I have something happening with class loading from within a server, and I don't
understand it.
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 exception
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 on.
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
--Chris
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