[concurrency-interest] Question about "happens-before" andreordering

Pete Soper Pete.Soper at Sun.COM
Fri May 19 13:32:01 EDT 2006


Jeremy Manson wrote:
> Pete Soper wrote:
> 
>>   JLS 17.4.5 says the edge is from the end of the constructor, which 
>> presumably can't be reached if an exception is thrown or the thread 
>> stops making progress through the constructor. If a more tricky 
>> definition of "end" is being used, then I shouldn't be surprised, but 
>> will be anyway.
> 
> 
> The section on finalization to which 17.4.5 points clarifies this to 
> mean that the constructor's "completion" happens-before the finalize 
> method.  In the context of the JLS, "completion" means completion via 
> normal means or via exceptional (also called abrupt) means.
> 
> So "end" here means via an exception, as well.  We probably should have

Oh, so what I asserted to Tom is wrong. Thanks for this clarification.

If a finalizer method might run on an object for which construction is 
incomplete in an arbitrary manner (by virtue of the ctor throwing an 
exception), it means writing a good finalizer method is more challenging 
than writing correct concurrent code for the uninitiated, or at least 
just as challenging. It tells me if I ever get tempted to write a 
finalizer method I'll put an edge between the last statement of the ctor 
and the first statement of the finalizer as the first lines of code 
written. And it's difficult to reconcile this with Bill Pugh's comment 
about 4034630 on the page Tom pointed to. Is the JLS being updated for 
Mustang to go along with the implementation change, or am I still not 
getting it?

-Pete

> used the word "completion" in 17.4.5 to make it clearer.
> 
> OTOH, if the thread hangs, then completion is never reached, as you 
> point out.
> 
>                     Jeremy
> 



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