Any way to share session state between different applications in tomcat?

We want to split a working application in two different .war files in order to be able to update one app without affecting the other. Each webapp will have different a UI, different users and different deploy schedule.

The easiest path seems to be sharing the same session, so if app A set session.setAttribute("foo", "bar") app B will be able to see it.

Is there a way to share the HttpSession state for both apps in the same Tomcat instance?

Our app is running on a dedicated Tomcat 5.5, there are no other apps running on the same tomcat instance, so any security concerns regarding the session sharing are not a problem. We're running multiple Tomcat instances, but the balancer is using sticky sessions.

If it's not possible or this session sharing is a really bad idea please leave a comment.

You should not share HttpSession; but you can share other objects. For example, you can register an object via JNDI and access the same object in all your apps (databases use this to pool connections).

One thing to be aware of is that two web apps will use different classloaders. If you want to share objects, they need to use the same version of the class from the same classloader (or else you will get LinkageErrors). That means either putting them in a classloader shared by both web apps (system classpath for example) OR using serialization to effectively drain and reconstitute the object in the right classloader with the correct version of the class.