10. 6 JSP1.2 and IterationTag
This chapter created iteration tags using BodyTag, but using BodyTag for iteration
includes within it a hidden performance hazard due to its buffering overhead.
As noted in chapter 6, when using BodyTag the JSP runtime places the body into
an intermediate buffer (the BodyContent object) and leaves it up to the tag to actually
do something with the results of the body execution. In our iteration tags, what
we did with these results was to copy them into the response flowing to the user,
thereby suffering needless buffering overhead. Granted, using the buffer made it
possible to develop ease of use techniques such as field placement, but if the JSP file
developer decides not to use field placement, why suffer the performance penalty?
10.6.1 IterationTag
This performance penalty was solved in JSP1.2 with the introduction of the IterationTag,
which can repeatedly execute its body for as long as it returns EVAL_BODY_AGAIN
from doAfterBody(). Hence, all we need do is take the iteration
framework that was developed in this chapter and have it work with the JSP1.2 IterationTag.
All our iteration-related code was part of a single class, IterationTagSupport,
which is where we implemented our doStartTag(), doBeforeBody(), and doAfterBody().
All the tags that work with the iteration framework have only to
extend IterationTagSupport and provide an implementation for a few methods.
At this point, we only need to port IterationTagSupport, which requires the
following steps:
- Remove any code portion related to the field placement (no buffering means
no field placement). .
- Return
EVAL_BODY_INCLUDE from doStartTag() so that the JSP runtime
includes the body's results into the stream flowing to the client.
- Export variables in
doStartTag() instead of doBeforeBody(), since IterationTag
does not have a doBeforeBody() method. .
- Return
EVAL_BODY_AGAIN from doAfterBody() as per the JSP1. 2 specification.
When we have finished, our iteration tags can take advantage of the IterationTag
interface and its improved performance. Listing 10.19 presents such an adaptation
of IterationTagSupport to the JSP1.2 IterationTag interface.
Listing 10. 19 IterationTagSupport adapted to the JSP1. 2 IterationTag
package book.iteration;
import book.util.LocalStrings;
import book.util.ExTagSupport;
import book.util.StringUtil;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.JspException;
public abstract class IncludedIterationTagSupport
extends ExTagSupport {
static LocalStrings ls =
LocalStrings.getLocalStrings(IncludedIterationTagSupport.class);
protected IterationSupport elementsList = null;
protected Object current;
public int doStartTag()
throws JspException
{
fetchIterationSupport();
if( elementsList.hasNext()) {
exportVariables();
return EVAL_BODY_INCLUDE;
}
return SKIP_BODY;
}
public int doAfterBody()
throws JspException
{
if(elementsList.hasNext()) {
exportVariables();
return EVAL_BODY_AGAIN;
}
return SKIP_ BODY;
}
protected abstract void fetchIterationSupport()
throws JspException;
protected void exportVariables()
throws JspException
{
current = elementsList.getNext();
pageContext.setAttribute( id, current);
}
protected void clearProperties()
{
id = null;
super.clearProperties();
}
protected void clearServiceState()
{
elementsList = null;
current = null;
}
}
IncludedIterationTagSupport presented in Listing 10.19 is much less complicated
than IterationTagSupport. This simplicity comes partially from the removal
of the field placement code, and partially from the fact that we no longer need to
handle the BodyContent buffer and write its content back to the user.
To summarize, all tags developed in this chapter should be able to run unmodified
in JSP1.2 (as BodyTag is supported there). However, tags wishing to take advantage
of the new IterationTag interface should abandon the field placement as a
means of populating the iterator's fields, since the tags can then extend our new IncludedIterationTagSupport
and gain performance improvements.
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