How should we integrate PayPal adaptive (and IPN) API in a Java webserver environment?

As I'm currently integrating a paypal layer in my spring based server (that should be usable by multiple clients asking me to implement their webstore):

And as there's surprisingly very few info to find on the Spring/Paypal topic, I could only find this article: http://blog.mushiengine.com/2010/08/25/paypal-adaptive-api-and-spring-3-rest-template-%E2%80%94-part-2/

I was wondering if there are perhaps some guidelines or best practices concerning the topic?

Any comments on the topic more than welcome!

Jochen