code: public static String getApplicationPropertiesPropertyConfigFilter(String applicationPropertyPid) {
          String applicationConfigFilter = null;
  
          StringBuilder applicationFilter = new StringBuilder(200);
          applicationFilter.append("(&");
          applicationFilter.append(FilterUtils.createPropertyFilter(CFG_SERVICE_FACTORY_PID, CFG_APP_PROPERTIES_PROPERTY));
          applicationFilter.append(FilterUtils.createPropertyFilter(CFG_CONFIG_PARENT_PID, applicationPropertyPid));
          applicationFilter.append(')');
  
          applicationConfigFilter = applicationFilter.toString();
  
          return applicationConfigFilter;
      }
docs: Get the filter string which will extract <property> elements nested inside a given <appProperties> element
  
  @param applicationPid The pid of the appProperties element in the config
  @return A filter string to be used by the OSGi ConfigurationAdmin service