How do Custom Tags Work?

Three features must be in place for custom tags can be used:

Tag Library

Collection of classes that provide the functionality of the custom tag.
Usually distributed in a .jar file (portability).
Increases maintainability.

Due to the relative simplicity, almost anyone can decipher the code.
Updating the code allows for one update in the central files, not in every page that uses this code.
New functionality can be added without changing the already existing pages.

Tag Handler

"Server-side object that is created to evaluate actions during the execution of a JSP page."
Controls the passing of information between a JSP page and a custom tag.
Can be thought of as a JavaBean.

taglib Directive

Ties the tag library description to the JSP page.
Sets the tag namespace prefix to be used in the JSP page.
<%@ taglib
uri-"http://www.javadesktop.com/taglib/htmltool.jar"
prefix="tool"
%>

Tag Library Descriptor

XML document that describes a tag library as a whole (individual tags, attributes, etc.).
Information can be found out about a tag library without having to instantiate objects or look at actual code.