The JSP Files - Parts 1 to 8: Tagged and Bagged

The JSP Files -- Part 8: Tagged and Bagged

The Beginning Of The End

Over the past few weeks, we've taken you on a guided tour of the intricacies of JSP, beginning with basics like conditional statements and loops, and quickly moving on to more complex things like form processing, session management, and error handling.

But all good things must come to an end - and so, in this final episode of The JSP Files, we'll be briefly touching on a few other facets of this powerful server-side scripting language.

Playing Tag

One of the most exciting features about JSP is the ability to build and use custom "tag libraries" in your JSP applications. A "tag library" is a reusable block of JSP code, typically written to replace Java code with easy-to-read-and-understand tags (similar in appearance to HTML markup). Once written, these tag libraries can be used over and over again, thereby bringing a new element of reusability to the language.

In addition to reusability, tag libraries also offer substantial advantages from the maintenance point of view. Since tag libraries are largely defined using XML-type markup, they make it possible to separate application presentation from application logic - which, in turn, implies that designers and developers working on Web applications can use a tag without worrying about how and why it works. This separation between program code and final layout is something most designers would kill to have - and it's now available to almost anyone who knows how to string together Java and JSP code.

Another advantage of the separation discussed above, is the ease of use when it comes to adding new features to a tag library, or making changes to existing features. Since a tag library is portable and reusable, a change made to it will immediately reflect in all JSP pages using that tag library. Similarly, a new feature added to the tag library becomes immediately available to all pages carrying that library.

Tag libraries are slowly catching on, with many JSP developers releasing custom tag libraries for free online. These libraries are typically designed for specific tasks - database connectivity, string manipulation and the like - and they provide a rich vein for JSP developers to mine for their own projects. After all, why waste time writing Java code to send email messages when you can find a feature-rich, properly-tested and free tag library to do the same thing online?

Looking Inside

A tag library is made up of several components. First comes the tag class itself, written in Java - this contains all the behind-the-scenes code which makes the tag function. This class will be referenced in the tag library descriptor, and constitutes the meat of the sandwich we're building.

Next, you need a "tag library descriptor", an XML file usually identified with the .TLD file extension. This TLD contains information on different aspects of the tag library: the name of the tag, and of the Java class associated with it, a short description of functionality, and optional attributes and body content.

Both these components are usually packaged into a JAR file for easy distribution.

Now, we're not going to get into the nitty-gritty of writing a tag library here - there are already a whole bunch of tutorials which discuss this in detail, and links to some are included at the end of this article. What we will do, however, is illustrate how a tag library can be incorporated into your JSP document, and how it interacts with the rest of your script.

In order to illustrate this, we'll be using the Jakarta Project's DATETIME tag library, designed to manipulate date- and time-stamps. You can download a copy of this library here, and you can find a number of other freeware tag libraries at this site, one of the larger repositories on the Web.

Once you've downloaded a copy of the library, you need to tell Tomcat about it before you can begin using it. The first step here is to decide the context within which you plan to use the library - for purposes of this example, we will assume the context is "/test/". Next, copy the TLD file from the distribution into the context's "web-inf/" directory, and the main JAR file into the context's "web-inf/lib/" directory.

The last step here is to open up the "web.xml" file which resides in the "web-inf/" directory, and alter it to reflect the new tag library - this is accomplished via the <taglib> directive.

<taglib> <taglib-uri>http://jakarta.apache.org/taglibs/                                    
datetime-1.0</taglib-uri>                                    
<taglib-location>/WEB-INF/datetime.tld</taglib-location>                                    
</taglib>

This element specifies the location of the tag library, as well as a unique URI used to reference it.
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