Form Validation

hi,
  I just have a question about the process of "Form Validation".If I have the following code---
---this.defineElement( new DefaultFormElement(lOG_NAME,FormType.STRING,null,new NotNullValidator("Login Name error"),false) );

And I also define a form validator later.When I call "form.map().validate(true)" and I didn't fill the "login" field,will the validation process stop here?
If not,and the ValidationException is also thrown in my form validator,will I catch the exception from the "NotNullValidator"?