Hi folks,
I'm hoping to open up some discussion on an important subject. I recently
D/L'ed the new beta of Resin servlet engine (2.0.b2), which implements a
custom-tag instance-pooling scheme, and right away I found that some of my
JSPs which use the utility:for tag were behaving unexpectedly. I had a test
page which called utility:for 3 times in succession:
<util:for iterations="3" begin="0" varName="index">
<%= index%> : <br />
</util:for>
<br />
<util:for iterations="3" begin="0" varName="index">
<%= index%> : <br />
</util:for>
<br />
<util:for iterations="3" begin="0" varName="index">
<%= index%> : <br />
</util:for>
which previously looped from 0-2 three times, produced the following output
under Resin:
0
1
2
3
4
5
on examining the source for the tag and several explanations of the
tag-pooling and spec from the creator of Resin I found the following
problems. The tag initializes its state when it's instance variables are
declared as follows:
public class ForTag extends BodyTagSupport {
private int iterations;
private String varName = "_count";
private int begin = 0;
etc...
Theoretically tags should be stateless, so these default values should be
being assigned in the doStartTag() method not in instance variable
declarations. Another part of the spec which was pointed out to me indicates
that successive calls to the same tag with the same attribute values may not
trigger the setter methods, here's the quote from the spec:
"From the spec, JSP 10.1, "Once properly set, all properties are expected
to be
persistent, so that if the JSP container ascertains that a property has
already
been set on a given tag handler instance, it needs not set it again.""
which once again implies that if any instance-specific initialization needs
to be done in a tag, it should be done in doStartTag() or some submethod
which will be called on every use of the tag instance.
My first thought was that release() should be implemented to reset state,
but the comments for the lifecycle diagram on page 165 of the spec indicate
that release is "intended to be for relleasing long-term data" and there is
no guarantee that properties are retained or not.
I know that this problem may seem remote and Resin-specific right now, but I
suspect that tag pooling will show up in the next release of almost every
servlet engine, as it is the next logical efficiency which can be optimized
in the servlet/JSP architecture. The behaviour which results from tag
instance pooling should be a consideration in the development and testing of
all of the Jakarta Taglibs tags.
