Groups > comp.lang.java.programmer > #10924

Re: reading the JLS (17.4.5)

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From	Lew <lewbloch@gmail.com>
Newsgroups	comp.lang.java.programmer
Subject	Re: reading the JLS (17.4.5)
Date	Tue, 20 Dec 2011 19:08:01 -0800 (PST)
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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.

-- 
Lew

> "happens-before" that if r1 == 1, then r2 would *have to*
> == 1, too.  Is there a happens-before relation between
> between setting n1 and reading n1 via write&read on the
> volatile v1 and each intra-thread ordering?
> 
> There is a slightly similar example later in the JLS about
> a final and a non-final set in a constructor, but that is
> different, in that the non-final is set after the final.
> I don't know for sure, if the ordering of the assignments
> was relevant in that example.

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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