RFR: 8288723: Avoid redundant ConcurrentHashMap.get call in java.time

[Attila Szegedi]

On Mon, 20 Jun 2022 15:01:55 GMT, liach <duke at openjdk.org> wrote:

>> src/java.base/share/classes/java/time/format/DateTimeTextProvider.java line 319:
>> 
>>> 317:                 store = prev;
>>> 318:             }
>>> 319:         }
>> 
>> You could do better here and use `computeIfAbsent` with `createStore` as its lambda. You could even change the signature of `createStore` to take `Entry<TemporalField, Locale>` as its parameter and then you could have this method be:
>> 
>>     private Object findStore(TemporalField field, Locale locale) {
>>         return CACHE.computeIfAbsent(createEntry(field, locale), this::createStore);
>>     }
>> 
>>     ...
>> 
>>     private Object createStore(Entry<TemporalField, Locale> entry) {
>>         ...
>>     }
>> 
>> This applies to most other changes you made, the one in `ZoneOffset` is the only one that's slightly different because there you have adjustment of `ID_CACHE` too.
>
> This behaves slightly different from the old initialization; the concurrent hash map blocks when the mapping function is run, just in case if non-blocking instantiation is what we want. If that's not a problem, I would prefer szegedi's suggestion.

@liach advance apologies for nitpicking: `ConcurrentHashMap` doesn't in general block while the mapping function runs. It can block _some_ concurrent updates, namely those that [hash to the same bin](https://urldefense.com/v3/__https://github.com/openjdk/jdk/blob/0f801fe6fd2fcc181121f9846f6869ca3a03e18a/src/java.base/share/classes/java/util/concurrent/ConcurrentHashMap.java*L324__;Iw!!ACWV5N9M2RV99hQ!JBDYAOKXAgaJoVrQyfzX6Cpak1HLEt3HG254q8COTDzpz7Ui1_Jm8iUTLqBdiluzP1PMfTEA4n31sDYGYuZr2t8$ ) and it won't block concurrent reads. I'm not saying the concern you raised isn't valid, just wanted to clarify that the situation is not nearly as grave as overall blocking; after all it ain't a `Collections.synchronizedMap` :-)

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PR: https://git.openjdk.org/jdk/pull/9208