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The service spring beans are configured in applicationContext.xml (not in action-servlet.xml) as singletons. Because of this, there should be no instance variables defined for your service other than static variables that you intend to be accessed across threads (users). For instance, in the example below, a static variable, log, is defined. This means all users will write to the same log file which is the behavior you want.
convertRFCMessage
If the message objects returned from the back end have both the TYPE and MESSAGE fields, then setting messageKeys is all the configuration you need to do (these are automatically dealt with). However, if some other type of object is returned instead that doesn't have these fields, you will need to add code in the convertRFCMessage method in your service implementation to convert these message objects to the RFCMessage type. Otherwise, the method convertRFCMessage should return null.