JavaServer Pages, 3rd Edition

Identifying the Tag Library in a JSP Page

To identify the library in JSP pages, you use a taglib directive like this:

<%@ taglib prefix="ora" uri="orataglib" %>

The container uses the uri attribute value to locate the TLD file for the tag library. The value must be either a symbolic name or a file path. A symbolic name is any string that is unique in the application. An HTTP URL is often used to be reasonably sure that it’s unique in any application. Even when an HTTP URL is used, the container uses it only as a symbolic name; it does not try to get the resource specified by the URL.

If the uri value is a symbolic name, it must be mapped to the actual location of the TLD file somehow. In JSP 1.2, a new auto-discovery mechanism was introduced to make this very easy. Here’s how it works. The TLD includes a <uri> element to define the default URI for the library:

<taglib> 
  ... 
  <uri>orataglib</uri> 
  ... 
</taglib>

When the web application is started, the container scans through the WEB-INF directory structure for files with .tld extensions and all JAR files containing files with .tld extensions in their META-INF directory. In other words, locating all TLD files. For each TLD, the container looks for the <uri> element and creates a map from the URI to the TLD that contains it. In your JSP page, you just have to place a taglib directive with a uri attribute value matching the URI in the TLD.

Prior to JSP 1.2, you had to define the mapping manually in the deployment descriptor for the application ...