[GitHub] [jena] afs edited a comment on issue #709: Add volatile to make method thread-safe

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afs edited a comment on issue #709: Add volatile to make method thread-safe
URL: https://github.com/apache/jena/pull/709#issuecomment-599228455
 
 
   I'm not a JMM expert either - it's good to thrash out the details. We both learn - the code is better. Unit testing concurrent code is insufficient - thinking and talking about concurrent code is needed.
   
   > The thread 2 then may still not see a completely initialized Set.
   
   meaning the set object from thread 1 is not visible properly yet? OK - I can see that now.
   
   Making `acceptedParams_` volatile puts in a happens-before point (write to a volatile). 
   I was hoping to avoid a volatile - more a style thing, the code is trying to create an immutable object while allowing `fusekiParams()` or `customParams()` to be overridden. That's feature not currently used but is there to allow additional params to be added by customizations.
   
   The fact that two objects can be created does not matter. If thread 1 creates a set, and uses it, and thread 2 sees null still, creates its own object and uses it, eventually overwriting `acceptedParams_` is not a problem (it is always the same set contents).
   
   Is there a better way to achieve subclass overrides to provide `customParams()`? If it weren't for that, everything could be done in class statics.
   
   Otherwise, volatile it is.
   

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