title: Differences between AuthenticationProvider and AuthenticationEntryPoint
body: <p>Sorry guys, maybe a silly question.</p>
  
  <p>But I need to implement some additional logic for authentication and authorization in my web app and I've not clear in mind where <code>AuthenticationProvider</code> and <code>AuthenticationEntryPoint</code> must be used.</p>
  
  <p>Looking for some examples I somethimes find that an <code>AuthenticationEntryPoint</code> is omitted in <code>security:http</code> section.</p>
  
  <p>But there are situations where also <code>AuthenticationProvider</code> is omitted (a default instance is provided by the framework?) and only a <code>UserDetailsService</code> implementation is needed.</p>
  
  <p>Please, can you clarify some basic concepts?</p>