Sharing login/SSO across multiple web apps

Hi all,

I have two web apps based on Spring MVC 3 and Spring Security. They are deployed on the same web server, under two different contexts of course:

http://localhost:8080/webapp1
http://localhost:8080/webapp2

Each app will ask the user to login (which is fine) but what I need is for both apps to accept a login on the other app. In other words, if you log in to webapp1 and then access the url for webapp2, you should not be required to authenticate yourself again.

I'm using a simple user service with users stored in a property file:

<sec:user-service properties="realm.properties"/>

Is this possible, and if so what is the simplest way to achieve this. A hack would be ok, adding new stuff to the infrastructure (CAS server etc) is less ok.

Any suggestions would be appreciated. I'm a bit stuck here.