apollo-client/src/main/java/com/ctrip/framework/apollo/spring/config/NamespaceHandler.java/BeanParser/resolveNamespaces
class BeanParser:    private String resolveNamespaces(Element element) {
      String namespaces = element.getAttribute("namespaces");
      if (Strings.isNullOrEmpty(namespaces)) {
        //default to application
        return ConfigConsts.NAMESPACE_APPLICATION;
      }
      return SystemPropertyUtils.resolvePlaceholders(namespaces);
    }