Generic JPA repository for multiple entities

Question

I have several entities and use Spring Data JPA repositories with specifications query my database. Therefore I created a generic class SpecBuilder to build my queries based on a query description (MyQueryDescriptor).

public class Specs {
  public static <T extends MyEntityIFace> Specification<T> myfind(final MyQueryDescriptor qDesc) {
    return new Specification<T>() {
      @Override
      public Predicate toPredicate(Root<T> root, 
               CriteriaQuery<?> criteriaQuery, CriteriaBuilder criteriaBuilder) {
        try {
          return SpecBuilder.mySpec(root, criteriaQuery, criteriaBuilder, qDesc);
        } catch (Exception e) {
          ...handle error...
        }
      }
    };
  }
}

My repositories:

public interface Entity1DAO extends Repository<Entity1,Long>, 
                                    JpaSpecificationExecutor {
}

and

public interface Entity2DAO extends Repository<Entity2,Long>, 
                                    JpaSpecificationExecutor {
}

Now there are 3 things I am not quite sure about:
1)
Is this use of a generic SpecBuilder a clean design?

2)
Is there a way to avoid writing those repository interfaces for each entity? Let's say a generic repository?

3)
The MyQueryDescriptor class has a method to return an instance of an Entity, which will be queried.
What would be a clean way to obtain the according repository based on the entity class, avoiding a switch case? I was thinking about putting an annotation with the specific repository class to each entity but it feels a bit smelly.
Should I create a factory and inject a map like

Entity1.class => Entity1DAO
Entity2.class => Entity2DAO

?

Answer 1

You can use entity inheritance and use Spring Expression Language (SpEL) to make repository issue calls on right entities. Like in my last update here