PR #22995 - OpenAPITools/openapi-generator

Description

This PR changes the way the java modelEnum.mustache templates generate the enums. The change is to introduce a HashMap that its filled during the static phase of the enum initialization, and is kept filled through the lifetime of the object.

The files that need revision are the modelEnum.mustache files.

This is my first PR on this project, so I kindly ask you some patience if I am missing any steps on your PR process :)

Thread safety

The initialization phase is done at static init time. No need for barriers.
The lookup is not hot. No need for barriers for the get.

Test

Unit

Two client codegen test have been updated to now be explicit about the use of toLowerCase(Locale.ROOT).

Perf

I have introduced in this PR a JMH benchmark for you to checkout the distinct ways i approached this perf. The benchmark class is at org.openapitools.perf.EnumLookupJmhBenchmark. This benchmark suite must be deleted before merge, as is there to provide a basis for discussion.
The results are

Benchmark                                                        (inputSize)   Mode  Cnt         Score         Error  Units
EnumLookupJmhBenchmark.hashMapLookupCaseInsensitive                    50000  thrpt    5  25144177.892 ±  820137.701  ops/s
EnumLookupJmhBenchmark.hashMapLookupWithOptional                       50000  thrpt    5  99265713.896 ± 2460266.752  ops/s
EnumLookupJmhBenchmark.hashMapLookupWithoutOptional                    50000  thrpt    5  99777930.270 ± 2376641.407  ops/s
EnumLookupJmhBenchmark.linearLookup                                    50000  thrpt    5  24858482.938 ±  367926.448  ops/s
EnumLookupJmhBenchmark.linearLookupCaseInsensitive                     50000  thrpt    5  12758767.698 ±  667129.809  ops/s
EnumLookupJmhBenchmark.treeMapLookupCaseInsensitiveWithOptional        50000  thrpt    5  14069454.341 ±  331263.365  ops/s

on the case sensitive case, a x4 speedup. On the case insensitive case a x2 speedup.

The benchmark can be run with ./mvnw -f perf/jmh/pom.xml -q clean package && java -jar perf/jmh/target/benchmarks.jar EnumLookupJmhBenchmark

Closes #22995

PR checklist

Read the contribution guidelines.
Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
Run the following to build the project and update samples:
```
./mvnw clean package || exit
./bin/generate-samples.sh ./bin/configs/*.yaml || exit
./bin/utils/export_docs_generators.sh || exit
```
(For Windows users, please run the script in WSL)
Commit all changed files.
This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
These must match the expectations made by your contribution.
You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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