Volatile variables and other variables

Question

The following is from classical Concurency in Practice:

When thread A writes to a volatile variable and subsequently thread B reads the same variable, the values of all variables that were visible to A prior to writing to the volatile variable, become visible to B after reading the volatile variable.

I am not sure I can really understand this statement. For example, what is the meaning of all variables in this context? Does this mean that using volatile also has side-effects to the usage of non-volatile variables?
It seems to me that this statement has some subtle meaning that I can not grasp.
Any help?

Answer 1

The answer to your question is in JLS #17.4.5:

A write to a volatile field (§8.3.1.4) happens-before every subsequent read of that field.

So if in one thread you have

aNonVolatileVariable = 2 //w1
aVolatileVariable = 5 //w2

And subsequently in another thread:

someVariable = aVolatileVariable //r1
anotherOne = aNonVolatileVariable //r2

You have the guarantee that anotherOne will be equal to 2, even if that variable is not volatile. So yes, using volatile also has side-effects to the usage of non-volatile variables.

In more details, this is due to 2 other guarantees provided by the Java Memory Model (JMM) in that same section: intra thread order and transitivity (hb(x,y) means x happens before y):

If x and y are actions of the same thread and x comes before y in program order, then hb(x, y).
[...]
If hb(x, y) and hb(y, z), then hb(x, z).

In my example:

• hb(w1, w2) and hb(r1, r2) (intra thread semantics)
• hb(w2, r1) because of the volatile guarantee

so you can conclude that hb(w1, r2) by transitivity.

And the JMM guarantees that all executions of a program will be sequentially consistent (i.e. will look like nothing has been reordered) if it is correctly synchronized with happens-before relationships. So in this specific case, the non-volatile read is guaranteed to see the effect of the non-volatile write.

Answer 2

It means if you write to ten non-volatile variable and write to a volatile one, all the non-volatile variables must be set before the volatile one.

If you read the volatile variable and all the non-volatile ones you can be sure that the order won't be swapped around.