Traditionally you'd deploy a bunch of apps to one container (Tomcat, or whatever) and they'd all share the same JVM. I'm not sure if jars are shared in this way or not. I think OSGI was supposed to allow deploy apps to reuse the same jars and resources but I don't know how commonly that is deployed.

Now it is more common to run Tomcat or Jetty embedded for your app so that they are isolated.