`AbstractSessionWebSocketMessageBrokerConfigurer` triggers eager instantiation of `SessionRepository` due to non-static declaration of `ApplicationListener`

[odrotbohm]

AbstractSessionWebSocketMessageBrokerConfigurer declares webSocketRegistryListener() returning an ApplicationListener. As this method is declared non-static, it requires the class to be instantiated, which in turn triggers the injection of autowired dependencies such as the SessionRepository. This might cause a cyclic dependency issue as the repository implementation such as MongoDB might bootstrap infrastructure that in turn needs access to all ApplicationListener instances.

Declaring webSocketRegistryListener() as static method prevents the class from being instantiated to use the bean instance and the subsequent dependency initialization procedure gets shortcut, preventing the cyclic dependency problem.