title: Android continuous running service
body: <p>I am trying to do simple chat application using android. </p>
  
  <ol>
  <li>Like whatapp, I want to run background service continuously whether activity is running or not running. </li>
  </ol>
  
  <p>2.When I get message, then there should be notification. </p>
  
  <ol>
  <li>Once clicked on notification, activity will be opened. </li>
  </ol>
  
  <p>Using which API, I can do above all those things?</p>