When I launch my site in hosted mode all variables that are declared as static preserves it's values among different requests. It works fine(?) when I upload it to google app servers.
I.e.
I have public static String action = "index"; and if I set its value to "someval" in another request it appears "someval" instead of "index".
Is it supposed to be so? Where could I search for this problem solution?
Using public static final String action = "index"; will ensure action's value doesnt change.
Instances of your app are kept around in memory for multiple requests, for efficiency. If an instance doesn't process any requests for a while, it gets shut down. So yes, you will see this behaviour, where changing a global in one request affects it in another. More importantly, the Java Servlet spec permits multiple threads of execution, so another request could be executing and modifying your globals at the same time as another request is.
In production, your app may be running simultaneously on many servers. Naturally, these don't have the same set of global variables, so changing the value in one will only affect subsequent requests to the same handler - not requests to other handlers.
In short, mutable globals in a webapp are generally a Bad Idea.