Spring Websocket+SockJS+STOMP 实现即时通信（五）—— UserRegistryMessageHandler与NoOpMessageHandler

---

UserRegistryMessageHandler

• 用来处理来自其他应用服务的用户注册表广播，同时定期地广播本地用户注册表的内容；
• 用户注册表的聚合信息，被维护在一个MultiServerUserRegistry成员变量中；
• 无需订阅MessageChannel，所以没有实现SmartLiftCycle接口；

处理来自其它应用服务的注册表广播

UserRegistryMessageHandler :

     
      public
      
     
      class
      
     
      UserRegistryMessageHandler
      
     
      implements
      
     
      MessageHandler
     , ApplicationListener<
     
      BrokerAvailabilityEvent
     > {
	@Override
	
     
      public
      
     
      void
      
     
      handleMessage
     (Message<?> message) 
     
      throws
      MessagingException {
		MessageConverter converter = 
     
      this
     .brokerTemplate.
     
      getMessageConverter
     ();
		
     
      this
     .userRegistry.
     
      addRemoteRegistryDto
     (message, converter, 
     
      getRegistryExpirationPeriod
     ());
	}
}

定期广播本地注册表

UserRegistryMessageHandler :


     
      public
      
     
      class
      
     
      UserRegistryMessageHandler
      
     
      implements
      
     
      MessageHandler
     , ApplicationListener<
     
      BrokerAvailabilityEvent
     > {
	
     
      private
      
     
      final
      UserRegistryTask schedulerTask = new 
     
      UserRegistryTask
     ();
	@Nullable
	
     
      private
      
     
      volatile
      ScheduledFuture<?> scheduledFuture;
	
     
      private
      
     
      long
      registryExpirationPeriod = TimeUnit.SECONDS.
     
      toMillis
     (20);
	@Override
	
     
      public
      
     
      void
      
     
      onApplicationEvent
     (BrokerAvailabilityEvent event) {
		if (event.
     
      isBrokerAvailable
     ()) {
			
     
      long
      delay = 
     
      getRegistryExpirationPeriod
     () / 2;
			
     
      this
     .scheduledFuture = 
     
      this
     .scheduler.
     
      scheduleWithFixedDelay
     (
     
      this
     .schedulerTask, delay);
		}
		
     
      else
      {
			ScheduledFuture<?> future = 
     
      this
     .scheduledFuture;
			if (future != null ){
				future.
     
      cancel
     (
     
      true
     );
				
     
      this
     .scheduledFuture = null;
			}
		}
	}
	
     
      private
      
     
      class
      
     
      UserRegistryTask
      
     
      implements
      
     
      Runnable
      {

		@Override
		
     
      public
      
     
      void
      run() {
			try {
				SimpMessageHeaderAccessor accessor = SimpMessageHeaderAccessor.
     
      create
     (SimpMessageType.MESSAGE);
				accessor.
     
      setHeader
     (SimpMessageHeaderAccessor.IGNORE_ERROR, 
     
      true
     );
				accessor.
     
      setLeaveMutable
     (
     
      true
     );
				Object payload = userRegistry.
     
      getLocalRegistryDto
     ();
				brokerTemplate.
     
      convertAndSend
     (
     
      getBroadcastDestination
     (), payload, accessor.
     
      getMessageHeaders
     ());
			}
			
     
      finally
      {
				userRegistry.
     
      purgeExpiredRegistries
     ();
			}
		}
	}
}

使用场景

启用代理中继，连接外部消息代理。 通常情况下，为集群模式，不止一个应用服务程序 —— 即不止一个代理中继，如下图所示：

使用条件

配置UserRegistryBroadcast目的地。
在AbstractMessageBrokerConfiguration类的userRegistryMessageHandler()Bean方法中，我们可以看到，当 UserRegistryBroadcast 目的地没被设置的时候，将会new NoOpMessageHandler()。所以必须满足两个条件：

1. 启用代理中继；
2. 配置UserRegistryBroadcast广播地址；

AbstractMessageBrokerConfiguration:

     
      public
      
     
      abstract
      
     
      class
      
     
      AbstractMessageBrokerConfiguration
      
     
      implements
      
     
      ApplicationContextAware
      {
    @Bean
	
     
      public
      MessageHandler 
     
      userRegistryMessageHandler
     () {
		if (
     
      getBrokerRegistry
     ().
     
      getUserRegistryBroadcast
     () == null) {
			
     
      return
      new 
     
      NoOpMessageHandler
     ();
		}
		SimpUserRegistry userRegistry = 
     
      userRegistry
     ();
		Assert.
     
      isInstanceOf
     (MultiServerUserRegistry.
     
      class
     , userRegistry, "
     
      MultiServerUserRegistry
      
     
      required
     ");
		
     
      return
      new 
     
      UserRegistryMessageHandler
     ((MultiServerUserRegistry) userRegistry,
				
     
      brokerMessagingTemplate
     (), 
     
      getBrokerRegistry
     ().
     
      getUserRegistryBroadcast
     (),
				
     
      messageBrokerTaskScheduler
     ());
	}
}

启用配置

WebSocketMessageBrokerConfigurer实现类:

@Configuration
@EnableWebSocketMessageBroker

     
      public
      
     
      class
      
     
      WebSocketConfigurer
      
     
      implements
      
     
      WebSocketMessageBrokerConfigurer
      {
	@Override
	
     
      public
      
     
      void
      
     
      configureMessageBroker
     (MessageBrokerRegistry config) {
		config.
     
      enableStompBrokerRelay
     ("topic").
     
      setUserRegistryBroadcast
     ("/topic/registry");
	}
}

---

NoOpMessageHandler

“无操作的消息处理器” —— 在上面AbstractMessageBrokerConfiguration类的userRegistryMessageHandler()Bean方法中，我们可以看到，当 UserRegistryBroadcast 目的地没被设置的时候，将会new NoOpMessageHandler()。

AbstractMessageBrokerConfiguration ：


     
      public
      
     
      abstract
      
     
      class
      
     
      AbstractMessageBrokerConfiguration
      
     
      implements
      
     
      ApplicationContextAware
      {
      
     
      private
      
     
      static
      
     
      class
      
     
      NoOpMessageHandler
      
     
      implements
      
     
      MessageHandler
      {
		   @Override
		   
     
      public
      
     
      void
      
     
      handleMessage
     (Message<?> message) {
		   }
	  }
}

说白了， NoOpMessageHandler 就是在没有配置UserRegistryBroadcast广播地址时，用来代替 UserRegistryMessageHandler 的 ——

1. 不订阅MessageChannel；
2. 不做任何Message处理；