apollo-core/src/main/java/com/ctrip/framework/apollo/core/MetaDomainConsts.java/MetaDomainConsts/initMetaServerAddress
class MetaDomainConsts:  private static void initMetaServerAddress(Env env) {
    if (metaServerProviders == null) {
      synchronized (LOCK) {
        if (metaServerProviders == null) {
          metaServerProviders = initMetaServerProviders();
        }
      }
    }

    String metaAddress = null;

    for (MetaServerProvider provider : metaServerProviders) {
      metaAddress = provider.getMetaServerAddress(env);
      if (!Strings.isNullOrEmpty(metaAddress)) {
        logger.info("Located meta server address {} for env {} from {}", metaAddress, env,
            provider.getClass().getName());
        break;
      }
    }

    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 all MetaServerProviders",
          metaAddress, env);
    }

    metaServerAddressCache.put(env, metaAddress.trim());
  }