@Override
    public String resolveIssuer(final Optional<OAuthRegisteredService> registeredService) {
        val filter = registeredService
            .filter(OidcRegisteredService.class::isInstance)
            .map(OidcRegisteredService.class::cast)
            .stream()
            .findFirst();
        return issuerService.determineIssuer(filter);
    }
This is {@link BaseOidcJsonWebKeyTokenSigningAndEncryptionService}.

@author Misagh Moayyed
@since 6.1.0
support/cas-server-support-oidc-core-api/src/main/java/org/apereo/cas/oidc/token/BaseOidcJsonWebKeyTokenSigningAndEncryptionService.java