methodwide
    response.setContentType("text/html");
    out = response.getWriter();
    uid             = request.getParameter("username");
    userpassword    = request.getParameter("userpassword");
    referringpage   = request.getParameter("refer");
    user                = new User();
    user.uid            = new String(uid);
    user.userpassword    = new String(userpassword);
    user.referringpage  = new String(referringpage);
//********************
// SERVLET request parameter, debug dump
// [the NPE occurred when I tried to do this before insantiating User]
  if (BizProperties.signinTrace)
  {
    user.msg.append(user.date + ": " + getClass().getName() + "\n");
    user.msg.append("***********\n");
    user.msg.append("referringpage: " + referringpage + "\n");
    user.msg.append("***********\n");
  }
//*************************
// authenticate
      if (!user.authenticate()) {
        http_head = user.setTarget(user.NOTAUTHENTICATED);
        WebTools.RedirectToFinalDestination(out, http_head);
      }
      else {
        http_head = user.setTarget(user.AUTHENTICATED);
        WebTools.RedirectToFinalDestination(out, http_head);
      }
//*******************************************
// after authentication, trace user object state
  if (BizProperties.signinTrace)
        user.go.msg.append(user.toString());
//*******************************************
 // signin
     try {
      user.signin();
    } catch (AppException e) {
              // This is a custom exception that is functioning fine
        report(e, FATAL, LOG, OUT);
        WebTools.redirectToSystemError(out);
    }
  } catch (Exception e) {
      report(e, FATAL, LOG, OUT);
      WebTools.redirectToSystemError(out);
   }
} // doPost()
} // end of class

[ May 31, 2002: Message edited by: Sanjay Anand ]

[Jessica Sant]

Originally posted by Sanjay Anand:
And, "L" (a name not in keeping with the JavaRanch custom by the way).

Actually we do allow FirstIntial <space> LastName but NOT FirstName <space> LastInitial
Thanks for looking out for us though!

[Sanjay Anand]

Terribly sorry about that.

[Thomas Paul]

There is no reason why this shouldn't work. Exception should catch everything. Perhaps there is a bug in the servlet container? Have you checked with Macromedia?

[Sanjay Anand]

No, I have not checked with Macromedia. I will do that. Thanks for the suggestion.
I am heartened to know that there isn't a bug in my code at any rate.
Sanjay

[L Goundalkar]

Hi
I have worked on JRun 4. Sometimes it gives exceptions like this and also ClassCastException for no bug in the code.
You need to shut down the servers and even the admin one and restart then through Windows Services. Then it will work fine.
I think its a bug in JRun container.
Same querries are unanswered in JRun user forum.

[peter brews]

Hi Jessica,
I would also like to know if there is any similar solution possible for EJB's. I have methods defined in EJB and after caching the exception i just need to show the users a generic error message instead of the actual one.
Thanks,
pete

[Kev D'Arcy]

I might be wrong here, but you could add this after your ]

catch(Exception e)

. An exception catch won't catch Null Pointers, you need to catch throwables to get nulls.

catch (Throwable t) {
      report(new Excption(t.toString()), FATAL, LOG, OUT);
      WebTools.redirectToSystemError(out);   
}  

[ June 04, 2002: Message edited by: Thomas Paul ]

[Geoffrey Falk]

Untrue; runtime exceptions are a subtype of java.lang.Exception. So NullPointerException will be caught by a catch(Exception e) {} statement.

[Thomas Paul]

You can prove it for yourself. Run the following code. The output will be "gotcha" which proves the nullPointerException was caught.

public class Test
{
public static void main(String args[]) {
try {
String s = null;
System.out.println(s.toString());
}catch (Exception e) {
System.out.println("gotcha!");
}
}
}