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

[Shaojin Wen]

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

We should place more processing logic in the pattern parsing stage, rather than the text parsing stage.

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PR Comment: https://git.openjdk.org/jdk/pull/28471#issuecomment-3599870510