@Service
public class OidcUserInfoService {
  // omitted
  private OidcUserInfo createOidcUserInfo(User user) {
    return OidcUserInfo.builder()
        .subject(user.getId().toString()) /* <= (HERE) */
        .preferredUsername(user.getUsername())
        .email(user.getUsername())
        .emailVerified(true)
        .name(user.getName())
        .build();
  }
  // omitted
}

@Component
public class JwtTokenCustomizer implements OAuth2TokenCustomizer<JwtEncodingContext> {

  private final OidcUserInfoService userInfoService;

  public JwtTokenCustomizer(OidcUserInfoService oidcUserInfoService) {
    this.userInfoService = oidcUserInfoService;
  }

  @Override
  public void customize(JwtEncodingContext context) {
    if (!isAccessToken(context) && !isIdToken(context) || isClientCredentials(context)) {
      var registeredClient = context.getRegisteredClient();
      var clientInfo = userInfoService.loadClientInfo(registeredClient.getId());
      context.getClaims().claims(claims -> claims.putAll(clientInfo));
    } else {
      var currentUser = context.getPrincipal().getName();
      if (isAccessToken(context)) {
        var userInfo = userInfoService.loadUserProviderInfo(currentUser);
        context.getClaims().claims(claims -> claims.putAll(userInfo));
      } else if (isIdToken(context)) {
        var userInfo = userInfoService.loadOidcUserInfo(currentUser);  /* <= (HERE) */
        context.getClaims().claims(claims -> claims.putAll(userInfo.getClaims()));
      }
    }
  }
  // omitted
}