RFR: 8315999: Improve Date toString performance [v7]

[温绍锦]

On Tue, 12 Sep 2023 13:54:34 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

> You haven't make the case for these changes, please describe the use cases when performance is a significant constraint on application performance. The changes largely just add more code to maintain without otherwise adding sufficient value.

Many scenarios require this optimization, such as:

1. The open source project [datax](https://github.com/alibaba/datax) is a data integration engine we maintain. When the input is a Date but the target type is String, the Date must be converted to String. In the flame graph below, commons-lang's FastDateFormat::format method consumes most of the time. It will be similar if replaced by DateTimeFormatter::format.
![image](https://github.com/openjdk/jdk/assets/1166785/1063aacf-c770-4c2d-9920-297be65ac6b0)


2. JSON libraries such as gson/jackson/fastjson need to convert Date/Instant into String, and format/toString usually takes up a relatively large proportion of time.

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PR Comment: https://git.openjdk.org/jdk/pull/15658#issuecomment-1715842265