I have an existing web app built around a Spring based business and DAO tier.

There is a new requirement to extend this so that I expose one of my spring business tier services as as web service and allow third party clients to connect. The wrinkle on this is that the spring business tier should be capable of pushing notifications via http from the server back to clients (in this case the client is a windows .NET thick client app).

I've seen a number of technologies referenced in this area: xfire, BlazeDS, Hessian. Can anybody suggest any best routes forward? Or better still, point me at some tutorials on this area?