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JSP Tag Libraries : Chapter 10: Iterating with Tags

10.2 Generalized iterating tags

In perusing the implementation of SimpleForeachTag it appears that most of the work done by the tag is not unique to it. In fact, other than the creation of the Iterator object in parseElements() all the other code was generic. True, some tags will not want to expose an iterator, and others may want to expose more than a single iterator as a scripting variable (for some other tag-specific purpose), but these tags are not representative of the majority. In most cases, tags will differ only in the objects they iterate (some will iterate over an Enumeration, others on Array, etc.) but the general structure will stay the same; a single iterator scripting variable will be exposed and updated for each element.

Based on this general iterating structure, we'll build:

• A generic iteration interface that lets the tag developer specify how to iterate over some set of objects. .
• A basic iterator tag that takes a generic iteration object (Enumeration, Array, etc.) and iterates on it.

Creating these two, generic components will then streamline the creation of various iteration tags. These specialized iteration tags will be custom-built, based on the type of Java object to be contained in the iterator, and the iterator type in which these objects are to be contained. For example, our SimpleForeachTag had an iterator type of java.util.Iterator, and contained in that iterator was a list of Strings. We are now going to build these two components (the class and interface) and modify SimpleForeachTag to use this new, more generic infrastructure.

10.2.1 A generic iteration interface

Before looking into the new ForeachTag, let's study the generic iteration infra-structure on which it is constructed, starting with the generic iteration interface as seen in listing 10.4.

Listing 10. 4 Source code for the generic iteration interface

package book.iteration; 

import javax.servlet.jsp.JspException; 

public interface IterationSupport { 
	public boolean hasNext() 
		throws JspException; 

	public Object getNext() 
		throws JspException; 
}

Why do we need another iteration/ enumeration interface, as Java already offers plenty. You may also wonder, why a JspException is thrown from the methods hasNext() and getNext(). Shouldn't a generic interface remove JSP related ties? We do this because we want to provide better JSP integration. Let's explore our motivation for this integration.

NOTE We could consider the option of defining a new exception type (such as IterationException) that the iteration support methods could throw; but why should we? This code is written for the JSP tags, and we are not going to reuse it. In 99 percent of all cases, you are going to throw a JspException as a result of the error. Based on this argument, we've rejected the new exception type idea, and continue to use JspException as our error-reporting vehicle.

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