To test it, run:

mvn -U -e gwt:codeserver -pl client -am

to execute the codeserver (just keep that running).

Then you can use

mvn -U jetty:run -pl server -am -Denv=dev

to run the application in developer mode (the URL is http://localhost:8080/).

From now on, whenever you modify the code inside the client module a simple reload on the browser will update the running code on the fly.

If you modify the code inside the server module you can update the server-side part of the running application by issuing

mvn -U compile -pl *-server -am