Scheduled websocket push with Springboot

Question

I want to create a simple news feed feature on the front end that will automatically update through websocket push notifications.

The technologies involved are:

• Angular for the general front-end application
• SockJS for creating websocket communication
• Stomp over webosocket for receiving messages from a message broker
• Springboot Websockets
• Stomp Message Broker (the java related framework)

What I want to achieve on the front end is:

1. Create a websocket connection when the view is loaded
2. Create s stomp provider using that websocket
3. Have my client subscribe to it
4. Catch server pushed messages and update the angular view

As far as the server side code:

1. Configure the websocket stuff and manage the connection
2. Have the server push messages every X amount of time (through an executor or @Scheduled?).

I think I have achieved everything so far except the last part of the server side code. The example I was following uses the websocket in full duplex mode and when a client sends something then the server immediately responds to the message queue and all subscribed clients update. But what I want is for the server itself to send something over Stomp WITHOUT waiting for the client to make any requests.

At first I created a spring @Controller and added a method to it with @SendTo("/my/subscribed/path") annotation. However I have no idea how to trigger it. Also I tried adding @Scheduled but this annotation works only on methods with void return type (and I'm returning a NewsMessage object).

Essentially what I need is to have the client initialize a websocket connection, and after have the server start pushing messages through it at a set interval (or whenever an event is triggered it doesn't matter for now). Also, every new client should listen to the same message queue and receive the same messages.

Answer 1

Before starting, make sure that you have the websocket dependencies in your pom.xml. For instance, the most important one:

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-websocket</artifactId>
    <version>${org.springframework-version}</version>
</dependency>

Then, you need to have your configuration in place. I suggest you start with simple broker.

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/portfolio").withSockJS();
    }

    @Override
    public void configureMessageBroker(MessageBrokerRegistry config) {
        config.setApplicationDestinationPrefixes("/app");
        config.enableSimpleBroker("/topic", "/queue");
    }

}

Then your controller should look like this. When your AngularJs app opens a connection on /portfolio and sends a subscription to channel /topic/greeting, you will reach the controller and respond to all subscribed users.

@Controller
public class GreetingController {
    
    @MessageMapping("/greeting")
    public String handle(String greeting) {
        return "[" + getTimestamp() + ": " + greeting;
    }
}

With regard to your scheduler question, you need to enable it via configuration:

@Configuration
@EnableScheduling
public class SchedulerConfig{}

And then schedule it:

@Component
public class ScheduledUpdatesOnTopic{

    @Autowired
    private SimpMessagingTemplate template;
    @Autowired
    private final MessagesSupplier messagesSupplier;

    @Scheduled(fixedDelay=300)
    public void publishUpdates(){
        template.convertAndSend("/topic/greetings", messagesSupplier.get());
    }
}

Hope this somehow clarified the concept and steps to be taken to make things work for you.