There generally is less reason or need to extend components located within the view layer than there is to extend components in the controller layer. Typically, views are written exclusively for an application. For example, it’s unlikely that a JSP page written for one application will be used within a different application. This is not always the case, but differences between look-and-feel and content make such reuse improbable. The one area within the Struts view layer where extensions often are created, however, is in the JSP tag libraries.
The custom tags provided by the Struts framework can be reused across applications and application domains. Therefore, it makes sense that customization and extensions are more likely with these components than with JSP pages. Because the tag handlers are regular Java classes, specialization is achieved through subclassing.
Although you can extend any tag, the HTML tag library is the one that you’ll most likely need to customize (mainly because the custom tags within this library have the greatest impact on the view content). Regardless of which tags you extend, you’ll need to create your own tag library to hold your tag extensions.
You could modify the Struts tag libraries and include your new tag class, but that would make upgrading to newer versions of the Struts framework much harder. You’re better off creating your own tag library that contains just your application’s tags.