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