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From: Knute Johnson <nospam@knutejohnson.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Volatile happens before question
Date: Tue, 17 Jan 2012 08:18:52 -0800
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On 1/17/2012 4:04 AM, raphfrk@gmail.com 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?
>
> So,
>
> 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.

Yes

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

Yes

> 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.

Yes

> (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 
programming.

-- 

Knute Johnson