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From Remy Maucherat <r...@apache.org>
Subject Re: svn commit: r491619 - in /tomcat/tc6.0.x/trunk: java/org/apache/catalina/core/ApplicationDispatcher.java webapps/docs/changelog.xml
Date Wed, 03 Jan 2007 13:13:53 GMT
Mark Thomas wrote:
> Remy Maucherat wrote:
>> markt@apache.org wrote:
>>> Author: markt
>>> Date: Mon Jan  1 11:31:29 2007
>>> New Revision: 491619
>>>
>>> URL: http://svn.apache.org/viewvc?view=rev&rev=491619
>>> Log:
>>> Port RD thread safety patch from TC5
>> I don't see any justification for this at the moment. Any further info ?
> 
> This started with http://marc.theaimsgroup.com/?t=116558790000003&r=1&w=2
> 
> The short version of the above thread is that a RD was obtained using
> ServletContext.getNamedDispatcher() and then re-used. When multiple
> requests used the RD at the same time, the request objects got mixed
> up (since the RD is not thread safe) which in turn lead to an
> occasional ISE when both threads tried to call getParameterMap() at
> the same time on the same Request object.
> 
> There was some debate whether or not the spec intended RDs to be
> re-used in this way. I couldn't see anything that explicitly
> dis-allowed it; hence I implemented the patch.

I remember the issue (not from this thread).

I don't understand how a RD could be be used by multiple threads, as no 
objects provided by the servlet container are thread safe. If you add 
some sort of thread safety somewhere, then users will demand that all 
the other objects to be thread safe, which is not going to happen. If 
they want to reuse the request dispatcher somehow and use it without 
checking, then they should sync on it. I thought the pattern was to 
obtain a RD every time (which is slower, but that's what I saw pretty 
much everywhere).

> There are a couple of places where the patch could be cleaner but I
> opted to make the changes in a consistent way throughout the code so
> it was clearer what the patch was doing. Cleaning this up is on my
> list of things to do but I wanted to give people a chance to consider
> the patch as-is first.

Rémy


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