VaadinSession.getSession() is sometimes null in session destroy listeners

Description of the bug / feature

In my usecase I want to keep track of all VaadinSessions. For that reason I am doing the following in the AppShell:

		serviceInitEvent.getSource().addSessionInitListener( ... );
		serviceInitEvent.getSource().addSessionDestroyListener(sessionDestroyEvent -> {
			if(sessionDestroyEvent != null && sessionDestroyEvent.getSession() != null) {
				//do smth with the sessionId
                               //this code is not reached
			}
		});

Expected behavior

I get the correct session when initialized, and I get the session before it is destroyed.

Actual behavior

The destroyed is session null. Problem: I can not track which session was destroyed.

Versions:

- Vaadin / Flow version: 18.0.3
- Java version: 11
- OS version: Windows 10
- Application Server (if applicable): Spring Boot with Tomcat

[javier-godoy]

In https://github.com/vaadin/flow/issues/6959 it was suggested to use VaadinRequest.getCurrent().getWrappedSession() instead of event.getSession().getSession()

[Legioth]

I couldn’t reproduce the symptoms that you’re describing. I created a new project with everything set as default from https://start.vaadin.com/ and made this addition to the Application class:

@Bean
public VaadinServiceInitListener serviceListener() {
    return new VaadinServiceInitListener() {
        @Override
        public void serviceInit(ServiceInitEvent serviceInitEvent) {
            System.out.println("Service init");
            serviceInitEvent.getSource().addSessionInitListener(sessionInitEvent -> {
                System.out.println("Session init: " + sessionInitEvent.getSession());
            });
            serviceInitEvent.getSource().addSessionDestroyListener(sessionDestroyEvent -> {
                System.out.println("Session destroy: " + sessionDestroyEvent.getSession());
            });
        }
    };
}

When the application is running, I can see log messages like these:

Session destroy: com.vaadin.flow.spring.SpringVaadinSession@3948cea6
Session init: com.vaadin.flow.spring.SpringVaadinSession@4a85427
Session destroy: com.vaadin.flow.spring.SpringVaadinSession@4a85427

I also looked at the framework code that creates those event objects and I couldn’t find any code path that wouldn’t have failed with a NullPointerException before an event without a session would have reached the listener.

Based on that, I have two potential things for you to look up:

1. Maybe there’s some other problem that means your listener is never called, rather than being called but going the wrong way in the if? You can find out that by adding the same kind of simple logging that I had in my test code.
2. Maybe there’s some add-on or such that introduces another code path than then one that I could find. Identifying that kind of case should be easy if you could share the stacktrace that leads to invoking the listener. You can find the stacktrace by adding this line to the beginning of the listener: new Exception().printStackTrace();.