error handling in web dynpro

hi everybody,

i want to know how to validate the field in web dynpro by

1) by ussing assistance class

2) allready present classes in abap and how .

Actually i have prepare a assistance class in se24 and put a message on '001'.now i want to check that this field which is a date field is less than the sy-datum.if the condition does not match then error message should be display on same page and the page should not be posted.

in my case the page is posting and the message is not displaying on the page.

thanks in advance.