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.
- Juergen Hoeller
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