<i18n dev> RFR: 8335791: Speed ​​up j.u.Formatter & String.format [v2]

[Shaojin Wen]

> String.format is widely used, and improving its performance is very meaningful. This PR can significantly improve the performance of String.format. Sorry, there are many changes, which will take a lot of time. I hope you can review it patiently.
> 
> 
> Improved performance includes the following:
> 
> ## 1. Write directly during the parse process to reduce object allocation.
> 
> In the current Formatter implementation, some objects do not need to be allocated, such as:
> 
> 
> class Formatter {
>     public Formatter format(Locale l, String format, Object ... args) {
>     	List<FormatString> fsa = parse(format);
>     	// ...
>     }
> 
>     static List<FormatString> parse(String s) {
>     	ArrayList<FormatString> al = new ArrayList<>();
> 
>         while (i < max) {
>             int n = s.indexOf('%', i);
>             if (n < 0) {
>                 // 
>                 al.add(new FixedString(s, i, max));
>             }
>         }
>     }
> }
> 
> In the process of parsing, the content that is not a Specifier is directly appended without going through FixedString. By directly printing the parsed FormatString object, there is no need to construct a `List<FormatString> fsa` to store it.
> 
> ## 2. Fast path print
> Use specialized FormatString implementations for single-character and single-width specifiers to avoid calling the large FormatSpecifier#print method.
> 
> ## 3. String.format directly calls j.u.Formatter
> String.format directly calls j.u.Formatter via SharedSecrets to improve performance

Shaojin Wen has updated the pull request incrementally with four additional commits since the last revision:

 - use unknownFormatConversion method construct UnknownFormatConversionException
 - uppercase static final field name & code format
 - add stable annotation
 - replace cast to pattern matching

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20055/files
  - new: https://git.openjdk.org/jdk/pull/20055/files/982e3f0e..27ff0b5d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20055&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20055&range=00-01

  Stats: 26 lines in 1 file changed: 1 ins; 5 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/20055.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20055/head:pull/20055

PR: https://git.openjdk.org/jdk/pull/20055