Understanding the conditions behind using volatile variable

Rohit Bajaj
Rohit Bajaj used Ask the Experts™
I came across the following in the book java concurrency in practice :
Frankly speaking i dont understand why all of these conditions are required.
Unable to find any reasons for all this.

Please help me understand it.

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Volatile means that the var has to be read from main memory so it can *be seen for both reads and writes with equal integrity.

(There's a bit more subtlety to it than that, but a read-up in the literature will explain it in more detail.)

* Sorry - edited typo.
Distinguished Expert 2018
Volatile is a weaker form of synchronisation (volatile variables only guarantee visibility, not atomicity). Variables can be cached in registers or CPU caches and thus can be hidden from other processors. A volatile variable on the other hand is shared by everyone since it is stored in main memory.

The way I understand it, the conditions you read mean that you should use volatile only when

1) atomicity is not a requirement and
2) the volatile variable does not participate in other state variables that calculate a result and
3) you do not need any locking

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