Spring root application context and servlet context confusion

Question

I know that I need to register classes annotated with @Controller in my servlet context to make my webapp accesible. Usually, I will do it the following way:

@Configuration
@EnableWebMvc
@ComponentScan({"foo.bar.controller"})
public class WebConfig extends WebMvcConfigurerAdapter {
    //other stuff like ViewResolvers, MessageResolvers, MessageConverters, etc.
}

All other configuration classes I added to my root application context. Here is how my dispatcher initializer usually look like:

public class DispatcherServletInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[] { RootConfig.class, ServiceConfig.class };
    }

    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[] { WebConfig.class };
    }

    @Override
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }
}

But things are getting more interesting when I started to use WebSockets. To get websockets working, you have to put WebSoketConfig.class to servlet context. Here is my example of WebSocketConfig:

@Configuration
@EnableScheduling
@EnableWebSocketMessageBroker
public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

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

    @Override
    public void configureClientInboundChannel(ChannelRegistration channelRegistration) {
        channelRegistration.taskExecutor().corePoolSize(4).maxPoolSize(8);
    }

    @Override
    public void configureClientOutboundChannel(ChannelRegistration channelRegistration) {
        channelRegistration.taskExecutor().corePoolSize(4).maxPoolSize(8);
    }

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

}

Also, I've created a service to send a message to the topic:

@Service
public class TimeServiceWsImpl implements TimeServiceWs {

    @Autowired
    private SimpMessagingTemplate messagingTemplate;

    @Override
    public void sentCurrentTime() {
        long currentTime = System.currentTimeMillis();
        String destination = "/topic/chatty";
        logger.info("sending current time to websocket /topic/time : " + currentTime);
        this.messagingTemplate.convertAndSend(destination, currentTime);
    }
}

I need to use this service in some other services (Autowire it). And now I'm in a deadlock:

1. If I'm trying to create TimeServiceWs bean inside root application context, as expected it doesn't see SimpMessagingTemplate bean and throws NoSuchBeanDefinitionException
2. If I'm trying to create TimeServiceWs bean inside servlet context, then I'm unable to autowire it to any another service, because root context can't see servlet context beans(as far as I know)
3. If I move all my configurations to servlet context, all beans are successfully created, but I get the following exception: java.lang.IllegalStateException: No WebApplicationContext found and can't access my webapp

What am I supposed to do? What should be inside root context? What should be inside servlet context? And could you please clarify the difference between these context one more time please?

If you will need any additional information, just let me know.

Answer 1

Most Spring MVC applications have one root context containing all service layer / DAO layer beans, and one servlet context per spring dispatcher servlet of the application, which contains (at least) the controllers of each servlet.

The idea being that is that one application might have several servlet dispatchers, for example one for URL /shopping/* and the other for URL /reporting/*, each with it's own set of controllers.

The controllers of one servlet dispatcher are isolated from each other, meaning although they are also Spring beans, they cannot be injected in each other.

Service layer and DAO beans in the root context are visible in all servlet contexts, so Service layer beans can be injected in any controller, but not the other way around.

The root context is said to be the parent of the controller servlet context/contexts.

It's all meant to be a mechanism of isolating groups of beans from each other to ensure no unmeant dependencies are possible.

Given this and going through the questions:

• If I'm trying to create TimeServiceWs bean inside root application context, as expected it doesn't see SimpMessagingTemplate bean and throws NoSuchBeanDefinitionException: Move the SimpleMessagingTemplate to the root context, it's a bean like a DAO that can be useful anywhere in the application so it should be in the shared root context.

• If I'm trying to create TimeServiceWs bean inside servlet context, then I'm unable to autowire it to any another service: If it's meant to be autowired to other services, leave it in the root context then.

  - If I move all my configurations to servlet context, all beans are successfully created, but I get java.lang.IllegalStateException: No WebApplicationContext found: Do the opposite, move basically all beans to the root context, and leave on the servlet context only the beans that are specific of that part of the application, many times only the controllers.