@Entity
open class User : DefaultJpaUser<TestUser, TestUserRole>() {

  @NotBlank
  override fun getUsername(): String? {
    return super.getUsername()
  }
}

@Entity
@Getter
@Setter
open class TestUserRole : DefaultJpaUserRole<TestUser, TestUserRole>() {
  var modified: Instant? = null}

public abstract class DefaultJpaUser<
    U extends DefaultJpaUser<U, UR>,
    UR extends DefaultJpaUserRole<U, UR>>
    extends JpaUser<Long, Long, DefaultJpaRole, UR, U> {
}

public abstract class DefaultJpaUserRole<
    U extends DefaultJpaUser<U, UR>,
    UR extends DefaultJpaUserRole<U, UR>>
    extends JpaUserRole<Long, Long, DefaultJpaRole, U, UR> {
}

@Getter
@Setter
@NoArgsConstructorpublic 
class JpaUserRole<
    UID,
    RID,
    R extends JpaRole<RID>,
    U extends JpaUser<UID, RID, R, UR, U>,
    UR extends JpaUserRole<UID, RID, R, U, UR>> {

  private UserRoleId<UID, RID> id = new UserRoleId<>();

  @NotNull  @ManyToOne  @MapsId("userId")
  private U user;

  @NotNull  @ManyToOne  @MapsId("roleId")
  private R role;

  public JpaUserRole(UserRoleId<UID, RID> id) {
    this.id = id;
  }

  @Embeddable  
  @Getter  
  @Setter  
  @NoArgsConstructor  
  @AllArgsConstructor  
  public static class UserRoleId<UID, RID> implements Serializable {
    @Serial private static final long serialVersionUID = 4710225845169939700L;

    private UID userId;
    private RID roleId;
  }
}

public class JpaUser<UID,
    RID,
    R extends JpaRole<RID>,
    UR extends JpaUserRole<UID, RID, R, U, UR>,
    U extends JpaUser<UID, RID, R, UR, U>>
    implements User<UID> {

  UID id;
  String username;
  String password;
  String email;
  Boolean emailConfirmed;

  Set<UR> roles = new HashSet<>();

  public Set<UR> getRoles() {
    return roles;
  }
}

var cb = entityManager.getCriteriaBuilder();
var q = cb.createQuery(Tuple.class);
var root = q.from(userRoleClass);
var userJoin = root.join("user");
q.where(cb.equal(userJoin.get("id"), user.getId()));
q.select(root.get("id"));
return entityManager.createQuery(q).getResultList()
    .stream().map(row -> row.get(0, Long.class).toString()).collect(Collectors.toList());