Re: JSP 1.2 tag pooling Bug in utility:for which may affect (many?) other tags...

[Eduardo Pelegri-Llopart <Ed...@Sun.COM>]

Mac Ferguson wrote:

> The behaviour which results from tag instance pooling should be a
> consideration in the development and testing of all of the Jakarta
> Taglibs tags.

100% agree.

We spent a fair amount of energy in JSP 1.2 to clarify the semantics of
tag pooling because it can make a significant difference in some
applications. Also, I believe that Tomcat is about to acquire tag
pooling.

	- eduard/o 

> 
> 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.

Re: JSP 1.2 tag pooling Bug in utility:for which may affect (many?) other tags...

["Craig R. McClanahan" <cr...@apache.org>]

On Fri, 11 May 2001, Eduardo Pelegri-Llopart wrote:

> Mac Ferguson wrote:
> 
> > The behaviour which results from tag instance pooling should be a
> > consideration in the development and testing of all of the Jakarta
> > Taglibs tags.
> 
> 100% agree.
> 

Likewise.  I got bit on some incorrect assumptions when Struts tags
started getting used in containers that pool tag instances as well.

> We spent a fair amount of energy in JSP 1.2 to clarify the semantics of
> tag pooling because it can make a significant difference in some
> applications.

In fact, part of that clarification was some lifecycle diagrams that make
these issues much more understandable.  In the JSP 1.2 PFD2 spec, check
out pages 164, 168, and 178.

> Also, I believe that Tomcat is about to acquire tag
> pooling.

It's work in progress.

> 
> 	- eduard/o 
> 

Craig


> > 
> > 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.
>