Google App Engine - InvalidClassException when logging in

Question

I'm using a custom class UserProfile to manage my sessions in Google app engine and i get the error (when users access the app's default servlet):

javax.servlet.ServletException: java.lang.RuntimeException: java.io.InvalidClassException: ro.expert.evt.shared.entities.ObjectModel; local class incompatible: 
stream classdesc serialVersionUID = -2824144882306533912, 
local class serialVersionUID = 6500787607817458947

I know the problem is that I updated my class and when I deployed my app it did not have a serialVersionUID.

My questions are:

1. If I set a generated serialVersionUID will the problem appear again ? do I have do regenerate it every time I modify my user class ?

2. How do I fix this for my already logged in users ? How do I invalidate their sessions ("clearing" their cookies should work )

Edit: Would clearing the whole memcache invalidate the users sessions ?

I should call getSession().invalidate() but i cannot run any code to catch the exception:

Caused by: java.lang.RuntimeException: java.io.InvalidClassException: ro.expert.evt.shared.entities.ObjectModel; local class incompatible: stream classdesc serialVersionUID = -2824144882306533912, local class serialVersionUID = 6500787607817458947
    at com.google.apphosting.runtime.SessionManagerUtil.deserialize(SessionManagerUtil.java:57)
    at com.google.apphosting.runtime.jetty.MemcacheSessionStore.getSession(MemcacheSessionStore.java:39)
    at com.google.apphosting.runtime.jetty.SessionManager.loadSession(SessionManager.java:327)
    at com.google.apphosting.runtime.jetty.SessionManager.getSession(SessionManager.java:301)
    at org.mortbay.jetty.servlet.AbstractSessionManager.getHttpSession(AbstractSessionManager.java:237)
    at org.mortbay.jetty.servlet.SessionHandler.setRequestedId(SessionHandler.java:246)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:136)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
    at com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:266)
... 16 more

Is there a method in my initial servlet that i can override so that i can catch the exception above and invalidate the user's session ?

Answer 1

Yes you have to provide explicit serialVersionUID value to avoid this error. You can keep value the same between deployments or/and class modifications.

As for existing users you can use versioning feature of GAE. Deploy with increased version, eg existing GAE app v. 1 so deploy new code as v.2 and then you can repoint default version to your new version...so existing users will still keep using old version...but new ones will hit new ver.