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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 09:21:17 -0800
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On 1/17/2012 8:49 AM, Knute Johnson wrote:
> On 1/17/2012 8:33 AM, markspace wrote:
>> On 1/17/2012 8:18 AM, Knute Johnson wrote:
>>
>>>> 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
>>
>>
>> I'm pretty sure Peter got the correct answer which is "no." I want to
>> stick my answers in a separate reply, so I'll just add that this is an
>> easy mistake to make and if both Peter and you hadn't already provided
>> some insights I probably would have flubbed it up myself.
>>
>> Reasoning about synchronization can be pretty hard. Make sure you
>> consider carefully all possibilities when analyzing multi-threaded
>> programs for bugs.
>
> OK, I'll bite, if a is false how can b be anything but 0?
>


And if b = 1 could be reordered before a = true then b would be flopping 
all over the place because as soon as a = true b would again have to be 0.

-- 

Knute Johnson