Servlet HttpSessionListener that automatically exposes the session mutex
when an HttpSession gets created. To be registered as a listener in
The session mutex is guaranteed to be the same object during
the entire lifetime of the session, available under the key defined
SESSION_MUTEX_ATTRIBUTE constant. It serves as a
safe reference to synchronize on for locking on the current session.
In many cases, the HttpSession reference itself is a safe mutex
as well, since it will always be the same object reference for the
same active logical session. However, this is not guaranteed across
different servlet containers; the only 100% safe way is a session mutex.