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Subject: RE: WebappClassLoader - don't understand what's happening
Date: Thu, 2 Jun 2005 14:50:28 +0200
Message-ID: 
 <52D9CED6CB3BFC4FA640275E0114E231037A47A2@BUDMLVEM05.e2k.ad.ge.com>
Thread-Topic: WebappClassLoader - don't understand what's happening
Thread-Index: AcVnb0JVDUAmT/C/RsmAnYJuo3swGAAAAzjA
From: "Herrmann, Sascha \(GE Healthcare\)" <Sascha.Herrmann@med.ge.com>
To: "Tomcat Users List" <tomcat-user@jakarta.apache.org>

> Any library having static methods like
> SomeLibraryClass.setXYZ(xyz);
> SomeLibraryClass.getXYZ(xyz);


Wow, wow, wow .... . You mean any class that has static methods gets =
shared??
Any class that calls static methods? Could you explain that a bit =
further, please?

> so i doubt usefull informations comes from it.=20

It works for all the other methods I put this code in. It seems that =
Tomcat assigns the CL to thread when it's
fetched from the pool.

It's definitely the case. We have a static log variable in the classes. =
It holds the log4j logger reference.
All the debug output from the class goes into the log file of the =
application that started first (the one that loaded the class).
Even the output when I use the app that was started second. That's how I =
noticed in the first place.

Are you familiar with the classloading stuff?

I was wondering ... when the WebappClassLoader doesn't find the class in =
its path, and then delegates the loading to the shared classloader ...
and when then the shared classloader loads the class .... who resolves =
the dependecies of this loaded class? Does the WebappClassLoader get a =
shot first,
or is it still the shared classloader?

What if the shared classloader loads a class, and then cannot resolve a =
referenced class (because it's in one of the app's WEB-INF\lib =
directories) ...
how does he determine which child WebappClassLoader he is to use? And =
who is registered in the JVM then as initiating classloader?

I am asking because we have classes in our shared\lib directory. Those =
classes use a factory pattern to instantiate classes (that can be in the =
WEB-INF\lib dirs). Now, I wonder how these classes are loaded, that the =
factory class in shared\lib needs?

Or am I thinking in the wrong direction? Does the WebappClassLoader =
always try first?

Damn, I wish I would know this classloading stuff better.

Do you have an idea why I cannot get logging output from the =
WebappClassLoaderS?

