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JSP Tag Libraries : Chapter 10: Iterating with Tags

EnumerationIterationSupport

The IterationSupport class supporting Enumerations, EnumerationIterationSupport, is very straightforward, since both the IterationSupport and Enumeration interfaces are so similar (see listing 10.8)

Listing 10. 8 EnumerationIterationSupport

package book.iteration; 

import java. util.*; 

public class EnumerationIterationSupport implements IterationSupport 
	{ 

	Enumeration elements; 
	public EnumerationIterationSupport(Enumeration e) 
	{ 
		elements = e; 
	} 
	
	public boolean hasNext() 
		throws JspException 
	{ 
		return elements. hasMoreElements();  
	} 
	

	public Object getNext() 
		throws JspException 
	{ 
	return elements.nextElement();  
	} 
}

Using Enumeration's method to determine if more elements exist.

Using Enumeration's method to retrieve the current object.

10.3.3 Implementing IterateTag

The next step is the implementation of IterateTag (listing 10.9) in which we'll see how all the wrappers, reflection logic, and our generic iteration framework combine in its creation (note that for clarity reasons we snipped the reflection code out of the code listing).

Listing 10. 9 Source code for the IterateTag handler class

package book.iteration; 

import java.beans.IntrospectionException; 
import java.lang.reflect.InvocationTargetException; 
import java.util.Enumeration; 
import java.util.Iterator; 
import book.reflection.ReflectionTag; 
import book.util.LocalStrings; 
import book.util.BeanUtil; 
import javax.servlet.jsp.PageContext; 
import javax.servlet.jsp.JspException; 

public class IterateTag extends IterationTagSupport { 

	static LocalStrings ls = 
		LocalStrings.getLocalStrings(IterateTag.class); 
	
	// Reflection related properties and properties setters 
	// were removed from this section. 
	
	protected void fetchIterationSupport() 
		throws JspException 
	{ 
		Object o = getPointed();  

		if( o instanceof Iterator) { 
			elementsList = 
				new IteratorIterationSupport(( Iterator) o); 
		} else if( o instanceof Enumeration) { 
			elementsList = 
				new EnumerationIterationSupport(( Enumeration) o); 
		} else if( o. getClass(). isArray()) { 
			elementsList = new ArrayIterationSupport( o); 
		} else { 
			// Throw an exception to inform that we cannot 
			// iterate this object 
		} 
	} 
	
	// The reflection code below this line 
	// was removed from this listing 
} 

getPointed() retrieves the object the tag should iterate on. This method is inherited from ExBodyTagSupport.

Gets the referenced object and wraps it within the appropriate IterationSupport implementation.

Apart from the reflection related code which we've omitted (we've seen how this code works in chapter 8), IterateTag's implementation consists of a single method implementation: fetchIterationSupport(). This method merely checks the object that is passed as the tag attribute and selects an appropriate iterator and IterationSupport wrapper, based on the object's type.

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