Groups > comp.lang.java.programmer > #10925

Re: reading the JLS (17.4.5)

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From	Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at>
Subject	Re: reading the JLS (17.4.5)
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Lew <lewbloch@gmail.com> wrote:
> Andreas Leitgeb wrote:
>> Given some (simplified) sample code:
>>   class Test {
>>      /*non-vol*/ int n1;
>>      volatile    int v1;
>> 
>>      void t1() {
>>        n1 = 1;
>>        v1 = n1; // v1 uses n1
>>      }
>> 
>>      void t2() {
>>        int r1=v1, r2=r1*n1; // r2 uses r1
>>        assert ! (r1 == 1 && r2 != 1) : "huh?";
>>      }
>>   }
>> Two threads T1 and T2 may at some point run t1() and t2()
>> respectively, then I should expect, per transitivity of
> You have to establish a /happens-before/ between the invocations
> of t1() an t2().
> If two threads begin to execute the methods, you cannot guarantee
> that t1() will /happen-before/ t2(), so the read of 'v1' in the
> latter could result in the default value.

Perhaps you found the/a sore spot of my (mis?)understanding.

*iff* a *volatile* read gets to see the result of a *volatile*
write, then doesn't that say anything about that the write must
have "happened-before" the read?

In my example, I didn't mean to imply, that r1 would always
turn out to be 1. But *if* it does, then could anything be
assumed about r2 ?

If the write to v1 (but not to n1) in t1() were in a
synchronized(this)-block, and so were the read of v1
(but not n1) in t2(), then would anything be implied
about r2 *if* observing r1 == 1 in T2?

In practice, I think that establishing a happens-before
based on observation of a read's value is quite essential,
but I may of course be wrong here. I don't even see how one
could determine the order in which synchronized blocks for
a common monitor are actually passed through by different
threads, other than by observing reads.

Thread

reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-18 12:10 +0000
  Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-18 06:57 -0800
    Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-20 17:54 +0000
      Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-20 10:50 -0800
        Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-20 12:12 -0800
          Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-21 08:54 +0000
            Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-21 10:56 -0800
              Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-21 12:02 -0800
  Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-18 09:21 -0800
    Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-20 18:35 +0000
      Re: reading the JLS (17.4.5) Lew <lewbloch@gmail.com> - 2011-12-20 19:08 -0800
        Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-21 08:37 +0000
          Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-21 10:46 -0800
          Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-21 12:09 -0800

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