That's really not that hard to find on the Internet. It says
"The classloader that a container uses to load a servlet in a WAR must allow the developer to load any resources contained in library JARs within the WAR
following normal J2SE semantics using getResource. It must not allow the WAR to override J2SE or Java servlet API classes. It is further recommended that the loader not allow servlets in the WAR access to the web container抯 implementation classes. It is recommended also that the application class loader be implemented so that classes and resources packaged within the WAR are loaded in preference to classes and resources residing in container-wide library JARs."