[Java][HttpClient] Fix memory leak with virtual threads (#21729)

[juriad]

Fixes #21729 - memory leak in Apache HttpClient when the result (status code and headers) were stored under the thread id. In a setup when virtual thread executor is used, each execution uses a different thread, and so the map grows without limit.

The fix replaces ConcurrentHashMap with ThreadLocal, which should assure that once the Thread ends, the result is deallocated. It solution should be as thread-safe as before.

@wing328 - you reviewed the PR adding the memory leak, would you care to take a look at this fix?

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

also cc @aaronforest-wf - the author of #19054

[Copilot]

Pull Request Overview

This PR fixes a memory leak in the Apache HttpClient implementation when using virtual threads. The issue occurred because status codes and response headers were stored in ConcurrentHashMaps indexed by thread ID, causing unlimited growth when virtual threads are used since each execution creates a new thread.

• Replaces ConcurrentHashMap with ThreadLocal for storing status codes and response headers
• Removes the now-unused ConcurrentHashMap import
• Updates getter methods to use ThreadLocal.get() instead of map lookups

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
modules/openapi-generator/src/main/resources/Java/libraries/apache-httpclient/ApiClient.mustache	Template file updated to use ThreadLocal instead of ConcurrentHashMap for thread-safe storage
samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java	Generated sample code updated with ThreadLocal implementation
samples/client/echo_api/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java	Generated sample code updated with ThreadLocal implementation

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[Copilot]

ThreadLocal.get() can return null if no value has been set for the current thread. This will cause a NullPointerException when auto-unboxing Integer to int. Consider using ThreadLocal.withInitial() or add null checking.

[juriad]

The same issue was always there before. It is an error to call the method before any call is made.

[Copilot]

ThreadLocal.get() can return null if no value has been set for the current thread. This could cause NullPointerException in calling code that expects a Map. Consider using ThreadLocal.withInitial() to provide a default empty map.

[juriad]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Thank you. I hope I fixed that. If even the committer should be aligned, I'll probably need to jump on my personal laptop.

[wing328]

let's give it a try.

thanks for the contribution