Reviews : Java Books :
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.

Although we developed it from our previous examples, this is close to a generic solution for list iteration, isn't it? It conceals the looping from the JSP and makes the successive list values and indices available. With minor changes, this class could take a Collection, or a Swing ListModel, and make it available to any JSP.

The main reason that this tag is so generic is that it doesn't generate markup. The more the JSP can control its output, the more useful a tag extension is.

Body Tags That Filter Their Content

Another idiomatic use of body tags is to perform filtering or other processing on their body content. This could be a simple text transformation, or could even interpret the tag's content as a custom language. The following simple example takes the tag's body content and writes it, reversed, into the calling JSP page.

Implementing the reversal is trivial; all we need to do is obtain the body content as a String, use it to initialize a StringBuffer, and call the StringBuffer's reverse() method before writing out the resulting String:

package tagext;

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

// Simple body tag to reverse its content
public class ReverseTag extends BodyTagSupport {
 
	/*
	* Called after processing of body content is complete.
	* We use it to obtain the tag's body content and write
	* it out reversed.
	*/
	public int doEndTag() throws JspTagException {
	BodyContent bodyContent = getBodyContent();
		// Do nothing if there was no body content
		if (bodyContent != null) {
			StringBuffer output = new StringBuffer(bodyContent.getString());
			output.reverse();
			try {
				bodyContent.getEnclosingWriter().write(output.toString());
			}
			catch (IOException ex) {
				throw new JspTagException("Fatal IO error");
			}
		}
 
	// Process the rest of the page
	return EVAL_PAGE;
	}
}

The tag library entry is also very simple. There are no variables requiring a TagExtraInfo class, and no attributes:

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