[concurrency-interest] ConcurrentHashMap and Unsafe usage

Paul Sandoz paul.sandoz at oracle.com
Wed May 23 21:13:08 EDT 2018



> On May 23, 2018, at 4:56 PM, Doug Lea via Concurrency-interest <concurrency-interest at cs.oswego.edu> wrote:
> 
> On 05/23/2018 07:40 PM, Kasper Nielsen via Concurrency-interest wrote:
>> Hi,
>> 
>> Now that most of the concurrent data structures in j.u.c have
>> transitioned to using VarHandle.
>> I was wondering why ConcurrentHashMap still uses Unsafe for its variables?
>> 
> 
> Bootstrapping. The VM code to compile VarHandles relies on
> ConcurrentHashMap. People have looked into somehow changing this, but
> nothing seems better than just allowing this to continue.
> 

Right. It seems possible and we have looked into it and other areas like in ThreadLocalRandom [*], but priority wise (for me at least) it always got bumped down in the priority queue of stuff to do.
 
Paul.

[*] One the challenge is to tease apart CMH usage from some areas of the java.lang.invoke implementation and replace with something similar but more focused. Another is to make VarHandle allocations lazy (e.g. if we could ldc a VarHandle and express that in the Java language and byte code, we can already do the latter with constant dynamic).


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