The Java Boutique: Java Books: Java Server Programming J2EE Edition

Anatomy of a Tag Extension

Before we return to our simple example, let's cover some basic theory of tag extensions.

A number of components are required to implement a tag extension. The minimal requirement is a tag handler and a tag library descriptor.

A tag handler is a Java bean implementing one of two interfaces defined in the javax.servlet.jsp.tagext package, Tag or BodyTag. These interfaces define the lifecycle events relevant to a tag; most importantly, the calls the class implementing the tag will receive when the JSP engine encounters the tag's opening and closing tags.

A tag library descriptor, as we have seen, is an XML document containing information about one or more tag extensions.

More complex tags will require an additional class extending the abstract class javax.servlet.jsp.tagext.TagExtraInfo to provide information about scripting variables that are made available to JSPs through the use of tags.

TagExtraInfo subclasses may also perform custom validation of tag attributes. Of course, the classes implementing a tag may require any number of helper classes, which will need to be packaged with the tag for it to be a complete deployable unit.

Before tags can be used in a JSP, the taglib directive must be used to import a tag library and associate the tags it contains with a prefix.

Let's look at each of these requirements in turn.

Tag Handlers

When the JSP engine encounters a tag extension in a JSP at translation time, it parses the tag library descriptor to find the required tag handler class, and generates code to obtain, and interact with, the tag handler. The Tag or BodyTag interfaces, one of which must be implemented by any tag handler, define callbacks that the servlet resulting from the JSP engine's code generation will make to the tag handler instance at runtime.

For performance reasons, JSP engines will not necessarily instantiate a new tag handler instance every time a tag is encountered in a JSP. Instead, they may maintain a pool of tag instances, reusing them where possible. When a tag is encountered in a JSP, the JSP engine will try to find a Tag instance that is not being used, initialize it, use it and release it (but not destroy it), making it available for further use. The programmer has no control over any pooling that may occur. The repeated use model is similar to a servlet lifecycle, but note one very important difference: tag handler implementations don't need to concern themselves with thread safety. The JSP engine will not use an instance of a tag handler to handle a tag unless it is free. This is good news: as with JSP authoring in general, developers need to worry about threading issues less often than when developing servlets.

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