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Date: Wed, 3 May 2006 16:32:13 +0200
From: "Leon Rosenberg" <rosenberg.leon@googlemail.com>
To: "Tomcat Users List" <users@tomcat.apache.org>
Subject: Re: Servlet inter-communication
In-Reply-To: <DDBBD1E00935D144AB9563D57EF98D622F7C07@raccoon.melandra.net>
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References: <DDBBD1E00935D144AB9563D57EF98D622F7C07@raccoon.melandra.net>

On 5/3/06, Peter Crowther <Peter.Crowther@melandra.com> wrote:
> > From: Leon Rosenberg [mailto:rosenberg.leon@googlemail.com]
> > Could you explain it a bit more? What's the difference whether I have
> > 10 instances of the same class or just one for reloading?
>
> In a Singleton:
>
> - the class holds a reference to the classloader;
>
> - the class holds a reference to the singleton instance;
>
> - the instance holds a reference to the class.
>
> My understanding is that classes cannot be GCed unless they have no
> instances, and classloaders are (obviously) not collected until they
> have no references from classes.  So singletons leave classes lying
> around in your JVM.  More annoyingly, classes always occupy PermGen
> space, so this junk class is now eating up precious space in PermGen -
> hence the possibility of an OOME if you reload your webapp too many
> times.  The solution is to implement appropriate listeners to make sure
> the instance is not referenced by the class before the webapp is
> unloaded.
>
> That's a quick recap from memory.  I thoroughly expect Chuck to jump
> into the conversation and point out where I've misinterpreted :-).

Ok, what you mean is, that the "instance" variable of the singleton is
static, and therefore can't be collected, because the class references
the object and the object references the class. I don't think that the
classloader plays a role here. Shouldn't the gc be able to resolve
cyclic references?
Even if it doesn't, is this a real problem? I mean reloading a webapp
in production environment is a greater nogo than using a singleton,
isn't it?

regards
Leon

>
>                 - Peter
>
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