I don't think your question is properly structured. Did you mean a webservice itself which can consume json sent to it by the webservice client?
I believe iDempiere is already webservice enabled. You can look at the org.idempiere.webservices module in the codebase. Over there the WEB-INF/web.xml says a thing or two about how the webservice is setup.
You can look at that and offer more questions.