JavaServer Pages, 3rd Edition

Creating the Tag Library Descriptor

Now you have a good idea about what the Java classes for tag library functions and both simple and classic tag handlers looks like. When the JSP container processes a page, it converts EL functions and custom action elements into code that creates and calls the correct classes. To do this, it needs information about which tag handler implements which custom action element, and which Java method corresponds to an EL function. It gets this information from the Tag Library Descriptor (TLD). As you will see in Chapter 22, the JSP container also uses the TLD information to verify that the attribute list for an action element is correct.

The TLD is an XML file with information about all custom actions and functions in a library. A JSP page that uses a custom tag library must identify the corresponding TLD and the namespace prefix used for the actions and functions in the page with the taglib directive:

<%@ taglib prefix="ora" uri="orataglib" %>
...
<ora:addCookie name="userName" value="${param.userName}" />
...
${ora:toCelsius(param.f)}

The uri attribute identifies the TLD, in one of several ways that I describe later in this section. The prefix attribute assigns a prefix to use for the action elements and functions included in the library.

The JSP container then uses the TLD to find the information it needs to generate code for invoking the correct class when it encounters action elements and functions with a matching prefix.

Example 21-13 shows a part ...