title: How to decode the OAuth2 access token to get the username by Resteasy
body: <p>I am a beginner to use the Resteasy to provide a OAuth2 service. Thanks very much!</p>
  
  <p>When I use the grant_type of client_crdentials,</p>
  
  <p>1st step: requesting the access token by a username and password.
  2nd step: put the access token into authorization header to call API.</p>
  
  <p>My question is, at the 2nd step, at the server side, how could I decode this access token to get the username that was used to request the access token in 1st step? Because, I want to know who is calling my API. Thanks very much!</p>