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.

The NameContext interface is trivial:

package tagext;

// Interface to provide context for nested tags
public interface NameContext {
	String getName();
}

So is the modification to VarHelloBodyTag (beyond making it implement NameContext) – simply the implementation of the new method:

//  Method from NameContext interface required to provide context to
// nested tags
public String getName() {
	return names.get(index).toString();
}

In a real application, we'd do some error checking here.

Now let's look at the new NameTag tag handler. Its main tasks are to obtain the context from an enclosing tag, and to retrieve the additional information. Note how it enforces correct nesting. There is a getParent() method in the Tag interface, but it is usually preferable to use findAncestorWithClass(). We don't want to limit the context in which we can use our tags. Can we guarantee that another tag or tags might not stand in the hierarchy between the two cooperating tags? If one or more do, and we have hard coded reliance on a particular parent tag class or interface, the nested tag will fail to find the ancestor it requires to provide its context.

For simplicity, I've hard coded the additional data in the class, in a hash table. For the sake of the example, imagine that this data is actually very expensive to retrieve. The extra information about people in the list will be their nationality, and the city they currently live in. My friends tend to travel quite a bit so these pieces of information may not be obviously related:

package tagext;

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

/**
*  Tag nested within a tag implementing the NameContext interface
*  to provide additional information about the relevant person.
*  This information will be exposed through the <i>nationality</i>
*  and <i>city</i> scripting variables.
*  <br/>This tag requires no attribute, as it initializes itself from
*  the appropriate ancestor tag.
*  <p/>This tag produces no markup.
*/
public class NameTag extends TagSupport {
	// Data store
	private HashMap  infoHash = new HashMap();

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