ReleaseMessageScanner：扫描变更配置，触发监听者们

6、ReleaseMessageScanner：扫描变更配置，触发监听者们

 这一步就是将第5步插入数据库的变更配置，每秒扫描一次，扫出来之后发送给监听者（还记得NotificationControllerV2 implements ReleaseMessageListener）

public class ReleaseMessageScanner implements InitializingBean {
  private static final Logger logger = LoggerFactory.getLogger(ReleaseMessageScanner.class);
  @Autowired
  private BizConfig bizConfig;
  @Autowired
  private ReleaseMessageRepository releaseMessageRepository;
  private int databaseScanInterval;
  private List<ReleaseMessageListener> listeners;
  private ScheduledExecutorService executorService;
  private long maxIdScanned;

  public ReleaseMessageScanner() {
    listeners = Lists.newCopyOnWriteArrayList();
    executorService = Executors.newScheduledThreadPool(1, ApolloThreadFactory
        .create("ReleaseMessageScanner", true));
  }

  @Override
  public void afterPropertiesSet() throws Exception {
  	//定时任务频率，默认1000ms
    databaseScanInterval = bizConfig.releaseMessageScanIntervalInMilli();
    //最大的ReleaseMessage编号
    maxIdScanned = loadLargestMessageId();
    //每秒走一次，扫描消息定时
    executorService.scheduleWithFixedDelay((Runnable) () -> {
      Transaction transaction = Tracer.newTransaction("Apollo.ReleaseMessageScanner", "scanMessage");
      try {
        scanMessages();
        transaction.setStatus(Transaction.SUCCESS);
      } catch (Throwable ex) {
        transaction.setStatus(ex);
        logger.error("Scan and send message failed", ex);
      } finally {
        transaction.complete();
      }
    }, databaseScanInterval, databaseScanInterval, TimeUnit.MILLISECONDS);

  }

  /**
   * add message listeners for release message
   * @param listener
   */
  public void addMessageListener(ReleaseMessageListener listener) {
    if (!listeners.contains(listener)) {
      listeners.add(listener);
    }
  }

  /**
   * Scan messages, continue scanning until there is no more messages
   */
  private void scanMessages() {
    boolean hasMoreMessages = true;
    while (hasMoreMessages && !Thread.currentThread().isInterrupted()) {
      hasMoreMessages = scanAndSendMessages();
    }
  }

  /**
   * scan messages and send
   * 扫描，触发监听
   * @return 是否有新的Message可以扫描
   *
   * @return whether there are more messages
   */
  private boolean scanAndSendMessages() {
    //current batch is 500
    List<ReleaseMessage> releaseMessages =
        releaseMessageRepository.findFirst500ByIdGreaterThanOrderByIdAsc(maxIdScanned);
    if (CollectionUtils.isEmpty(releaseMessages)) {
      return false;
    }
    //触发监听
    fireMessageScanned(releaseMessages);
    int messageScanned = releaseMessages.size();
    //记录最新一条Id
    maxIdScanned = releaseMessages.get(messageScanned - 1).getId();
    return messageScanned == 500;
  }

  /**
   * 获取最大ReleaseMessage编号
   * find largest message id as the current start point
   * @return current largest message id
   */
  private long loadLargestMessageId() {
    ReleaseMessage releaseMessage = releaseMessageRepository.findTopByOrderByIdDesc();
    return releaseMessage == null ? 0 : releaseMessage.getId();
  }

  /**
   * Notify listeners with messages loaded
   * @param messages
   */
  private void fireMessageScanned(List<ReleaseMessage> messages) {
    for (ReleaseMessage message : messages) {
    	//将每条消息循环通知所有订阅者
      for (ReleaseMessageListener listener : listeners) {
        try {
        	//触发监听
          listener.handleMessage(message, Topics.APOLLO_RELEASE_TOPIC);
        } catch (Throwable ex) {
          Tracer.logError(ex);
          logger.error("Failed to invoke message listener {}", listener.getClass(), ex);
        }
      }
    }
  }

总结：

1、启动一个定时任务（每秒一次）

2、根据最大编号查询数据库500条（按id倒叙）

3、设置最新的最大编号

4、触发监听者们--还记得下面这个监听吗com.ctrip.framework.apollo.configservice.ConfigServiceAutoConfiguration.MessageScannerConfiguration#releaseMessageScanner

 /**
	   * 监听器注册，namespaces变更后接收消息
	   * @return
	   */
    @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;
    }
  }