title: How internally UserDefinedRepository implements method by extending JpaRepository in Spring Boot?
body: <p>Suppose I have interface UserDetailRepository which extend JpaRepository in which I have 1 method </p>
  
  <p><code>findByUsername</code> which I have not implemented, still I get the result. How internally spring boot manages to implement method?</p>
  
  <pre><code>public interface UserDetailRepository extends JpaRepository&lt;User, Integer&gt; {
  
       Optional&lt;User&gt; findByUsername(String name);
  
  }
  </code></pre>