Migrate concurrent linked map cache to Caffeine

[jamesfredley]

Description

What was found

Problem	Impact
Cache module still uses concurrentlinkedhashmap-lru	Older dependency, less maintained
Codebase 3 recommended Caffeine migration	No current PR
Public cache API must stay stable	Internals can move first

What changed

Area	Change
Implementation	GrailsConcurrentLinkedMapCache internals use Caffeine
Manager	Keep public manager API
Dependency	Prefer Caffeine over concurrentlinkedhashmap-lru path
Docs / tests	Cache config note + behavioral Spock coverage

Out of scope / follow-up

Topic	Status
Renaming public classes away from ConcurrentLinkedMap	Later
Full Spring Cache abstraction rewrite	Not this PR
Benchmark suite vs old LRU	Follow-up

Related MD topics

Source	Topic
Codebase 3	Caffeine migration from concurrentlinkedhashmap-lru

Contributor Checklist

Issue and Scope

• Dependency modernization starter.
• Public cache API preserved.
• Single focused migration seed.
• Targets 8.0.x.

Code Quality

• Tests updated.
• Focused cache tests run.
• No mass reformatting.
• AI starting point labeled.

Licensing and Attribution

• Apache License 2.0.
• Contributor rights confirmed.
• ai-generated-starting-point label applied.

Documentation

• Cache docs updated.
• Scope explained.

Assisted-by: Sisyphus:xai/grok-4.5 [gpt-coding]

[Copilot]

Pull request overview

This pull request migrates the GrailsConcurrentLinkedMapCache implementation from concurrentlinkedhashmap-lru to Caffeine, aiming to modernize the cache internals while keeping the existing public cache manager API stable for Grails applications.

Changes:

• Replaced the cache’s internal store with a Caffeine Cache and exposed its asMap() view as the ConcurrentMap native cache.
• Switched the grails-cache module dependency from concurrentlinkedhashmap-lru to com.github.ben-manes.caffeine:caffeine.
• Updated cache documentation and migrated the cache unit test from JUnit-style assertions to Spock.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
grails-doc/src/en/guide/cache/cacheUsage/cacheConfiguration.adoc	Updates cache configuration documentation to describe the Caffeine-backed bounded cache manager.
grails-cache/src/test/groovy/grails/plugin/cache/GrailsConcurrentLinkedMapCacheTests.groovy	Rewrites tests as Spock specs and adjusts expectations for the new eviction behavior.
grails-cache/src/main/groovy/grails/plugin/cache/GrailsConcurrentLinkedMapCacheManager.groovy	Updates class-level documentation to reflect the new bounded-cache intent.
grails-cache/src/main/groovy/grails/plugin/cache/GrailsConcurrentLinkedMapCache.java	Implements Caffeine-backed storage and updates capacity/size/hottest-keys behavior accordingly.
grails-cache/build.gradle	Replaces the old LRU cache dependency with Caffeine.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[testlens-app]

🚨 TestLens detected 1 failed test 🚨

Here is what you can do:

1. Inspect the test failures carefully.
2. If you are convinced that some of the tests are flaky, you can mute them below.
3. Finally, trigger a rerun by checking the rerun checkbox.

Test Summary

CI - Groovy Joint Validation Build / build_grails > :grails-cache:test

Test	Runs	Flakiness
GrailsConcurrentLinkedMapCacheTests > returns hottest keys from cache eviction policy	❌	0% 🟢

🏷️ Commit: e01d6d8
▶️ Tests: 6736 executed
⚪️ Checks: 60/60 completed

Test Failures

GrailsConcurrentLinkedMapCacheTests > returns hottest keys from cache eviction policy (:grails-cache:test in CI - Groovy Joint Validation Build / build_grails)

Condition not satisfied:

cache.hottestKeys.contains(2)
|     |           |
|     |           false
|     [12, 11, 10, 9, 8, 7, 6, 5, 4, 18]
<grails.plugin.cache.GrailsConcurrentLinkedMapCache@5715f434 name=inaccessible capacity=inaccessible caffeineStore=inaccessible store=inaccessible allowNullValues=inaccessible>

	at grails.plugin.cache.GrailsConcurrentLinkedMapCacheTests.returns hottest keys from cache eviction policy(GrailsConcurrentLinkedMapCacheTests.groovy:136)

Muted Tests

Select tests to mute in this pull request:

• GrailsConcurrentLinkedMapCacheTests > returns hottest keys from cache eviction policy

Reuse successful test results:

• ♻️ Only rerun the tests that failed or were muted before

Click the checkbox to trigger a rerun:

• Rerun jobs

---

Learn more about TestLens at testlens.app.

[jdaugherty]

@jamesfredley have you discussed this with @matrei? I thought I remember him doing something similar here (#15091)

[jdaugherty]

This PR represents an architectural shift that was not discussed before being submitted. I do not think it should proceed until we've discussed it as a team.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 19.5852%. Comparing base (b00e83e) to head (e01d6d8).
⚠️ Report is 8 commits behind head on 8.0.x.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                8.0.x     #15961         +/-   ##
===================================================
- Coverage     49.5403%   19.5852%   -29.9550%     
+ Complexity      16922        308      -16614     
===================================================
  Files            1999         61       -1938     
  Lines           93754       3472      -90282     
  Branches        16420        601      -15819     
===================================================
- Hits            46446        680      -45766     
+ Misses          40145       2662      -37483     
+ Partials         7163        130       -7033     

see 1939 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[jamesfredley]

Good call to check - I looked at #15091, and it's complementary rather than overlapping.

#15091 (@matrei, "fix(deps): clarify and update usage of caffeine") is dependency cleanup: it removes unused direct Caffeine deps from grails-core / grails-data-simple and adds explanatory comments in the modules that do use Caffeine (grails-web-common, grails-web-url-mappings). It changes no files in grails-cache and doesn't touch GrailsConcurrentLinkedMapCache, concurrentlinkedhashmap-lru, or the cache implementation. So it did not perform this migration - #15961 is the functional swap of the cache internals. The two just need dependency-resolution coordination once both are in flight.

On the "architectural shift, not discussed as a team" point - fair, and I'll take it to the dev list / weekly before proceeding rather than merging unilaterally. For grounding, the migration is a documented codebase-improvement recommendation (pre-release review, Codebase 3):

• concurrentlinkedhashmap-lru is 1.4.2 (2015, archived), and its own author built Caffeine as the successor.
• It's used only by grails-cache's GrailsConcurrentLinkedMapCache.
• Caffeine is Spring's preferred cache provider, so migrating aligns grails-cache with the Boot 4 ecosystem, and lets us deprecate/remove the archived dependency.

The plan I'd bring to the discussion: swap the internals of GrailsConcurrentLinkedMapCache to Caffeine behind the existing cache API, add behavioral tests, and stage the old dependency for removal - keeping public behavior stable. I'll hold merge until we've talked it through as a team.