RFR: 8372460: Use EnumMap instead of HashMap for DateTimeFormatter parsing to improve performance [v2]

[Chen Liang]

On Tue, 25 Nov 2025 06:44:57 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:
> 
>   copyright

I think instead of checking each component printer parser, we should check the public methods on `DateTimeFormatterBuilder` that can take a `TemporalField` and track the `onlyChronoField` there.

This is better because this is where users can actaully pass in non-ChronoField. For example, I last time discovered text printer parser, and now have discovered DefaultValueParser is problematic too.

So I believe guarding where users can pass custom TemporalField and adding a boolean field on a DateTimeFormatterBuilder to keep track of this is better.

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Changes requested by liach (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28471#pullrequestreview-3521038435