Custom ClassLoader - trying to use different version of SNMP library than WebLogic Server 8.1 uses

Question

Problem: my J2EE ear file uses the AdventNet third-party library to do
SNMP work. WebLogic Server 8.1 also uses this AdventNet third-party
library to do its own SNMP work. The problem is the version used by
WebLogic 8.1 is older than the version I use, so my code tries to run
and finds the wrong version of the library that WebLogic 8.1 has supplied.
Possible solution: I plan to have a custom classloader (derived from
java.lang.ClassLoader) that gets hooked in to my threads using
Thread.setContextClassLoader(). This custom classloader would look for
the AdventNet library jars in a spot I specify so it would find the
correct ones. Since I want to change the usual classloader behavior of
"look in parent classloaders first, then child classloader" to "look in
child classloader first, then in parent classloaders" I can't just
override ClassLoader.findClass(). I, at a minimum, have to override
ClassLoader.loadClass() to look in my classloader first. I have
questions about this:
1. What other methods do I have to override? For example, which of the
resource-related methods do I need to override so that resources are
searched for first in my classloader then in parent classloaders?
2. I was thinking of using a URLClassLoader as a helper to my
classloader, invoking its method(s) from my custom classloader's
method(s) to actually load the classes from URLs that are not on the
standard classpath. I was planning to set it up with a custom parent
classloader that can't find anything, so that the helper URLClassLoader
would only ever find classes/resources in the URLs I provide to it. Does
this approach make sense? Have you seen anything like this done before?
3. Is there any way around this problem besides a custom ClassLoader? A
buggy custom ClassLoader would have problems which AFAIK would be
difficult to track down as ClassLoader problems.
Thanks in advance for any help you can provide.

Answer 1

Alvin wrote:
Hi,
I am experiencing the same problme and
even I tried to put the AdventNet jar files
before weblogic.jar I still cannot get it
to work
Would you help me out here.
Thanks,
-AlvinI finally figured out a way to do it using a custom classloader that
looked in the directory where I kept the version of AdventNet I wanted
before looking in the normal places classloaders look.
How it works is I hook two classloaders at the bottom of the chain of
classloaders. My custom BlockingClassLoader is hooked as a child of the
normal chain of classloaders, then a URLClassLoader is hooked under
that. The BlockingClassLoader's findClass method checks if the class
starts with a package prefix I want to control (like "com.adventnet.").
If it does, then it acts like it can't find the class. This makes its
child, the URLClassLoader, try to find the class. The URLClassLoader
is given the URL of places to look for classes that match the directory
I keep the version of AdventNet I want in. When I load a class, I
directly tell the URLClassLoader to load it so that it is loaded as I want.
There's more to it than this. I had to override some other methods in
my BlockingClassLoader and do some other stuff. I'm not sure if I can
share the source code, as it was developed on company time and thus is
owned by the company. Feel free to ask questions though.
I haven't tried this solution very long (basically just unit tested it)
but it looks promising.