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From Leon Rosenberg <rosenberg.l...@googlemail.com>
Subject Re: Singletons in Tomcat(6.0)
Date Tue, 01 Dec 2009 14:20:24 GMT
everything below is just my knowledge :-)
comments inlined

On Tue, Dec 1, 2009 at 2:01 PM, jkv <j.kumaravel@gmail.com> wrote:
>
> Hello,
>
> Is using singleton patterns in Tomcat (in servlets programming and deploying
> them in tomcat) a really bad idea?? I came accross many forum posts and
> wikis that warn about of OOM errors. One really useful post is
> http://wiki.apache.org/tomcat/OutOfMemory
> http://wiki.apache.org/tomcat/OutOfMemory  and this is really worth taking a
> look!!
> We are having a web application and have four or five singleton patterns,
> they are really necessary as singletons patterns, such as Xml parsers and
> Applications properties reader. I have some questions when they say. "server
> being unable to reclaim the space for the entire webapp class loader".
>
> 1) Will I be getting OOM errors only when I redeploy a war file (with
> singleton patterns) several times without re-starting my Tomcat? Or I will
> be getting the OOM errors even if I didnot redeploy the war file and the
> server crashes after some days??
>

Yes. The OOME are only redeploy related, they don't affect normal
production sites.

> 2) I dont think we can GC a conventional singleton pattern by using a
> ServletContextListener, as the private instance variable is hard coded and
> we cannot access it from outside(and set it to null) as its never visible to
> the outside world unless we try using reflection??

Use the 'new' singleton pattern with enums. This should be free of OOMEs:

public enum MySingletonClass{
   INSTANCE;
   //functionality....
}

access it simply by:
MySingletonClass.INSTANCE.method()...

regards
Leon

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