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From David Smith <>
Subject Re: Class loading from within a servlet: interface definition not found
Date Mon, 01 May 2006 12:11:33 GMT
I think you are assuming the classloader
org.something.DatabaseRequestHandler is using is the webapp's local
classloader.  I doubt this is the case.  Those with more knowlege of the
classloader architecture may chime in, but the classloader being used is
most likely the shared classloader instead.  If possible, move the
handler.jar from shared/lib to WEB-INF/lib of your webapp and the
problem should go away.


Christopher Piggott wrote:

>I have something happening with class loading from within a server, and I don't understand
>My servlet defines some classes called:
>	org.something.server  -- the class that extends HttpServlet
>	org.something.DatabaseRequestHandler -- an interface
>When you make a request to the server, it figures out what handler class to load.  Then
it attempts to load it:
>	Class handlerClass = Class.forName(handlerClassNameString);
>QUESTION: looking at the little diagram in the Tomcat 5.5 Class Loader HOW-TO, when I
do the above (Class.forName) I am using the Webapp class loader, correct?
>The handler classes are all in handlers.jar which is in $CATALINA_BASE/shared/lib.  The
actual exception that I get is:
>	2006-04-28 17:05:55 StandardWrapperValve[xdb]: Servlet.service() for servlet xdb threw
>	java.lang.NoClassDefFoundError: org/something/DatabaseRequestHandler
>It took me a few minutes to realize that this was not a ClassNotFound exception, but something
else.  Reading docs I figured out that the most probably cause is that the servlet's classloader
was able to find the handler class, but then it failed to load one of the things it depended
on, specifically it was able to find handlerClass but not the interface class that it depends
>Trouble is: the interface .class file is right there in WEB-INF/classes/com/something/DatabaseRequestHandler.class
just like it should be.  So, I am stumped why the class loader would be able to find the handler
class in $CATALINA_BASE/shared/lib but NOT the interface that it implements which is in WEB-INF/classes.
>At the suggestion of a nice gentleman on freenode, I tried turning on the -verbose:class
option in the jvm, and watched it log some interesting stuff to catalina.out.  Unfortunately
it doesn't shed any light on what is happening, beyond the exception and stack trace in the
error log.  I don't want to post the whole stack trace (unless someone thinks it would help
to see it) but I will say that it includes catalina.loader.WebappClassLoader as well as catalina.loader.StandardClassLoader
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