UIScope and RouteScope does not follow lifecycle of UI instance but actually the attach/detach

[samulivaadin]

Consider scenario where a proxy server breaks a websocket connection and then the "Resynchronizing UI by client’s request" happens. Internally ui.getInternals().getStateTree().prepareForResync() gets called.

What happens next is that the same existing UI instance will receive a detach event and then attach event. This will cause all UIScope and RouteScope beans to be cleaned on detach event and then re-created. So UIScope is not bound to actual UI instance lifecycle, but attach/detach lifecycle.

Documentation (https://vaadin.com/docs/latest/integrations/spring/scopes) talks only about UI lifecycle and nothing about attaching or detaching so either the documentation or implementation has to change.

Also javadoc of UI.onDetach says "Called before the UI is removed from the session. A UI instance is detached exactly once, either..." which is incorrect in this case. UI is not removed from session and onDetach can be called multiple times on same instance because of resync causes detach & attach without creating a new instance.

Observed with Vaadin 23.2.6.

[mvysny]

The Vaadin-Spring documentation at https://vaadin.com/docs/latest/integrations/spring/scopes states that:

• UIScope to target beans to a browser window or tab opened by the user
• The UIService bean instance is the same inside the same UI

Which is not the case as this ticket shows. Resync in a single tab should keep UI-scoped Spring bean instances, since the UI instance stays the same during the resync.

However, this feels like a bug in Vaadin:

• UI Javadoc states that UI instance is detached exactly once, however
• ui.getInternals().getStateTree().prepareForResync() clearly is capable of detaching the UI multiple times on resync.

A ticket should also be filed at https://github.com/vaadin/flow/issues to not to detach/attach UI in case of resync; fixing that ticket will then fix this ticket in turn.

[samulivaadin]

Here's a simple application to demonstrate this. Without a proxy breaking the websocket is not exactly the same, but the app calls same internals as in real case and the symptoms are reproducible.

scopes.zip

[mvysny]

This is related: it's very hard to implement Tab scope in Vaadin atm: vaadin/flow#13468