Let's say I want my corporate server to communicate with Google App Engine and vice versa. I know that GAE does not support JMS,RMI etc. What is the best alternative for this kind of communication? Use task queue? (I think HTTP get() is not suitable for this kind of communication).
Both my corporate server and GAE application use Spring framework.
XMPP is a powerful and flexible messaging protocol, and this article shows how to do the GAE side of it in both Java and Python. For XMPP implementations (in Java and others) outside of GAE, see this SO question.
For accessing from GAE a lot of bulky secure data that lives behind your corporate firewall, Google recommends implementing the Secure Data Connector (I'm pointing specifically to the URL of the Java tutorial for SDC with GAE).
Use any of a number of HTTP based RPC protocols: REST, JSONRPC, SOAP, etc.
You say "I think http get() is not suitable for this kind of communication" - why not?