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Think of a lockable object as a blackboard (lockable) in a class room containing a teacher (writer) and many students (readers).
While a teacher is writing something (exclusive lock) on the board:
Nobody can read it, because it’s still being written, and she’s blocking your view => If an object is exclusively locked, shared locks cannot be obtained.
Other teachers won’t come up and start writing either, or the board becomes unreadable, and confuses students => If an object is exclusively locked, other exclusive locks cannot be obtained.
When the students are reading (shared locks) what is on the board:
They all can read what is on it, together => Multiple shared locks can co-exist.
The teacher waits for them to finish reading before she clears the board to write more => If one or more shared locks already exist, exclusive locks cannot be obtained.
