Which Java technology should be used to build client-server applications on the server side?

I'm new in the Java world a开发者_StackOverflow社区nd I'm totally confused with the sheer number of Java server-side technologies. I need to build application with Java on the server side and Flex-AIR on the client. All communication between them must be on sockets and in AMF format.

Which technology would be suitable for this?

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JPA+Spring. You can create service layer with Spring, and use it from you flex client.

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I think tha given your constraints you should be looking at Blaze: http://opensource.adobe.com/wiki/display/blazeds/BlazeDS/