Re: Singletons in Tomcat(6.0)
everything below is just my knowledge :-)
comments inlined
On Tue, Dec 1, 2009 at 2:01 PM, jkv j.kumara...@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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RE: Singletons in Tomcat(6.0)
From: jkv [mailto:j.kumara...@gmail.com] Subject: Singletons in Tomcat(6.0) Is using singleton patterns in Tomcat (in servlets programming and deploying them in tomcat) a really bad idea?? No, it's fine. One really useful post is http://wiki.apache.org/tomcat/OutOfMemory Unfortunately, the section on singletons in that page is completely bogus. Keeping an instance of a class in a static variable of that class does not inhibit garbage collection in any way in a current JVM. I suspect the author was using a GC method in an older JVM where class unloading was disabled, or he simply didn't wait long enough for the class to be discarded (calling System.gc() won't do it). The comment about starting extra threads is also erroneous. As long as your webapp manages the threads by using a ServletContextListener to shut them down when the webapp is stopped, there's no problem. - Chuck THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY MATERIAL and is thus for use only by the intended recipient. If you received this in error, please contact the sender and delete the e-mail and its attachments from all computers. - To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org For additional commands, e-mail: users-h...@tomcat.apache.org
Re: Singletons in Tomcat(6.0)
Caldarale, Charles R wrote: From: jkv [mailto:j.kumara...@gmail.com] Subject: Singletons in Tomcat(6.0) Is using singleton patterns in Tomcat (in servlets programming and deploying them in tomcat) a really bad idea?? No, it's fine. One really useful post is http://wiki.apache.org/tomcat/OutOfMemory Unfortunately, the section on singletons in that page is completely bogus. Deleting it now... I'll add some brief notes on some real problems. Mark - To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org For additional commands, e-mail: users-h...@tomcat.apache.org
