apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/environment/DefaultPortalMetaServerProvider.java/DefaultPortalMetaServerProvider/initializeDomains
class DefaultPortalMetaServerProvider:    private Map<Env, String> initializeDomains() {

        // add to domain
        Map<Env, String> map = new ConcurrentHashMap<>();
        // lower priority add first
        map.putAll(getDomainsFromPropertiesFile());
        map.putAll(getDomainsFromOSEnvironment());
        map.putAll(getDomainsFromSystemProperty());

        // log all
        return map;
    }