[Java] Cache DoFn invoker constructors per instance

[lofifnc]

Summary

ByteBuddyDoFnInvokerFactory currently resolves a DoFn's input and output type descriptors before every generated-constructor cache lookup. Runners that create a new invoker at each bundle boundary therefore repeat reflective generic-type resolution even when they reuse the same DoFn instance and the constructor cache hits.

This change adds a weak, identity-keyed cache from each live DoFn instance to its selected generated constructor. A new or deserialized instance still resolves its descriptors and uses the existing (DoFn class, input type, output type) constructor cache. Later invoker creation for that instance skips descriptor resolution.

The constructor value does not reference its DoFn key, so weak keys remain collectible. Identity equality is intentional: separate instances that compare equal can still expose different generic descriptors.

The patch preserves the generic cache-collision fix from #37355. It also strengthens that regression test by using equal DoFn instances with different descriptors.

Fixes #39309.

Validation

• ./gradlew :sdks:java:core:test --tests org.apache.beam.sdk.transforms.reflect.DoFnInvokersTest
• ./gradlew :sdks:java:core:check

Both pass on Java 21. The full core check includes formatting, compilation, unit tests, Checkstyle, SpotBugs, Javadocs, and shaded-JAR validation.

---

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• Mention the appropriate issue in your description (Fixes #39309).
• Update CHANGES.md with noteworthy changes.
• This is not a large contribution; an ICLA is not required for its size.

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request optimizes the ByteBuddyDoFnInvokerFactory by caching the generated constructor for each DoFn instance. Previously, runners creating new invokers at bundle boundaries would repeatedly perform expensive reflective type descriptor resolution. By implementing an identity-keyed cache, the system now reuses the resolved constructor for the same DoFn instance, significantly reducing overhead while maintaining correctness for separate instances that might share equal values but different generic types.

Highlights

• Performance Optimization: Introduced a weak, identity-keyed cache for DoFn invoker constructors to avoid redundant reflective generic-type resolution.
• Cache Implementation: Utilized MapMaker with weakKeys to ensure that DoFn instances are not leaked while maintaining identity-based caching.
• Regression Testing: Added a new test case to verify that type descriptor resolution is performed only once per DoFn instance.

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[gemini-code-assist]

Code Review

This pull request introduces caching for Java DoFn type descriptor resolution when creating cached invokers in ByteBuddyDoFnInvokerFactory. By utilizing a weak, identity-keyed cache (byteBuddyInvokerConstructorInstanceCache), it avoids expensive repeated resolution of input and output type descriptors for the same DoFn instance. Unit tests have been added to verify that type descriptor resolution is cached per DoFn instance and that identity is used for caching. There are no review comments, so I have no additional feedback to provide.

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[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[lofifnc]

The only failing check is Java IOs Direct. Its Java 11 step fails because javac 11 rejects -Xlint:-this-escape. The PR's exact base commit fails in the identical step:
https://github.com/apache/beam/actions/runs/29276895305/job/86908120778

All PR-related checks passed, including Java PreCommit, Spotless, SpotBugs, and all 2,186 published tests. This is a base-branch CI regression, not introduced by this change.

[lofifnc]

assign set of reviewers

[github-actions]

Assigning reviewers:

R: @kennknowles for label java.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.00%. Comparing base (42daed3) to head (5e0cc1a).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #39310      +/-   ##
============================================
- Coverage     57.00%   57.00%   -0.01%     
- Complexity     3627     3630       +3     
============================================
  Files          1186     1186              
  Lines        190593   190593              
  Branches       3768     3768              
============================================
- Hits         108644   108643       -1     
- Misses        78471    78472       +1     
  Partials       3478     3478              

Flag	Coverage Δ
java	72.48% <ø> (-0.01%)	⬇️

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[Abacn]

Thanks, since #39309 mentioned benchmark numbers. Have you verified that this fixes the performance issue? To run a pipeline against modified Beam code, one can refer to this doc:

https://github.com/apache/beam/blob/master/contributor-docs/code-change-guide.md#run-your-pipeline-with-modified-beam-code

[Abacn]

Probably mention this is for performance