JavaServer Pages Pocket Reference

Attribute Validation

In the previous example, the UsePropertyTagExtraInfo class sets the varName and className properties of the VariableInfo bean to the values of the id and className attributes specified by the page author in the JSP page. This is done using another simple class named TagData, passed as the argument to the getVariableInfo() method. The TagData instance is created by the web container to provide the TagExtraInfo subclass with information about all the action attributes specified by the page author in the JSP page.

A TagData instance is also passed as an argument to the TagExtraInfo isValid() method. This method is called by the web container during the translation phase to allow you to implement validation rules for the custom action’s attributes. The container can perform simple validation based on the information available in the TLD about which attributes are required. But a custom action may have optional attributes that are mutually exclusive or that depend on each other. That’s when you have to implement the isValid() method in a TagExtraInfo subclass and provide your own validation code.

The TagData class has two methods of interest. The getAttributeString() method simply returns the specified attribute as a String. But some attributes’ values may be specified by a JSP expression—a so-called request-time attribute—instead of a string literal. Since such a value is not known during the translation phase, the TagData class provides the getAttribute() method ...