[Springframework-developer] BindException changes

[Ben A. <ben...@ac...>]

I'm using SimpleFormController to process a form containing a parameter 
that is converted to a Long value in the form backing object. The Long 
has a check digit and thus there are both number format exceptions as 
well as check digit related exceptions possible. By default such errors 
will cause the following messages to be displayed to the user in the 
errors list:

Failed to convert property value of type [java.lang.String] to required 
type [java.lang.Long] for property 'luhn'; nested exception is 
java.lang.IllegalArgumentException: Cannot parse number: Unparseable 
number: "aString"

or

Property 'luhn' threw exception; nested exception is 
java.lang.IllegalArgumentException: Business key must be a valid Luhn 
identifier

Neither response is suitable for display to users. I would rather 
replace these with my own more user-friendly message. One approach 
discussed at http://forum.springframework.org/viewtopic.php?t=1098 (last 
post by ojolly) is for the PropertyEditor to set the object to null, 
then detect something was actually entered in the request parameter in 
onBindAndValidate and then add a friendly error. That's OK, but then you 
lose whatever the user actually typed in as the PropertyEditor set it to 
null. This is unacceptable as quite often the data entered will be close 
to what it should have been (eg an unwanted fractional portion for an 
integer, a mis-typed digit for a checksum-based number, a property that 
is simply too long etc).

I ended up writing a MyBindException which subclasses BindException and 
provides these two methods:

    public void removeError(ObjectError error) {
        this.errors.remove(error);
    }
   
    public void removeErrors(List objectErrors) {
        Iterator iter = objectErrors.iterator();
        while (iter.hasNext()) {
            ObjectError objectError = (ObjectError) iter.next();
            removeError(objectError);
        }
    }

My controller then does this:

    protected void onBindAndValidate(HttpServletRequest request,
            Object command, BindException errors) throws Exception {
        Person person = (Person) command;
        if (errors.hasFieldErrors("luhn")) {
            // Cast is safe as createBinder caused MyBindException to be 
used
            MyBindException myErrors = (MyBindException) errors;
            errors.rejectValue("luhn", "LUHN_INVALID", "A valid Luhn is 
required.");
        } ....

This works great. It consumes the unwanted, uninformative user feedback 
and replaces it. It also avoids needing to look at HttpServetRequest 
parameters. The problem is to achieve this I've needed to write 
MyBindException, copy nine methods to it (as the errors List is private 
in the BindException superclass), write a MyServletRequestDataBinder to 
set it up, and then override createBinder in the controller to use 
MyServletRequestDataBinder. It seems a lot of work, so I was hoping we 
could add the above two methods to BindException? Alternatively, at 
least making the errors List protected in BindException will save the 
ugly code duplication.

Alternatively, if there's another way entirely of achieving this, I'd be 
grateful if someone would let me know.

Best regards
Ben