Java - Google App Engine - InvalidClassException when I change a class that was stored in session scope

Question

I updated my User class, and now whenever someone that had the old version of the User class stored in their session scope accesses my site, I get an InvalidClassException.

javax.servlet.ServletException: java.lang.RuntimeException: 
java.io.InvalidClassException: User; local class incompatible: 
stream classdesc serialVersionUID = 4949038118012519093, 
local class serialVersionUID = -971500502189813151

How do I stop this error from happening for those users? I could probably invalidate everyone's sessions every time I want to update a class that gets stored in session scope, but is there a better way, so that my user's don't have to login again?

Answer 1

You can add

private final long serialVersionUID=4949038118012519093;

to your class definition. New class should have the same serializable fields in the same order of course.