apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/environment/PortalMetaDomainService.java/PortalMetaDomainService/getMetaServerAddressCacheValue
class PortalMetaDomainService:    private String getMetaServerAddressCacheValue(
        Collection<PortalMetaServerProvider> providers, Env env) {

        // null value
        String metaAddress = null;

        for(PortalMetaServerProvider portalMetaServerProvider : providers) {
            if(portalMetaServerProvider.exists(env)) {
                metaAddress = portalMetaServerProvider.getMetaServerAddress(env);
                logger.info("Located meta server address [{}] for env [{}]", metaAddress, env);
                break;
            }
        }

        // check find it or not
        if (Strings.isNullOrEmpty(metaAddress)) {
            // Fallback to default meta address
            metaAddress = DEFAULT_META_URL;
            logger.warn(
                    "Meta server address fallback to [{}] for env [{}], because it is not available in MetaServerProvider",
                    metaAddress, env);
        }
        return metaAddress.trim();
    }