JEP draft: Scope Locals

[Peter Levart]

On 15/05/2021 19:50, Peter Levart wrote:
> Another question: Suppose there are two inheritable ScopeLocal 
> variables with bound values in scope (A and B) when I take a snapshot:
>
> var snapshot = ScopeLocal.snapshot();
>
> now I pass that snapshot to a thread which does the following:
>
> ScopeLocal
>     .where(C, "value for C")
>     .run(() -> {
>         System.out.printf("A:%s B:%s C:%s\n", A.isBound(), 
> B.isBound(), C.isBound());
>         ScopeLocal.runWithSnapshot​(() -> {
>             System.out.printf("A:%s B:%s C:%s\n", A.isBound(), 
> B.isBound(), C.isBound());
>         }, snapshot);
>         System.out.printf("A:%s B:%s C:%s\n", A.isBound(), 
> B.isBound(), C.isBound());
>     });
>
> What would this code print?
>
> ...in other words, does runWithSnapshot replace the whole set of bound 
> values or does it merge it with existing set? 


...let me answer this myself, after checking current implementation. The 
answer is: "It depends on whether C is an inheritable ScopeLocal or 
non-inheritable". If I understand the code correctly there are two 
independent sets of bindings: inheritable and non-inheritable. 
snapshot() retrieves the current inheritable set and runWithSnapshot​() 
replaces current inheriatable set with the snapshot for the execution of 
given Runnable and afterwards swaps previous inheritable set back.

So if C is inheritable, the output would be:

A:false B:false C:true
A:true B:true C:false
A:false B:false C:true

...but if C is non-inheritable, the output would be:

A:false B:false C:true
A:true B:true C:true
A:false B:false C:true

This seems consistent. In other words, non-inheritable bindings are 
never transferred from thread to thread automatically or by 
snapshot/runWithSnapshot. I can see that snapshot/runWithSnapshot was 
meant as a mechanism to "simulate" inheritance of bindings when 
execution is transferred from one thread to another which is not a newly 
started child thread.

Regards, Peter

Peter