Re: Volatile happens before question

Knute Johnson <>
Tue, 17 Jan 2012 08:18:52 -0800
On 1/17/2012 4:04 AM, wrote:

The spec says that all writes to volatiles can be considered to happen
before all subsequent reads. What does "subsequent" mean, is that
with regards to real time?


Thread 1
int b = 0;
volatile boolean a = false;
b = 1;
a = true;

Thread 2
if (a) {
   System.out.println("The value of b is " + b);

Since the setting of a to true happens before the reading of a as
true, the println must happen after b is set to 1.


This means that either nothing will be printed or "The value of b is
1" will be printed.


Does this work in reverse too?

For example,

Thread 1
int b = 0;
volatile boolean a = false;
a = true;
b = 1;

Thread 2
int bStore = b;
if (!a) {
   System.out.println("The value of b is " + bStore);

Will this always print either "The value of b is 0" or nothing.


(bStore = b) happens before (read a as false)
(read a as false) happens before (set a = true) [is this valid?]
(set a = true) happens before (set b = 1)

So, bStore = b happens before set b = 1, so bStore = 0.

Only if a is false

Effectively, the rule would be "A read to a volatile happens before
the write to that volatile that overwrites the value that was read".

I don't think so but I'm not really sure what you mean.

However, that wasn't clear from the spec. I think since read/writes
to volatiles are synchronization actions, then when running the
program, they can be considered to have happened in some ordering
(consistent with program order in the threads). As long as the
program works no matter what the ordering is picked, then it is fine.

Not sure what you really mean here. There is also a side effect of
writing a volatile variable and that is that all other variable writes
that happened before in that thread are visible to any other thread that
subsequently reads the volatile variable. "Locking can guarantee both
visibility and atomicity; volatile variables can only guarantee
visibility" Java Concurrency in Practice, Brian Goetz. You should buy a
copy of that book if you are going to get serious about concurrent


Knute Johnson

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