apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/ConfigServiceAutoConfiguration.java/ConfigServiceAutoConfiguration/releaseMessageScanner
class ConfigServiceAutoConfiguration:    @Bean
    public ReleaseMessageScanner releaseMessageScanner() {
      ReleaseMessageScanner releaseMessageScanner = new ReleaseMessageScanner();
      //0. handle release message cache
      releaseMessageScanner.addMessageListener(releaseMessageServiceWithCache);
      //1. handle gray release rule
      releaseMessageScanner.addMessageListener(grayReleaseRulesHolder);
      //2. handle server cache
      releaseMessageScanner.addMessageListener(configService);
      releaseMessageScanner.addMessageListener(configFileController);
      //3. notify clients
      releaseMessageScanner.addMessageListener(notificationControllerV2);
      releaseMessageScanner.addMessageListener(notificationController);
      return releaseMessageScanner;
    }