Head First Servlets and JSP, 2nd Edition

The Container builds a map

Before JSP 2.0, the developer had to specify a mapping between the <uri> in the TLD and the actual location of the TLD file. So when a JSP page had a taglib directive like this:

<%@ taglib prefix="mine" uri="randomThings"%>

The Deployment Descriptor (web.xml) had to tell the Container where the TLD file with a matching <uri> was located. You did that with a <taglib> element in the DD.

The OLD (before JSP 2.0) way to map a taglib uri to a TLD file

<web-app>
...
 <jsp-config>
   <taglib>
     <taglib-uri>randomThings</taglib-uri>
     <taglib-location>/WEB-INF/myFunctions.tld</taglib-location>
   </taglib>
</jsp-config>
</web-app>

Note

In the DD, map the <uri> in the TLD to an actual path to a TLD file.

The NEW (JSP 2.0) way to map a taglib uri to a TLD file

No <taglib> entry in the DD!

The Container automatically builds a map between TLD files and <uri> names, so that when a JSP invokes a tag, the Container knows exactly where to find the TLD that describes the tag.

How? By looking through a specific set of locations where TLDs are allowed to live. When you deploy a web app, as long as you put the TLD in a place the Container will search, the Container will find the TLD and build a map for that tag library.

If you do specify an explicit <taglib-location> in the DD (web.xml), a JSP 2.0 Container will use it! In fact, when the Container begins to build the <uri>-to-TLD map, the Container will look first in your DD to see if you’ve made any <taglib> entries, and if you have, it’ll use ...