10. 2. 2 IterationTagSupport
Let's look at the basic iteration tag class, IterationTagSupport, and how it uses IterationSupport.
Before taking a look into the implementation of IterationTagSupport
as presented in listing 10. 5, let's consider how we would like it to work.
What should IterationTagSupport do?
Most emphatically, the generic iteration tag class should automatically take care of
iteration-related issues such as flow control, as well as exporting default iterator
variables. In addition, it must be able to:
- Create an
IterationSupport object out of the elements provided as a tag
attribute. This can be accomplished by defining a method that our specialized iteration tags
can override and that IterationTagSupport will call during its
doStartTag(). By specialized tag we mean the special version of the tag that
is custom built to handle a particular iterator type and a particular type of object in that iterator.
- Export a different set of JSP variables. Whenever
IterationTagSupport
wants to export its iterator value, it should call yet another method that can be overridden by the specialized tag (but the default implementation of the
variable exportation method should export only a single iterator).
IterationTagSupport's implementation
IterationTagSupport was created with a few methods that may be overridden by
specialized iteration tags.
Listing 10. 5 Source code for the generic iteration tag handler
package book.iteration;
import book.util.LocalStrings;
import book.util. ExBodyTagSupport;
import javax.servlet.jsp.JspException;
public abstract class IterationTagSupport
extends ExBodyTagSupport {
static LocalStrings ls =
LocalStrings.getLocalStrings( IterationTagSupport. class);
IterationSupport elementsList = null;
public int doStartTag()
throws JspException
{
fetchIterationSupport();
if( elementsList.hasNext()) {
return EVAL_BODY_TAG;
}
return SKIP_BODY;
}
public void doInitBody()
throws JspException
{
exportVariables();
}
public int doAfterBody()
throws JspException
{
try {
getBodyContent().writeOut( getPreviousOut());
getBodyContent().clear();
} catch( java.io.IOException ioe) {
// User probably disconnected ...
// Log and throw a JspTagException
}
if( elementsList.hasNext()) {
exportVariables();
return EVAL_BODY_TAG;
}
return SKIP_BODY;
}
protected abstract void fetchIterationSupport() 
throws JspException;
protected void exportVariables() 
throws JspException
{
pageContext.setAttribute( id, elementsList. getNext());
}
protected void clearProperties() 
{
id = null;
super.clearProperties();
}
protected void clearServiceState() 
{
elementsList = null;
}
}
First override point. The specialized tag must implement this method to create and set
an IterationSupport object The first method that tags can and must override is fetchIterationSupport().
This abstract method is the location wherein the overriding
tag should implement the creating and setting of the IterationSupport object and
any specialized iteration tag must provide such objects to make the generic infrastruc-ture
work. If problems rise within fetchIterationSupport(), it can throw a JspException
that the generic implementation will pass to the JSP runtime.
Second override point. The specialized tag may want to export additional objects The
second method that can be overridden is exportVariables(), which is where the
generic iteration tag exports the iterator (based in the id attribute). An overriding
tag may override this method to add more variables. For example, a certain tag
iterates a hash table and wants to export both the key to the table and the value itself.
In this case you would like to add the exportation of the value variable along with
the default iterator.
Override if you have additional attributes in the specialized tag (you probably do).
Override if you have additional service state in the specialized tag.
Listing 10.5 shows that the general structure of IterationTagSupport is very
similar to the one presented in SimpleForeachTag. The tag is merely a generic
iteration infrastructure with several methods to override as explained in the annotations.
Note also that IterationTagSupport extends our now familiar ExBodyTagSupport,
and therefore inherits its functionality.
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