apollo-core/src/main/java/com/ctrip/framework/apollo/core/dto/ApolloConfigNotification.java/ApolloConfigNotification/addMessage
class ApolloConfigNotification:  public void addMessage(String key, long notificationId) {
    if (this.messages == null) {
      synchronized (this) {
        if (this.messages == null) {
          this.messages = new ApolloNotificationMessages();
        }
      }
    }
    this.messages.put(key, notificationId);
  }