RE: RE: RE: Problems with class loader

[Ju...@gmx.de]

Hi Charlie,

You are right saying that it is a consequence of how classes are loaded....
Still, this can be an error... ;-)

I think I could live with the fact that as long as the "container" of the
serialized classes was just a HashTable.  This would, of course, require
that I always keep the current version of the HashTable.class file in my
application (where is definitely does not belong: but we agree on this)..

What makes me feel a little unconfortable is the following: I can readily
do something like this:

   hashTable.add(new PrivateInfo());

but I cannot do:

   hashTable.add(new PrivateInfo());
   hashTable = deserialize(serialize(hashTable));

just a few lines later (which should almost be a no-op).  Though I can
explain this because I know what happens in detail, I have a hard time to
explain this to someone who is not into so many technical details as we
are...

What I would expect from this code is the following: when an Object is read
off a stream, the classloader in charge should be the application
classloader.  The same classloader should be loading and instantiating all
remaining classes.  This way, I would behave the same as having written the
instantiation code myself.

Having written this, I just realized that this could break code: I some
common library would depend on knowing that some of its dependend classes
are loaded by the same classloader, and the same class - well, not really
the same, but rather similar, but with the same name - would be also part
of the application, we would break existing code. :-(

Conclusion for the time being: the topic is (1) compex, (2) difficult, (3)
not so easy to come by as hoped/expected.  This is a small deployment
nightmare! :-)

I have to admit: I'm stuck and clue-less as to how to go on... :-(


Jürgen

--------------------------------------------------------------------------
From: ccox@cincom.com
To: tomcat-user@jakarta.apache.org
Date: Fri, 30 Aug 2002 11:24:14 -0400
Subject: RE: RE: RE: Problems with class loader

I guess I wouldn't consider it a bug, but rather a consequence of how
classes are loaded and which class loader holds the files in question.
But I see your point in that you don't want Hashtable(and more) in all your
webapps...

you can open a bug for this and one of the developers(I'm not a developer)
will look at it. Feel free to include my responses and I will help best I
can.

To restate, the classes stored in the Hashtable(and Vector,etc) would have
to be in the same classloader as the Hashtable(and Vector,etc) when
serializing a class containing a Hashtable.

Classloaders can only delegate requests to one 'parent'. So the common
classloader either delegates to the 'system' classloader(which it does) or
to the 'webapp' classloader(then which one of those?)

I guess you would need to provide more detail as to why this would still be
a problem for you. Other than having the java classes(Hashtable,etc) in
your
webapp instead of in the JVM's Bootstrap and having to maintain this with
each JVM upgrade, I don't see why this wouldn't work.

You *may* be able to move classes that will be in the hashtable to the
/jre/lib/ext directory for your JVM, but that's not ideal either as they
won't be able to see your other classes loaded by tomcat, but those classes
loaded by tomcat will see them.

