I've used both webapp2 + GAE for python and a number of Java/JEE webapp frameworks. The python WSGI framework could keep users logged in while I redeploy the app while none of the Java web framework that I tried could do it. If I redeploy, users of the Java webapp will get logged out but if I use GAE for python then a redeployment doesn't log users out. Is this a general "feature" of python vs Java webapps and is this true in general or just a coincidence that the ones I tried had these features? Is there a name for this feature, ability to keep users logged in between redeployments and also keeping caches (e.g. memcache) without getting the cache erased for updating the app=
You're not looking hard enough. Pretty much all java servers that I know of are able to persist sessions. Its just a case of reading the docs and setting up the config.
See this answer for tomcat.