JavaServer Pages, 3rd Edition

Using a TagExtraInfo Class for Validation

The TagLibraryValidator is the most powerful validation mechanism, but it comes at the price of complexity. You need to be pretty well versed in XML to validate the use of your tag library. I recommend that you give it a shot and make it your first choice, but it may be overkill for a small library with modest validation needs. If that’s the case, you can develop TagExtraInfo subclasses for the individual custom actions that need validation. A TagExtraInfo subclass can validate the use of the action element attributes. Optional attributes may be mutually exclusive—if one is used, the other must not be used—or using one optional attribute may require another optional attribute be used as well. A TagExtraInfo subclass can verify rules like this, but it can’t verify that a custom action is used correctly in the JSP page relative to other actions.

After the JSP container has checked everything it can on its own and has used the TagLibraryValidator classes for all libraries used in the page, it looks for TagExtraInfo declarations for each custom action element used in the page:

<tag>
  <name>myOptionalAttributesAction</name>
  <tag-class>com.foo.MyOptionalAttributesTag</tag-class>
  <tei-class>com.foo.MyOptionalAttributesTEI</tei-class>
    ...
</tag>

If it finds a <tei-class> element for an action, the container creates a TagData instance with the attribute values specified in the action element and calls the TagExtraInfo validate( ) method:

public ValidationMessage[] ...