A quick tip on how to add a (Vaadin) session init listener: first, create a VaadinServiceInitListener as documented at VaadinServiceInitListener tutorial.

Afterwards, use this code to init the session:

public class ApplicationServiceInitListener
        implements VaadinServiceInitListener {

    @Override
    public void serviceInit(ServiceInitEvent event) {
         // ...
        event.getSource().addSessionInitListener(e -> {
            // init the session: e.getSession()
        });
    }
}

A bug report #417 to enhance the official docs have been opened - please upvote.

Init’ing the lower-level Servlet Session

Then you need @WebListener-annotated class implementing HttpSessionListener.

Spring

Documented in this StackOverflow answer.

Simply create a class ApplicationServiceInitListener as above, but don’t forget to annotate it with @Component.

Adding @EventListener on SessionInitEvent doesn’t work, implementing ApplicationListener<SessionInitEvent> doesn’t work.

MAKE SURE to remove any stray META-INF/services/com.vaadin.flow.server.VaadinServiceInitListener file, otherwise serviceInit() will be called but SessionInitListener WILL NOT.