RE: DO NOT REPLY [Bug 16749] - Struts EL tag handlers cannot be reused by containers

["PILGRIM, Peter, FM" <Pe...@rbos.com>]

> -----Original Message-----
> From: Craig R. McClanahan [mailto:craigmcc@apache.org]
> On Tue, 4 Feb 2003 bugzilla@apache.org wrote:
> 
> >
> > Struts EL tag handlers cannot be reused by containers
> >
> 
> It sounds like these tags definitely have a problem.  There's 
> been some
> recent discussion on TOMCAT-USER about designing tags that 
> work reusably
> as well.
> 
> The bottom line:
> 
> * The container can reuse instances of tag handlers
>   when it sees that exactly the same set of attributes
>   is used (not necessarily the same values).
> 
> * This kind of reuse can happen whether or not the
>   container implements "tag pooling".
> 
> * When a tag handler instance is reused, the JSP page
>   compiler can decide if it has already set a particular
>   property on that instance, and omit the second set call.
>   For example, in:
>     <foo:bar a="1" b="2/>
>     <foo:bar a="1" b="3"/>
>   The second setA() call can be omitted, since the container
>   knows that it already called setA("1") the first time.
> 

Yep!

String a;
public void setA( String a ) {this.a = a;}
public String getA() { return a;}

> * If a tag instance is going to be reused, the doEndTag()
>   call on the first use is going to be followed by the
>   doStartTag() call of the second use.  Any per-use cleanup
>   activity needs to happen at the end of doEndTag().
> 
> * The net effect of all this is an important restriction -- it's
>   not legal for a tag handler to modify the values stored by
>   the setter calls from the page, *anywhere* in the path from
>   doStartTag() through doEndTag().

Are the many tags that are badly behaved. This make modification
on the value `a' in doStartTag() or doEndTag()

If the tag is doing this

	a += "foo ("+b+") bar";

then it cannot work as reusable tag.

To my knowledge, many tags are doing this. I have never written
tags that modify the input attributes. If I had I would use
automatic variables in local scope. 

> * The container will call release() after it has used the
>   instance for the last time.

--
Peter Pilgrim,
Struts/J2EE Consultant, RBoS FM, Risk IT
Tel: +44 (0)207-375-4923




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