Spring Boot WebSocket：我如何知道客户何时取消订阅？

Question

我目前有一个简单的WebSocket和STOMP设置，客户端连接到一个主题（带有ID）。控制器立即响应所要求的内容，并设置一个变量，指示客户端订阅了什么。一个用@Scheduled注释的方法现在每隔几秒就向客户端发送它所请求的内容。

在客户端第一次连接之前，预定的方法不会执行任何操作。但是，在第一次订阅之后，无论客户是否订阅，它都将继续发布。

@Controller
public class ServiceWebSocketController {

    @Autowired
    private ServiceService serviceService;

    @Autowired
    WebSocketSessionController webSocketSessionController;

    @Autowired
    private SimpMessagingTemplate simpMessagingTemplate;

    private Set<Long> services = new HashSet<>();

    @SubscribeMapping("/service/{serviceId}")
    public ServiceDTO subscribe(@DestinationVariable("serviceId") final Long serviceId) throws SQLException {
        System.out.println("Subscribed to Service with ID: " + serviceId);
        services.add(serviceId);
        return serviceService.getServiceWithProperties(serviceId).orElseThrow(() -> new ResourceNotFoundException("Service", "id", serviceId));
    }

    @Scheduled(fixedDelay = 2000)
    public void service() throws SQLException {
        services.removeIf(serviceId -> !webSocketSessionController.hasSubscriptionTo("/topic/service/" + serviceId));

        // Publish specified Service data to each anonymously subscribed client.
        services.forEach(serviceId -> {
            try {
                System.out.println("Publishing Service with ID: " + serviceId);
                // We don't use .convertAndSendToUser here, because all our clients are anonymous.
                simpMessagingTemplate.convertAndSend("/topic/service/" + serviceId, serviceService.getServiceWithProperties(serviceId));
            } catch (SQLException e) {
                e.printStackTrace();
            }
        });
    }
}

如何判断客户是否取消订阅？如果存在类似于@UnsubscribeMapping的东西，我可以简单地再次将currentSubscriptionServiceId变量设置为null，从而阻止调度方法连续发布数据。

Answer 1

你可以像这样听SessionUnsubscribeEvent这个活动：

@Controller
public class SessionUnsubscribeListener implements ApplicationListener<SessionUnsubscribeEvent> {

   @Override
   public void onApplicationEvent(SessionUnsubscribeEvent event) {
       GenericMessage message = (GenericMessage) event.getMessage();

       String simpDestination = (String) message.getHeaders().get("simpDestination");

       if ("/topic/service".equals(simpDestination)) {
           // do stuff
       }
   }
}