public interface EntityRepository extends EntityRepositoryCustom, CrudRepository<Entity, String> {
}

public interface EntityRepositoryCustom {
    public Page<Entity> findEntitisOnCondition(Entity t, Pageable pageable);
}

public class EntityRepositoryCustomImpl<Entity> extends SelfDefinedRepositoryImpl<Entity> {
    @Override
    protected String getSOmething() {
        ...
    }
}

public interface SelfDefinedRepository<T, ID extends Serializable>  extends CrudRepository<T, ID> {
    Page<T> findEntitisOnCondition(T t, Pageable pageable);
}

public abstract class SelfDefinedRepositoryImpl<T, ID extends Serializable> implements SelfDefinedRepository<T, Serializable>{

    public Page<T> findEventsOnCondition(T t, Pageable pageable) {
        ...
        getSomething();
        ...
    }

    protected abstract String getSomething();

}