While building a web application using external jars you may frequently hit issues of having multiple jars containing different versions of same class in the class path and thus unable to run your app. In case of weblogic server you can always force the server to pick classes from your included libraries instead of from other places.
To do it just put the following lines in the weblogic.xml
In case weblogic.xml does not exist in your app then right click web project and click new. Go to deployment descriptors and choose weblogic.xml. This should generate a weblogic.xml file for you in case of jdeveloper.