[ISSUE #10400] Cache Version.values() in MQVersion to avoid repeated array allocation

[qianye1001]

Summary

Cache Version.values() in a static final field to eliminate repeated defensive-copy array allocations in hot-path methods getVersionDesc() and value2Version().

Problem

Version.values() returns a defensive copy of a 943-element enum array on every call. These two methods call it 6 times total, allocating ~7.5KB of heap per invocation. Under high throughput (broker heartbeats, client connections), this creates unnecessary GC pressure.

Fix

• Added private static final Version[] VERSION_VALUES = Version.values(); (initialized once at class load)
• Rewrote both methods to use a local reference to the cached array
• Public API signatures unchanged — pure internal optimization, zero behavioral change

Changes

File	Change
common/.../MQVersion.java	Cache enum array, rewrite 2 methods

Related Issue

Closes #10400

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This PR was automatically generated.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.92%. Comparing base (a705dbc) to head (5dd98cc).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop   #10401   +/-   ##
==========================================
  Coverage      47.91%   47.92%           
- Complexity     13248    13250    +2     
==========================================
  Files           1376     1376           
  Lines         100536   100539    +3     
  Branches       12983    12983           
==========================================
+ Hits           48172    48181    +9     
+ Misses         46424    46419    -5     
+ Partials        5940     5939    -1     

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[qianye1001]

Review by rocketmq-reviewer-bot

Summary

Cache Version.values() result in a private static final field to eliminate repeated defensive-copy array allocations (~7.5KB per call) in getVersionDesc() and value2Version() hot-path methods.

Findings

• [Info] MQVersion.java:23 — private static final Version[] VERSION_VALUES = Version.values(); is initialized once at class loading time. The enum array is immutable after JVM startup (no new enum constants can be added at runtime), so caching is safe. ✅

• [Info] Both methods — Using a local variable Version[] versions = VERSION_VALUES; before accessing .length and indexing is a good practice. It avoids repeated field reads and is JIT-friendly. ✅

• [Low] MQVersion.java:23 — The cached array is mutable (Java arrays are always mutable). Since VERSION_VALUES is private and only used read-only within this class, the risk is negligible. However, if any future code were to modify the array (e.g., VERSION_VALUES[0] = null), it would corrupt all lookups. This is a very low risk given the current usage, but worth noting for future maintainers.

• [Suggestion] Consider adding a brief comment on the cached field explaining why Version.values() is cached (defensive copy avoidance) — this helps future developers understand the optimization intent without needing to trace the git history.

• [Info] The PR description mentions 943 enum elements and ~7.5KB per allocation. This is a meaningful optimization for high-throughput broker scenarios (heartbeats, client connections). The fix is minimal, well-isolated, and follows a standard Java enum caching pattern.

Cross-repo Note

No cross-repo impact. MQVersion is a common utility class; the change is purely internal performance optimization with no API or behavioral changes.

Verdict

Approve — Clean, minimal performance optimization. The "cache values() in static final + use local variable" pattern is a well-established Java idiom for enum-heavy hot paths. No API changes, no compatibility risk, no cross-repo impact. The only suggestion is to add a brief comment on the field for future maintainability.

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Automated review by rocketmq-reviewer-bot