Sascha


> -----Original Message-----
> From: delbd [mailto:delbd@oma.be]=20
> Sent: Thursday, June 02, 2005 2:32 PM
> To: Tomcat Users List
> Subject: Re: WebappClassLoader - don't understand what's happening
>=20
>=20
> Le Jeudi 2 Juin 2005 13:49, Herrmann, Sascha (GE Healthcare) a =E9crit =
:
> > Thanks for your reply.
> >
> > It could be. How is a class "pushed" to a shared library?
>=20
> Any library having static methods like
> SomeLibraryClass.setXYZ(xyz);
> SomeLibraryClass.getXYZ(xyz);
>=20
> >
> > I used the following code:
> >
> >       System.out.println( "BoneDBTransactionImpl2  LOADING=20
> > ***************************************" ); System.out.println(=20
> > "Thread Classloader is: " +=20
> > Thread.currentThread().getContextClassLoader() );=20
> System.out.println(=20
> > "Hash: " +
> > Thread.currentThread().getContextClassLoader().hashCode() );=20
> > System.out.println( "Class: " +
> >=20
> Thread.currentThread().getContextClassLoader().getClass().getName() );
> > System.out.println( "---------------------------     " );
> >       System.out.println( "Class Classloader is: " +
> > BoneDBTransactionImpl2.class.getClassLoader() );=20
> System.out.println( "Hash:
> > " + BoneDBTransactionImpl2.class.getClassLoader().hashCode() );=20
> > System.out.println( "Class: " +
> >=20
> BoneDBTransactionImpl2.class.getClassLoader().getClass().getName() );=20
> > System.out.println( "BoneDBTransactionImpl2  LOADING=20
> > ***************************************" );
> >
>=20
> Tss tss tss, the Threads instances are started by=20
> catalina-http and shared inside a Thread pool amongst tomcat=20
> application if am not wrond, so i doubt usefull informations=20
> comes from it.=20
> Only usefull information should comes from
> BoneDBTransactionImpl2.class.getClassLoader() as this is the=20
> classloader which will load any class needed by=20
> BoneDBTransactionImpl2.
>=20
>=20
> > I put this into a static initializer block of a class that=20
> is in a jar=20
> > in both WEB_INF\lib directories. I also added this code the=20
> > constructor of that class. And to make sure to see what=20
> loader belongs=20
> > to what app I also added the code to some methods of=20
> classes that are=20
> > either in app A or in app B and not in both.
> >
> > I start app A. The output shows that
> >
> > - the class is loaded in app A by WebappClassLoader with hash A
> > - the instance is created in app A using the Class loaded by=20
> > WebappClassLoader with hash A. - Thread context classloader is=20
> > WebappClassLoader with hash A.
> >
> > Then I start app B. The output shows that
> >
> > - the instance is created in app B using the Class loaded by=20
> > WebappClassLoader with hash A - Thread context classloader is=20
> > WebappClassLoader with hash B.
>=20
> Strange indeed, so you mean when starting app B, the static block of
> BoneDBTransactionImpl2 is never executed? If that's the case=20
> this indeed mean the class was already loaded before. Either=20
> this mean you have another lib in webapp B requiring=20
> BoneDBTransactionImpl2 and you didn't see the static init=20
> message 50 lines before ;), either you indeed have something shared.
>=20
> One easy way to check
> In webapp A, create a servelt which will set to 1 a static boolean in
> BoneDBTransactionImpl2 and in webapp B, set another servlet=20
> which will read the value of this boolean.=20
> first access B servlet to ensure boolean is set to false=20
> acces A servlet to switch boolean to true re-access B servlet=20
> to check if boolean has switched value in B webapp
>=20
> If that's the case, this really mean class is shared by=20
> webapp class loaders, serious problems ahead for tomcat team ;)
>=20
>=20
>=20
> >
> > The class exists in jar files in both WEB-INF\lib=20
> directories, but not in
> > any other lib directory. And - if the class was loaded by a shared
> > classloader, shouldn't the xxx.class.getClassLoader() return the
> > StandardClassLoader?
> >
>=20
> dunno
>=20
> > I must admit that I am not too familiar with the deeper=20
> concept of J2EE
> > classloading apart from the information found on some web pages.
> >
>=20
> me too ;) however, separation in webapps mean every ressource=20
> in a .war is not=20
> to be shared with any other webapp.
>=20
>=20
> > So, if the class indeed is loaded by WebappClassLoader A,=20
> how can it be
> > "pushed" to become a shared one? And how can I prevent this=20
> from happening?
> > Or am I making a mistake reading the output? I am sure that=20
> the class is
> > not loaded again.
>=20
> see message above, i just didn't know how you concluded the=20
> class was shared,=20
> one common error is to give an instance of some object to a=20
> shared library=20
> and retreive it in another webapp, not understanding this=20
> object you are=20
> manipulating is a stranger ;) not your case scenario you describe.
>=20
> >
> > I checked the sources in Apache's CVS. I noticed that there=20
> is quite some
> > debugging put into WebappClassLoader. But I can't get my=20
> server to log
> > these messages. I have the log4j and commons logging jars=20
> in my common\lib
