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From Godmar Back <god...@gmail.com>
Subject Re: Q: how to obtain notification when a WebApp is unloaded/reloaded?
Date Fri, 23 Apr 2010 02:48:22 GMT
Following up on an earlier conversation about how to obtain notification
when a WebApp is unloaded/reloaded [1], I was told that registering a
ServletContextListener is the only possibility (that is, there is no runtime
API.)

However, registering a ServletContextListener doesn't work and leads to me
being unable to start the web application.

To recap: I have an application with context reloadable=true. I continuously
recompile this application during development. The application starts a
worker thread I need to shut down when a newly compiled version of the
application is loaded (that is, when new versions of the .class files show
up in the WEB-INF/classes directory.

Right now, I'm getting messages that the listener class is missing (it isn't
- it's just a new .class file after recompilation.)

Apr 22, 2010 10:31:25 PM org.apache.catalina.loader.WebappClassLoader
modified
SEVERE:     Resource
'/WEB-INF/classes/org/libx/editionbuilder/GCHelper$ShutdownListener.class'
is missing
Apr 22, 2010 10:31:25 PM org.apache.catalina.core.StandardContext reload
INFO: Reloading this Context has started
Apr 22, 2010 10:31:25 PM org.apache.catalina.core.StandardContext start
SEVERE: Error listenerStart
Apr 22, 2010 10:31:25 PM org.apache.catalina.core.StandardContext start
SEVERE: Context [/gbed] startup failed due to previous errors

My web.xml contains:
    <listener>

<listener-class>org.libx.editionbuilder.GCHelper$ShutdownListener</listener-class>
    </listener>

So - what is the correct way to register a listener that is executed when an
application is reloaded due to a recompilation?

Does Tomcat get confused when the listener class itself is part of the
to-be-reloaded web application?

Thanks.

 - Godmar


[1] See
http://mail-archives.apache.org/mod_mbox/tomcat-users/201004.mbox/%3Cu2s719dced31004132122oc656c456ya25e1a4ba4d4a81c@mail.gmail.com%3E
and the messages in that thread.

---------- Forwarded message ----------
From: Godmar Back <godmar@gmail.com>
Date: Wed, Apr 14, 2010 at 10:20 AM
Subject: Re: Q: how to obtain notification when a WebApp is
unloaded/reloaded?
To: Tomcat Users List <users@tomcat.apache.org>


On Wed, Apr 14, 2010 at 10:12 AM, Pid <pid@pidster.com> wrote:

> > For instance, if you look at
> >
> http://java.sun.com/products/servlet/2.3/javadoc/javax/servlet/ServletContextListener.html
> > it says:
> >
> > "To recieve (sic) notification events, the implementation class must be
> > configured in the deployment descriptor for the web application."
>
> Web applications are largely configured by the web.xml file in
> app/WEB-INF.  Servlets, listeners etc are all configured in it.
>
>
Thank you for your confirmation.  I thought I was going nuts, after having
waded through various *Facade classes, hoping to find an API method I could
call at runtime.

I have added a ServletContextListener, but it is very much a solution I
strongly dislike. The reason is that my application is layered on top of
another application (ZK), and I don't really want to touch web.xml.
'web.xml' describes how ZK is configured to run inside Tomcat or another
J2EE server. My applications runs on top of ZK, and having to go and made
changes to the underlying deployment descriptor violates basic principles of
layering.

It also creates a maintenance problem (unless an application can have
multiple .xml files that are combined to form a deployment descriptor).
Whenever ZK is updated, a new version of web.xml will be installed, and I
would then have to merge my <listener> declaration into the new file.

Just out of curiosity, what is the rationale for the (apparently deliberate)
lack of an runtime API?

 - Godmar

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