How to use Spring Security RemoteTokenService with Keycloak

I found a solution by myself just after formulating this question here. Sometimes it helps to try to express a problem.

The solution is to override the DefaultAccessTokenConverter to teach him how to read the "realm_access" field. Its ugly but it works:

@Override
public void configure(ResourceServerSecurityConfigurer resources) throws Exception {

    resources.resourceId("demo-client");
    RemoteTokenServices tokenServices = new RemoteTokenServices();
    tokenServices.setCheckTokenEndpointUrl(
        "http://localhost:8280/auth/realms/demo-realm/protocol/openid-connect/token/introspect");
    tokenServices.setClientId("demo-client");
    tokenServices.setClientSecret("80e19056-7770-4a4a-a3c4-06d8ac8792ef");
    tokenServices.setAccessTokenConverter(new KeycloakAccessTokenConverter());
    resources.tokenServices(tokenServices);

}
private class KeycloakAccessTokenConverter extends DefaultAccessTokenConverter {

    @Override
    public OAuth2Authentication extractAuthentication(Map<String, ?> map) {
        OAuth2Authentication oAuth2Authentication = super.extractAuthentication(map);
        Collection<GrantedAuthority> authorities = (Collection<GrantedAuthority>) oAuth2Authentication.getOAuth2Request().getAuthorities();
        if (map.containsKey("realm_access")) {
            Map<String, Object> realm_access = (Map<String, Object>) map.get("realm_access");
            if(realm_access.containsKey("roles")) {
                ((Collection<String>) realm_access.get("roles")).forEach(r -> authorities.add(new SimpleGrantedAuthority(r)));
            }
        }
        return new OAuth2Authentication(oAuth2Authentication.getOAuth2Request(),oAuth2Authentication.getUserAuthentication());
    }
}

Via keycloak admin console you can create a token mapper of type User Realm Role with claim name "authorities" for your client "demo-client". Then the access token contains the role names in this attribute and no custom DefaultAccessTokenConverter is needed.