> > directory and my log4j.properties (root logger set to debug) in my
> > common\classes directory. I also set the debug to "4"=20
> everywhere in my
> > server.xml. I see some catalina debug messages, but I cannot see the
> > messages from the WebappClassLoader. So what am I doing=20
> wrong? I believe
> > the debug output would be very helpful in understanding the problem.
>=20
> Yes have same problem here to get usefull log messages from=20
> Realms. Seems you=20
> have to modify directly the logging level in your java home,=20
> logging.properties (and setting it to debug for whole jvm is=20
> VERY verbose)
>=20
> >
> > Sascha
> >
> > > -----Original Message-----
> > > From: delbd [mailto:delbd@oma.be]
> > > Sent: Thursday, June 02, 2005 12:33 PM
> > > To: Tomcat Users List
> > > Subject: Re: WebappClassLoader - don't understand what's happening
> > >
> > > This indeed should not happen as this would break sun specs.
> > > I would be quite surprised it it was at all possible :/ How
> > > did you conclude the webappclassloader is sharing the class
> > > between webapps in your case?
> > >
> > > Are you sure you are not mistakenly using a shared class?
> > >
> > > Not only does every webapp has his very own
> > > webappclassloader, but if you reload the webapp (using the
> > > manager/html interface) this should also create a new
> > > webclassloader and reload all classes (did you check it was
> > > the case?).
> > >
> > > Am also using tomcat 5.5.7 here, with jdk 1.4.2_05-b04 and we
> > > didn't notice such a problem. We can deploy several webapps,
> > > all using a library in WEB-INF/lib which is crating
> > > singletons. All webapp has it's own sets of singletons.
> > >
> > > Could it be that you are in fact doing such scenario?
> > >
> > > - create an object of class A in webapp1 using webappclassloader1
> > > - push this object to some library shared between webapplications
> > > - retrieve the object from shared library in webapp2 (as this
> > > is via a shared library, it's possibile to do and you and up
> > > manipulating in webapp2 an object who's classloader is
> > > webappclassloader1)
> > > - in webapp2, calls something like a.createB() to get an
> > > object of class B and be surprised that the class of B was
> > > created using webappclassloader1
> > >
> > > Le Jeudi 2 Juin 2005 10:59, Herrmann, Sascha (GE=20
> Healthcare) a =E9crit :
> > > > We are facing a problem with Tomcats WebappClassLoader.
> > > > We deploy several of our own applications on one Tomcat server.
> > > >
> > > > It seems that the WebappClassLoaderS are not isolated but share
> > > > classes instead. Why would that be?
> > > >
> > > > Judging from the (well, outdated) classloader=20
> documentation, this
> > > > should never happen.
> > > > If an application tries to use a class, and the class is in
> > >
> > > a jar in
> > >
> > > > its WEB_INF\lib directory, then the class should be=20
> loaded by the
> > > > application's very own WebappClassLoader. It should not be
> > >
> > > available
> > >
> > > > to other WebappClassLoaderS, isn't that correct? I quote:
> > > >
> > > > "All unpacked classes and resources in the /WEB-INF/classes
> > >
> > > directory
> > >
> > > > of your web application archive, plus classes and=20
> resources in JAR
> > > > files under the /WEB-INF/lib directory of your web application
> > > > archive, are made visible to the containing web
> > >
> > > application, **but to no others**."
> > >
> > > > Instead of sharing the class, the second WebappClassLoader
> > >
> > > should load
> > >
> > > > the class again, shouldn't it?
> > > > So why does a WebappClassLoader here re-use a class already
> > >
> > > loaded by
> > >
> > > > the WebappClassLoader of another application?
> > > >
> > > > We're running 5.5.7 on JDK 142_08.
> > > >
> > > > Thanks in advance!
> > > >
> > > >
> > > > Sascha Herrmann
> > > > Engineering (Java)
> > > > GE Healthcare
> > > > Technologies
> > >
> > >=20
> ---------------------------------------------------------------------
> > >
> > > > To unsubscribe, e-mail:=20
> tomcat-user-unsubscribe@jakarta.apache.org
> > > > For additional commands, e-mail:=20
> tomcat-user-help@jakarta.apache.org
> > >
> > > --
> > > David Delbecq
> > > Royal Meteorological Institute of Belgium
> > >
> > > -
> > > Is there life after /sbin/halt -p?
> > >
> > >=20
> ---------------------------------------------------------------------
> > > To unsubscribe, e-mail: tomcat-user-unsubscribe@jakarta.apache.org
> > > For additional commands, e-mail:=20
> tomcat-user-help@jakarta.apache.org
> >
> >=20
> ---------------------------------------------------------------------
> > To unsubscribe, e-mail: tomcat-user-unsubscribe@jakarta.apache.org
> > For additional commands, e-mail: tomcat-user-help@jakarta.apache.org
>=20
> --=20
> David Delbecq
> Royal Meteorological Institute of Belgium
>=20
> -
> Is there life after /sbin/halt -p?
>=20
> ---------------------------------------------------------------------
> To unsubscribe, e-mail: tomcat-user-unsubscribe@jakarta.apache.org
> For additional commands, e-mail: tomcat-user-help@jakarta.apache.org
>=20
>=20

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