RE: Classes in tomcat\server\lib folder sometimes are visible to web application

[Gary Hirschhorn <gh...@fetch.com>]

Hi Chuck,

Thank you very much for your feedback. Your comments about reflection
were right on the money and have helped me resolve the underlying issue.
It turns out that my initial assertion about our server/lib classes
being visible to our web app was incorrect -- it was just that our
custom classloader was SOMETIMES loading these classes from a different
location, and therefore no error was reported.  This situation was the
result of other problems we had, but your comments have helped me
develop a much cleaner solution.  

In order to help others, here is our resolution:

Background: We have a simple custom ClassLoader that uses a number of
helper classes to determine which jars on our system should be loaded
from some well-known location (instead of just using jars in the
WEB-INF/lib folder). We bundle the custom ClassLoader and the helper
classes in a jar together. The custom ClassLoader obviously needs to be
loaded by Tomcat, and the helper classes are used not only by our custom
ClassLoader, but by our web app as well. 

First attempt: Since the helper classes are also used by the web-app
itself, our initial plan was to place this jar in common/lib.  However,
this results in Tomcat errors because the custom class loader can't
exist in common/lib because it extends WebappClassLoader, which exists
in server/lib and is not visible to common/lib. 

Second attempt: So we moved the combined jar to server/lib. Tomcat now
uses the custom ClassLoader fine, but our web app can't see the helper
classes, because they are in the Tomcat-private shared/lib folder.

Third attempt: We place a duplicate of the combined jar in the
well-known location for our custom ClassLoader to find. For the most
part, this works okay, but it is very klugey, sometimes leads to errors,
and is hard to debug. This is because we now have the same helper
classes being loaded twice by different ClassLoaders. 

Final solution:  Break the custom ClassLoader and helper classes into
two jars.  The custom ClassLoader goes in server/lib, and the helper
classes go in common/lib.  We now no longer need to duplicate the jars
-- all classes are loaded by only one ClassLoader.  And the one part of
our system where the wep-abb needs to call a custom ClassLoader-specific
method is handled by reflection rather than by casting the ClassLoader
and making a direct call.

Once again, thanks for spending the time to respond so thoroughly and
accurately.

Regards,
Gary



-----Original Message-----
From: Caldarale, Charles R [mailto:Chuck.Caldarale@unisys.com] 
Sent: Friday, October 17, 2008 11:27 AM
To: Tomcat Users List
Subject: RE: Classes in tomcat\server\lib folder sometimes are visible
to web application

> From: Gary Hirschhorn [mailto:ghirschhorn@fetch.com]
> Subject: RE: Classes in tomcat\server\lib folder sometimes
> are visible to web application
>
> The Catalina ClassLoader in server/lib creates Webapp1 ClassLoader
> specifically for webapp1.

It's actually org.apache.catalina.loader.WebappLoader that creates each
webapp's classloader, but the result is the same as you surmised.

> the only way for this to work in practice is that these specific
> implementations are referenced in Webapp1 as Interfaces which are
> defined NOT in server/lib, but rather by jars higher up in hierarchy,
> such as common/lib.

Essentially correct; in addition to Interface classes, there are also
abstract classes in the top-level jars.

> a) if my webapp code has any specific reference (e.g. the
> Class name is in my code as a variable or parameter type)
> to a class in server/lib, I should get a ClassDefNotFoundError.

Correct.

> b) however, if my webapp has a reference to an Interface defined
> in common/lib and at runtime is handed a Class that implements
> this class, I may not get a ClassDefNotFoundError.

Not just may not, but should not.

Be aware that Tomcat uses a fair amount of reflection to avoid having
direct references at compile and load time; depending on what your code
needs, you may have to do something similar.

 - Chuck


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