apollo-client/src/main/java/com/ctrip/framework/apollo/internals/DefaultMetaServerProvider.java/DefaultMetaServerProvider/initMetaServerAddress
class DefaultMetaServerProvider:  private String initMetaServerAddress() {
    // 1. Get from System Property
    String metaAddress = System.getProperty(ConfigConsts.APOLLO_META_KEY);
    if (Strings.isNullOrEmpty(metaAddress)) {
      // 2. Get from OS environment variable, which could not contain dot and is normally in UPPER case
      metaAddress = System.getenv(ApolloClientSystemConsts.APOLLO_META_ENVIRONMENT_VARIABLES);
    }
    if (Strings.isNullOrEmpty(metaAddress)) {
      // 3. Get from server.properties
      metaAddress = Foundation.server().getProperty(ConfigConsts.APOLLO_META_KEY, null);
    }
    if (Strings.isNullOrEmpty(metaAddress)) {
      // 4. Get from app.properties
      metaAddress = Foundation.app().getProperty(ConfigConsts.APOLLO_META_KEY, null);
    }

    if (Strings.isNullOrEmpty(metaAddress)) {
      logger.warn(
          "Could not find meta server address, because it is not available in neither (1) JVM system property 'apollo.meta', (2) OS env variable 'APOLLO_META' (3) property 'apollo.meta' from server.properties nor (4) property 'apollo.meta' from app.properties");
    } else {
      metaAddress = metaAddress.trim();
      logger.info("Located meta services from apollo.meta configuration: {}!", metaAddress);
    }

    return metaAddress;
  }