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：
在这里插入图片描述

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：

启用代理中继；
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 ——

不订阅MessageChannel；
不做任何Message处理；