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Reviews : Java Books :
Professional Java Server Programming J2EE Edition : Chapter 12

Professional Java Server Programming J2EE Edition
Chapter 12

Title: Professional Java Server Programming J2EE Edition
ISBN: 1861004656
US Price: $ 64.99
Canadian Price:
C$ 97.95
UK Price: £ 46.99
Publication Date: September 2000
Pages: 1633
© Wrox Press Limited, US and UK.

A Simple Tag

Before we look at the API supporting tag extensions and the supporting infrastructure in detail, let's implement a simple tag. The simplest case is a tag without attributes or body content, which outputs some HTML. We'll add some dynamic content to prove that the tag is alive. Say we want to see the following output: 

Hello world.
My name is <tag handler implementation class> and 
it is now <date and time>

We'll call our simple tag hello. Here's how we might use it in a JSP. I've named this simple example hello.jsp. The first line is a declaration used to import the tag library, which we will discuss in detail later: 

<%@ taglib uri="/hello" prefix="examples" %>

<html>
	<head>
		<title>First custom tag</title>
	</head>
	<body>
	 
		This is static output.
	 
		<p />
		<i>
			<examples:hello></examples:hello>
		</i>
		This is static output again.
	</body>
</html>

All we need to do to implement the tag is to define a tag handler (a Java class implementing the tag's functionality), and provide the tag library descriptor. We can then import the tag library into any JSP that requires it.

The tag handler class must react to callbacks from the JSP engine when the tag is encountered in JSPs. The most important of these are doStartTag(), called when the opening of the tag is encountered, and doEndTag(), called when the closing tag is encountered. The implementation of HelloTag is quite simple, as most of the work of implementing the custom tag is done by the TagSupport superclass provided by the JSP 1.1 API. Don't worry if some of the details are puzzling: we'll look at this class, as well as the API it uses, in more detail in a moment. Note that the tag has access to the PageContext object of each JSP that uses it. 

package tagext;

import java.io.IOException;
import java.util.Date;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;

// Implementation of a tag to generate a single piece of HTML
public class HelloTag extends TagSupport {

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