Question

I am coding a web-app in Java-EE and find myself with a very unexpected result when I am trying to display errors on user-input.

The app is built on the JSP/Servlet/Form/Bean model. Basically, the JSP stores data in the request, and transfers it to the servlet. Then the servlet transfers the request raw to the form, which then reads the data, performs the necessary checks and returns the bean to the servlet.

Most of the fields must have specific values, some others must simply be non-null.

I have written error detection code to secure inputs however I find myself with a very strange result:

• when the field is non-null but the value is incorrect (say, an hour located outside the 00:00-23:59 range), it does return the proper error, along with the error message, stored in a HashMap, and I can access it in my JSP.
• However, when the field is null, it returns the message, probably stores it in the HashMap as well (I know this because the ${!empty errors.dataErrors} test returns true and the error field is displayed in my JSP) but there’s no way to access the values of the errors

I have searched through my code but still can’t find where the error comes from. Here are snippets of it if someone knows where the problem comes from

doPost method from the servlet:

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
{
    NewBookingForm form = new NewBookingForm();
    Booking booking = form.registerBooking(request);
    String VUE;

    request.setAttribute("booking", booking);
    request.setAttribute("errors", form);

    this.getServletContext().getRequestDispatcher(VIEW).forward(request, response);
}

the map is a field in the NewBookingForm class, declared and initialized outside the registerBooking method like this private Map<String,String> dataErrors = new HashMap<String,String>(); and it has a private setter (for access within the class) and a public getter (for access in the Servlet and in the JSP)

inside the form class, I use this function to get the field values:

private static String getFieldValue(HttpServletRequest request, String fieldName)
{
    String value = request.getParameter(fieldName);
    if (value == null || value.trim().length() == 0){return null;}
    else{return value;}
}

After getting the values with a series of calls like String fieldDepartureStation = getFieldValue(request, FIELD_DEPARTURE_STATION); at the beginning of my method, I then check them using try/catch blocks like this

try
{validation.departureStation(fieldDepartureStation);}
catch(Exception e)
{setDataErrors(FIELD_DEPARTURE_STATION, e.getMessage());}

The validations method within the validation class are a bit different if the data must have specific value-ranges or must simply be non-null.

In the former case, they are something like this:

public void departureTime(String time) throws Exception
{
    if (!validationRETime(time)) { throw new Exception("Please input a time with the hh:mm pattern"); }
}
....
private boolean validationRETime(String strTime)
{
    String regExp = "^([01][0-9]|2[0-3])[:][0-5][0-9]$"; // hh:mm
    if (strTime.matches(regExp))
    {
        return true;
    }
    else
    {
        return false;
    }
}

whereas in the latter case they are simply

public void departureStation(String station) throws Exception
{
    if (station.equals(null)) { throw new Exception("Please input a departure station"); }
}

Finally, in my JSP, I use the following code to display errors:

<c:if test="${!empty errors.dataErrors}">
     <p>Errors</p>
     <c:forEach items="${errors.dataErrors}" var="message">
           <p><c:out value="${message.value}" /></p>
     </c:forEach>
</c:if>

And it does display the Error paragraph when I purposedly enter incorrect values, but the <c:forEach> is only looping and displaying the error messages when the wrong field is non-null but with an incorrect value. Thus with a field that only needs to be non-null, I never get the message (but I do get the error)

These are all the things I could think of that could possibly go wrong, but I have yet to discover where they did and if someone could help me, I’d be very glad.

Answer 1

The problem is in your departureStation method: –

public void departureStation(String station) throws Exception
{
    if (station.equals(null)) { 
        throw new Exception("Please input a departure station"); 
    }
}

Your test for null value is itself triggering a NPE. So, as soon as station.equals(null) is executed for station = null, a NPE exception is thrown, which is then propagated to the caller. So, your if block will not even be executed. And hence you are not throwing the Exception as you might be thinking.

Now, also note that, the NPE that is thrown does not contain any message. So, e.getMessage() will return null on it.

Now, let’s move back to the caller: –

try
{validation.departureStation(fieldDepartureStation);}
catch(Exception e)
{setDataErrors(FIELD_DEPARTURE_STATION, e.getMessage());}

Here you are doing the biggest Crime in the world of Exception Handling, by using a catch block for Exception. Since Exception is the super class of all the exceptions, it will handle all the exceptions in the same way. So, it consumes the NPE, and passes it to setDataErrors().

So, you will of course get the errors, but, the value e.getMessage() will be null. And that is why you are not seeing any message. You can even test it by logging the value of e.getMessage() in the catch block above.

---

Solution ??

Just change your null check with this one: –

if (station == null) { 
    throw new Exception("Please input a departure station"); 
}

And everything will be ok. I think, you will have to do this change in all of your methods. Always perform the null check using == operator.