Before getting into the details of using the Validator framework, it's necessary to give an overview of how Validator works. Recall that without Validator, you have to code all of your form data validations into the validate( ) methods of your Form Bean objects. Each Form Bean field that you want to perform a validation on requires you to code logic to do so. Additionally, you have to write code that will store error messages for validations that fail. With Validator, you don't have to write any code in your Form Beans for validations or storing error messages. Instead, your Form Beans extend one of Validator's ActionForm subclasses that provide this functionality for you.
The Validator framework is set up as a pluggable system of validation routines that can be applied to Form Beans. Each validation routine is simply a Java method that is responsible for performing a specific type of validation and can either pass or fail. By default, Validator comes packaged with several useful validation routines that will satisfy most validation scenarios. However, if you need a validation that is not provided by the framework, you can create your own custom validation routine and plug it into the framework.
It's important to point out that while traditionally the validator-rules.xml file stores validation routine definitions and the validation.xml file applies validation routines to Form Beans, both configuration files are governed by the same DTD. Thus their contents could technically be combined into one file.