cannot load RPC service interface from external jar

I make a jar file which contain client side code and a RPC service, serviceAsync interface.

After that, I create an new GWT project and create a RemoteServiceServlet that implements the RPC service in the external jar but there are some error message when I start the dev server:

java.lang.NoClassDefFoundError: xxx/client/XXXService

p.s. I have create a gwt.xml file in the jar:

<module><source path="client"></source></module>

and add line to gwt.xml in new project:

<inherits name='xxx/client/XXX'/>

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