apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/environment/PortalMetaDomainService.java/PortalMetaDomainService/getMetaServerAddress
class PortalMetaDomainService:    public String getMetaServerAddress(Env env) {
        // in cache?
        if (!metaServerAddressCache.containsKey(env)) {
            // put it to cache
            metaServerAddressCache
                .put(env, getMetaServerAddressCacheValue(portalMetaServerProviders, env)
            );
        }

        // get from cache
        return metaServerAddressCache.get(env);
    }