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For SAP apps, the service calls RFCs via sap2java using sap2java generated proxies. The service converts the proxy data into biz model objects. The service class should be a subclass of SAPServiceSupport which provides methods to call the back end and process errors. If an error message is returned from the SAP RFC at this point, processing will stop and the user will either stay on the same page displaying the error message if an action input has been defined correctly in the struts-config or they will be sent to the GlobalKeys.FAIL jsp page with a MessageRuntimeException. In the rare case that you don't want to stop processing and display the error message you should wrap your service call in a try...catch statement intercepting any MessageRuntimeException messages. See ApplicationResources for documentation on the error handling.
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