Java Server Pages

Validating Syntax

The TLD for a tag library contains information about the attributes each action element supports. Therefore, the JSP container can help by verifying that the custom action is used correctly by the page author, at least with respect to the attributes.

When the JSP container converts a JSP page to a servlet, it compares each custom action element to the specification of the action element in the TLD. First, it makes sure that the action name matches the name of an action specified in the TLD corresponding to the action element’s prefix. It then looks at the attribute list in the page and compares it to the attribute specification in the TLD. If a required attribute is missing, or an attribute is used in the page but not specified in the TLD, it reports it as an error so the page author can correct the mistake.

But for some actions, it’s not that simple. Some attributes may depend on the presence of other attributes. Attributes may be mutually exclusive, so that if one is used, the other must not be used. Or an optional attribute may require that another optional attribute is used as well. To be able to verify these kinds of dependencies, the JSP container asks the tag handler’s TagExtraInfo subclass for assistance.

After the JSP container has checked everything it can on its own, it looks for a TagExtraInfo subclass, defined by the <teiclass> element, for the action. If one is defined, it puts all attribute information in an instance of the TagData class and calls the ...