Spring Controller needs to be moved from client to the server side service

MessageRestController needs to be moved from client to the server side service

I noticed this when I tried running your project.  I had to move that class over from the client to the service to get it all working.

Also, your README should mention that the url to get the response is:
localhost:8888/a-bootiful-client/message

Your documentation implies that but doesn't literally say it.