Unit test spring bean implementation as bean

I have a Spring Boot application, using Spring 4, in this app I have an interface and an implementing class:

interface Sorter{
   void start();
   List<MailingListMessage> getMessages();
}

and

@Service
public class SorterImpl implements Sorter {

    List<MailingListMessage> messages;

     @Override
     public void start() {

     }

     @Override
     public List<MailingListMessage> getMessages() {
         return null;
     }
 }

What I want to to is test the implementation directly but still as a bean with autowiring and everything.

I want to do that because when I need to assert something that for example relates to messages, I have to make the method public as you can see in the Impl class.

Is there a way in Spring to properly write tests for implementations directly and still have them work as beans?

Thanks.