title: Get OAuth access token from Google AppEngine server Java
body: <p>All what I need is in the title, I'm looking for the process to get an access token from OAuth for my google app engine account (It's to use API which isn't in the google libraries).</p>
  
  <p>I already create a OAuth client ID on my google console, but now I don't know what to do, how to get my access token?</p>
  
  <p>PS: It's to use in the proximity beacon API from the server directly</p>