Charlie

> -----Original Message-----
> From: Juergen.Praska@gmx.de [mailto:Juergen.Praska@gmx.de]
> Sent: Thursday, August 29, 2002 11:05 AM
> To: tomcat-user@jakarta.apache.org
> Subject: RE: RE: RE: Problems with class loader
>
>
> Hi Charlie,
>
> I gave it a try.  And guess what: it worked!  Now, in this
> very simple case
> one can argue that adding the collection classes to the
> application's lib
> directory is not a bad thing to do.  But what would one do if
> the scenario
> wasn't that simple? (Actually, it isn't!)
>
> Anyway, I consider this still a bug in the classloader hierarchy!
>
> So, where do we go from here?
>
>
> --- Jürgen
>
> NB: As for your concerns with loading the classes more than
> once: I think
> that's what classloaders are for: I would always let it be
> their problem... ;-)
>
> NB(2): Actually, the problem already shows up when an application is
> serializing a hashtable that contains an object of a class
> that is defined in the
> application only. - Yet one reason why I think that's a bug
> in the classloader
> hierarchy.
>
> ----------- original post ------------
>
> ok, StandardClassLoader means that the class is trying to be
> loaded from
> \tomcat\lib. If it were being loaded from \web-inf\lib, it would be
> WebappClassLoader throwing the error.
>
> >From your stack trace, it looks as if it is loading the Hashtable
> ok(through
> reflection), but it is the contents of the hashtable that
> can't be loaded.
> This would make sense since you put the Hashtable in \tomcat\lib.
>
> How about if you try to put Hashtable in the \web-inf\lib so
> that the first
> reflection call will find it in the current(webapp)
> classloader, then the
> second reflection call(from hashtable) will occur in your webapp's
> classloader. Of course being a java class, I'm not sure if it
> will have
> problems being(or can be) loaded multiple times.
>
> Charlie
>
> > -----Original Message-----
> > From: Juergen.Praska@gmx.de [mailto:Juergen.Praska@gmx.de]
> > Sent: Friday, August 09, 2002 9:07 AM
> > To: tomcat-user@jakarta.apache.org
> > Subject: RE: RE: Problems with class loader
> >
> >
> > see intermixed, too
> >
> > From: ccox@cincom.com
> > To: tomcat-user@jakarta.apache.org
> > Date: Fri, 9 Aug 2002 08:42:05 -0400
> > Subject: RE: Problems with class loader
> >
> > see intermixed
> >
> > >
> > > Hello,
> > >
> > > we have problems with the tomcat class loaders.
> > >
> > > scenario:
> > > Tomcat 4.0.4, jdk1.3
> > > 2 Applications
> > >
> > > App1:webapps/App1/WEB-INF/lib/x.jar
> > > App2:webapps/App2/WEB-INF/lib/x.jar (the same .jar-file)
> > >
> > > x.jar: a.class, b.class, c.class
> > >
> > > b.class has a Hashtable
> (com.sun.java.util.collections.Hashtable) as
> > > member-variable. (Hashtable is in the directory
> > $CATALINA_HOME/lib/).
> >
> > why is hashtable defined in $CATALINA_HOME/lib ? Is there a
> reason why
> > allowing the JVM to provide the java classes is a problem?
> > This shouldn't
> > really matter in this case, but could cause problems keeping
> > in sync with
> > the current JVM version.
> > --> yes, this jar-file doesn't have to defined there but this
> > is not the
> > problem
> >
> > > In the Hastable are Objects of the c.class.
> > >
> > > class a in App1 serialize class b and save it in persistence
> > > (poet-)classes
> > > in directory ($CATALINA_HOME/lib/).
> > so, this poet(I'm not familiar with it) is located in
> > $CATALINA_HOME/lib ?
> > Is this the class that tries to create an instance of b.class?
> > --> no, class a in x.jar tries to create an instance with the
> > ObjectInputStream
> >
> > If so this is your problem. Classes in the /tomcat/lib or
> > /common/lib can
> > *NOT* see classes in the /web-inf/lib directories. Try
> moving x.jar to
> > /tomcat/lib and see if it fixes your problem.
> > --> I can`t do it because there are dependances to other
> > classes in the
> > application-directory.
> >
> > Alternatively you could move poet to each web-inf/lib with
> > x.jar, but they
> > need to stay together.
> > --> this is also impossible because the poet-classes can only
> > be loaded from
> > one class loader.
> >
> > jürgen
> >
> > Charlie
> >
> > > In App2 class a load the serialized class from the
> > > persistence classes and
> > > deserialize it (--> class b).
> > > Then, a ClassNotFoundException (class c) is thrown (see
> > lower). Why???
> > >
> > > class b and the Hashtable are loaded, but not class c.
> > >
> > > It is as follows: ?
> > > The applicationClassLoader finds the class b and then the
> > > StandardClassLoader finds the Hashtable and then (however??) the
> > > StandardClassLoader try to load class c. And of course the
> > > class loader
> > > can't find the class c!
> > >
> > >
> > > thanks
> > > jürgen
> > >
> > >
> > >  at
> > > org.apache.catalina.loader.StandardClassLoader.loadClass(Stand
> > > ardClassLoade
> > > r.java:1127)
> > >  at
> > > org.apache.catalina.loader.StandardClassLoader.loadClass(Stand
> > > ardClassLoade
> > > r.java:992)
> > >  at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:313)
> > >  at java.lang.Class.forName0(Native Method)
> > >  at java.lang.Class.forName(Class.java:195)
> > >  at
> > java.io.ObjectInputStream.resolveClass(ObjectInputStream.java:654)
> > >  at
> > > java.io.ObjectInputStream.inputClassDescriptor(ObjectInputStre
> > > am.java:918)
> > >  at
> java.io.ObjectInputStream.readObject(ObjectInputStream.java:366)
> > >  at
> java.io.ObjectInputStream.readObject(ObjectInputStream.java:236)
> > >  at
> > java.io.ObjectInputStream.inputObject(ObjectInputStream.java:1186)
> > >  at
> java.io.ObjectInputStream.readObject(ObjectInputStream.java:386)
> > >  at
> java.io.ObjectInputStream.readObject(ObjectInputStream.java:236)
> > >  at
> > >
> >
> com.sun.java.util.collections.Hashtable.readObject(Hashtable.java:773)
> > >  at java.lang.reflect.Method.invoke(Native Method)
> > >  at
> > > java.io.ObjectInputStream.invokeObjectReader(ObjectInputStream
> > > .java:2213)
> > >  at
> > java.io.ObjectInputStream.inputObject(ObjectInputStream.java:1410)
> > >  at
> java.io.ObjectInputStream.readObject(ObjectInputStream.java:386)
> > >  at
> > > java.io.ObjectInputStream.inputClassFields(ObjectInputStream.j
> > > ava:2262)
> > >  at
> > > java.io.ObjectInputStream.defaultReadObject(ObjectInputStream.
> > > java:519)
> > >  at
> > java.io.ObjectInputStream.inputObject(ObjectInputStream.java:1411)
> > >  at
> java.io.ObjectInputStream.readObject(ObjectInputStream.java:386)
> > >  at
> java.io.ObjectInputStream.readObject(ObjectInputStream.java:236)

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