Programming Jakarta Struts, 2nd Edition [Electronic resources] نسخه متنی

A.7 The Struts Validator

The Struts Validator is a validation framework
that supports both Struts and non- Struts applications. It was
designed with Struts in mind, though, and it has many nice features
that are desirable for the types of applications typically built
using Struts. Chapter 11 covers the
Struts Validator in detail.

One thing worth mentioning as far as upgrading goes is earlier versions of the Validator framework relied on the RegExp package from Jakarta. This has been switched to Jakarta's Chapter 11 for more details on why this change was made and how it affects the configuration of the Validator.