RFR: 8372460: Use EnumMap instead of HashMap for DateTimeFormatter parsing to improve performance [v4]

[Roger Riggs]

On Mon, 1 Dec 2025 02:33:33 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> This PR optimizes the parsing performance of DateTimeFormatter by replacing HashMap with EnumMap in scenarios where the keys are exclusively ChronoField enum values.
>> 
>> When parsing date/time strings, DateTimeFormatter creates HashMaps to store intermediate parsed values. HashMap has more overhead for operations compared to specialized map implementations.
>> 
>> Since ChronoField is an enum and all keys in these maps are ChronoField instances, we can use EnumMap instead, which provides better performance for enum keys due to its optimized internal structure.
>> 
>> Parsing scenarios show improvements from 12% to 95%
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   bug fix

Spreading out and duplicating the state across multiple classes isn't very satisfactory.
Since non-ChronoField is very unlikely, I'd suggest a more localized change confined to Parsed.
Always create the initial EnumMap and refactor the `fieldValues.put()` calls to a private utility method to catch the ClassCatchException and upgrade the map to a HashMap.
That should retain the performance improvements without any extra overhead or non-local code changes for all of the normal cases.

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PR Comment: https://git.openjdk.org/jdk/pull/28471#issuecomment-3596762057