How do we catch errors in Servlets?

Question

Hello, i 'm using Servlets and JDBC.
i'm having a form.html where the user inserts data in the input fields.
It's a registration page.
I would like to check for possible user errors.
For example if the user has not typed anything in a required field like field "password", so the field is null.
I would like to inform the user that this field is null and he/she shall type in it again.
(I'm talking generally not only for a registration page but to any page that requires correct data)
*1.* Shall i use Javascript?
*2.* Shall i direct the user to an error page where the page describes the error, what the user has to do in order to correct it and direct him back to the previous page?
*3.* Shall i only use if/else?
I would appreciate if sm could give me some advice on this matter or post me some links related to catch errors in servlets.
Thanks, in advance!
Edited by: g_p_java on Jul 11, 2009 12:15 PM

Answer 1

Here is a possible approach to this issue:
* use javascript to validate all textfields on the form client-side every time the user clicks the submit button. If it fails, put a message in red next to the problem textfield describing what the problem is. Dont allow submit if there are validation issues.
* More than one validation may apply to a given textfield. For instance, the field cant be empty is checked before its checked for an invalid date.
* Check to see if its an invalid date format first (example: mm/dd/yyyy), then check to see if its an invalid date (non leap year for example).
* recheck all validation back on the server (necessary if user disabled javascript or a hacker bypasses your client side validation). Redraw the page and populate it, but put messages next to the problem fields as you did for javascript.
* The invalid date format and invalid date should pass if given an empty value since the its the empty value validation (if it applicable for that field) that is repsonsible for that test.
*since you will be applying this validation over and over again for all your JSP pages for this project and future projects, it may be a good idea to create reusable code for it. You may also research google for good approaches.
Your javascript code should be from a single javascript file that is included into each JSP page so it can be reused.
@see http://www.xs4all.nl/~sbpoley/webmatters/formval.html
* All textfields should be validated throughly so your database never gets bad data (data is king).
* common things to check for:
invalid date format, invalid date, empty textfield, invalid email address, invalid telephone number format, invalid zip code, etc, etc, etc