public interface UserRepository extends JpaRepository<User, Long>, IUserRepository<User>{
   User findByUsername(String username);
}

public interface IUserRepository<T> {
   public List<T> findAllValidEtSiteAndStructure();
}

public class UserRepositoryImpl<T> implements IUserRepository<T> {

@PersistenceContext
private EntityManager em;

private Class< T > type;

@Override
public List<T> findAllValidEtSiteAndStructure() {
    final Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
    if (authentication instanceof UserAuthentication) {
        final User currentUser = ((UserAuthentication) authentication).getDetails();
        return (List<T>) em.createQuery("FROM " + type.getName()+ " WHERE site=:site AND structure=:structure AND valid=:valid")
                .setParameter("site", currentUser.getInstallation().getSite())
                .setParameter("structure", currentUser.getInstallation().getStructure())
                .setParameter("valid", true)
                .getResultList();
    }
    return null;
}
}

public interface UserRepository extends JpaRepository<User, Long> {

   User findByUsername(String username);

   List<User> findByStructureAndSiteAndValid(Structure structure, Site site, boolean valid);
}

public abstract Class getType();

public class StudentRepository extends UserRepositoryImpl<Student> {
  public Class getType() {
    return Student.class;
  }
}

ResolvableType resolvableType = ResolvableType.forClass(UserRepository.class).as(JpaRepository.class);
System.out.println(resolvableType.getGeneric(0));//User
System.out.println(resolvableType.getGeneric(0));//Long