Java Enterprise Best Practices

Consider Developing a Tag Library Validator

To make the page author’s life as easy as possible, you should consider implementing a tag library validator for your custom tag library. This type of validator was introduced in JSP 1.2, and it can do a much better job verifying that all custom actions are used correctly than the previous mechanism (the TagExtraInfo class) could.

You implement a tag library validator by extending javax.servlet.jsp.tagext.TagLibraryValidator and provide an implementation for the abstract validate( ) method:

public ValidationMessage[  ] validate(String prefix, String uri, PageData pd)

The container calls this method when it translates the page into a servlet. The parameters provide the validator with the tag library prefix, URI (declared by the taglib directive in the page), and, most importantly, a PageData instance. Through the PageData instance, the validator gains access to an XML representation of the complete page (called an XML view). With access to the full page in a format that’s easy to parse, it can make sure the actions are properly nested and appear in the right order, in addition to what’s always been possible: checking the proper use of all attributes for each individual custom action element.

The validate( ) method returns an array of ValidationMessage objects. Each object represents a validation error, containing a text message plus a unique element ID that the container can use to map the error message to the location of the corresponding ...