title: Spring Data JPA underlying mechanism without implementation
body: <p>I started to read this tutorial: <a href="https://spring.io/guides/tutorials/bookmarks/" rel="nofollow">spring boot tutorial</a></p>
  
  <p>In this I read that under model module they implemented POJOs and Repository interfaces. -> <a href="https://github.com/joshlong/bookmarks/tree/tutorial/model/src/main/java/bookmarks" rel="nofollow">tutorial on github</a></p>
  
  <p>In Repository interfaces I found two methods without implementations:
  findByUsername,
  findByAccountUsername.</p>
  
  <p>My questions are:</p>
  
  <ol>
  <li>How does it work when those methods in repository interfaces have no
  implementations and those are not inherited from any super class?</li>
  <li>Does it work with name conventions and reflections?</li>
  <li>Does Spring Data has inmemory database to work with?</li>
  </ol>