@Override
    public String determineIssuer(final Optional<OidcRegisteredService> registeredService) {
        val issuer = registeredService
            .filter(svc -> StringUtils.isNotBlank(svc.getIdTokenIssuer()))
            .map(OidcRegisteredService::getIdTokenIssuer)
            .orElseGet(() -> properties.getCore().getIssuer());
        LOGGER.trace("Determined issuer as [{}] for [{}]", issuer,
            registeredService.map(RegisteredService::getName).orElse("CAS"));
        return StringUtils.removeEnd(issuer, "/");
    }
This is {@link OidcDefaultIssuerService}.

@author Misagh Moayyed
@since 6.4.0
support/cas-server-support-oidc-core-api/src/main/java/org/apereo/cas/oidc/issuer/OidcDefaultIssuerService.java