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From "Caldarale, Charles R" <Chuck.Caldar...@unisys.com>
Subject RE: Exeptions after upgrading to tomcat 7
Date Sun, 26 May 2013 14:05:56 GMT
> From: Leon Rosenberg [mailto:rosenberg.leon@gmail.com] 
> Subject: Re: Exeptions after upgrading to tomcat 7

> First we are talking about a ThreadLocal, not Thread.

Read the logs again; besides the ThreadLocal abuse, there are two threads left lying around:

> > SEVERE: The web application [/solr] appears to have started a thread named
> > [localhost-startStop-1-SendThread(zookeeper2:2181)] but has failed to stop
> > it. This is very likely to create a memory leak.
> > SEVERE: The web application [/solr] appears to have started a thread named
> > [localhost-startStop-1-EventThread] but has failed to stop it. This is very
> > likely to create a memory leak.

> Maybe I'm missing something here, but I never seen how ThreadLocal does
> real harm, neither I can imagine it, please enlighten me ;-)

Abusing ThreadLocal (which is what's going on here) can have very serious side effects, depending
on what's stored via the reference.  For example, if something specific to the request or
session is kept in the ThreadLocal and not refreshed on the next unrelated request, inadvertent
data sharing and potential corruption can occur.  During one of the Tomcat get-togethers,
we had discussed associating ThreadLocal instances with sessions rather than Threads, but
that would have been a good bit of work.  We settled on just purging such threads from the
pool.

Libraries written for non-thread-pool environments should be documented as such, and used
only with great caution in server environments.

 - Chuck


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