in my use case i have to invoke existing Web Services. How can i represent the invocation of this Web Services in a Sequence Diagram?
At the moment i simply have inserted, in my sequence diagram, the object "xxxxx" with stereotype <<Web Services>> and i have put synchronous messages to it with the name of the services i except to have. Do i need to introduce any JAXB layer/object or wrapping design pattern?
Depends on what level of abstraction you're working on. If your sequence diagram explains how SuD is communicating with third-parties - JAXB is not important. It's not necessary to know for a reader how exactly Java will be converted to XML. What is important is how and when data are being sent between components.
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In my assignment also i need to do that. I have shown the Web Services client, the message class(request) , the response class and the Web Services as a Entity. I provided a note in the sequence diagram that i am not showing tool generated classes and JAXB classes. That's it.