KAFKA-19761 Gradle Shadow plugin version upgrade: 8.3.9 -->> 9.3.1

[dejan2609]

Prologue:
#19513 (comment)

💡 Rationale: Gradle 10 will be released in Q4 2026 or Q1 2027
and we can't expect that Shadow plugin version 8 will be compatible:

Release notes for Shadow plugin:

• https://gradleup.com/shadow/changes/#931-2026-01-06

---

I did some testing, and results look just fine ✅:

• gradle builds ./gradlew :jmh-benchmarks:shadowJar and ./gradlew releaseTarGz are successful
• generated distributions (kafka_2.13-4.3.0-SNAPSHOT-site-docs.tgz and
  and kafka_2.13-4.3.0-SNAPSHOT.tgz) have identical file numbers (for
  trunk and this branch)
• Kafka cluster starts and messages can be produced and consumed:
  https://kafka.apache.org/quickstart

---

⚠️ disclaimer notes:

• solution is generated via Gemini CLI
• Gemini prompt is based on Jira ticket (previously created by me):
  KAFKA-19761

[dejan2609]

@chia7712 Please review when you have some time.

[dejan2609]

I'll rebase this onto trunk (to include a new, updated Gradle version into this branch/PR).

[dejan2609]

Hi @apoorvmittal10,

Can you please allocate some time to provide a review ?

[dejan2609]

Few tests are flaky/failing:

[dejan2609]

Gradle task that ends up with one failed test works as expected on my machine:

dejan@dejan-HP-ProBook-450-G7:~/kafka$ date
Sat 13 Dec 16:21:53 CET 2025
dejan@dejan-HP-ProBook-450-G7:~/kafka$ java -version 
openjdk version "17.0.17" 2025-10-21
OpenJDK Runtime Environment Temurin-17.0.17+10 (build 17.0.17+10)
OpenJDK 64-Bit Server VM Temurin-17.0.17+10 (build 17.0.17+10, mixed mode, sharing)
dejan@dejan-HP-ProBook-450-G7:~/kafka$ ./gradlew :streams:upgrade-system-tests-24:test -q
Starting build with version 4.3.0-SNAPSHOT (commit id 4196380b) using Gradle 9.2.1, Java 17 and Scala 2.13.18
Build properties: ignoreFailures=false, maxParallelForks=8, maxScalacThreads=8, maxTestRetries=0
dejan@dejan-HP-ProBook-450-G7:~/kafka$ date
Sat 13 Dec 16:22:18 CET 2025
dejan@dejan-HP-ProBook-450-G7:~/kafka$

💡 I will try to execute entire test suite on Github Action CI server once more.

[dejan2609]

All tests are fine now ✅

@chia7712 can you allocate some time to review this one ?

[clolov]

Thanks for the change and apologies for the delay!

It may be because my expertise in Gradle is not my strong suit, but I would encourage you to give a brief overview of why all the changes in the PR are required!

The real problem is that if we just bump Gradle Shadow from 8 to 9 we run into

* What went wrong:
Execution failed for task ':jmh-benchmarks:shadowJar'.
> Cannot expand ZIP '/Users/REDACTED/Documents/kafka/core/build/classes/java/test' as it does not exist.

which appears to be because :core only has build/classes/scala while all other dependencies have build/classes/java and Gradle Shadow 9 appears to be more strict in requiring that directory to exist.

I am a bit on the fence for this change. On one hand, I don't like depending on .sourceSets.test.output in some places in the build.gradle file and on configurations/artifacts in others. On the other hand, I also don't like generating an empty folder (i.e. build/classes/java) just to satisfy a stricter expectation.

What are the opinions of others in the community?

[dejan2609]

Hi @clolov and thanks for a review/comment ! You are rasing some valid questions here.

But first: can you please share your environment details (thing is that Gradle build task :jmh-benchmarks:shadowJar works just fine on my end):

dejan@box:~/kafka$ date
Fri  9 Jan 16:24:51 CET 2026
dejan@box:~/kafka$ uname -r
6.8.0-90-generic
dejan@box:~/kafka$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 24.04.3 LTS
Release:	24.04
Codename:	noble
dejan@box:~/kafka$ java -version 
openjdk version "25.0.1" 2025-10-21 LTS
OpenJDK Runtime Environment Temurin-25.0.1+8 (build 25.0.1+8-LTS)
OpenJDK 64-Bit Server VM Temurin-25.0.1+8 (build 25.0.1+8-LTS, mixed mode, sharing)
dejan@box:~/kafka$ git log -2 --oneline 
70dac31e9f (HEAD -> KAFKA-19761, origin/KAFKA-19761) KAFKA-19761 Gradle Shadow plugin version upgrade: 8.3.9 -->> 9.3.1
2f90af70ee (upstream/trunk, upstream/HEAD, origin/trunk, origin/HEAD, trunk) MINOR: Remove onPartitionsDeleted from GroupCoordinator interface (#21263)
dejan@box:~/kafka$ ./gradlew clean :jmh-benchmarks:shadowJar -q
Starting build with version 4.3.0-SNAPSHOT (commit id 70dac31e) using Gradle 9.2.1, Java 25 and Scala 2.13.18
Build properties: ignoreFailures=false, maxParallelForks=8, maxScalacThreads=8, maxTestRetries=0
MessageGenerator: processed 1 Kafka message JSON file(s).
MessageGenerator: processed 1 Kafka message JSON file(s).
MessageGenerator: processed 5 Kafka message JSON file(s).
MessageGenerator: processed 4 Kafka message JSON file(s).
MessageGenerator: processed 2 Kafka message JSON file(s).
MessageGenerator: processed 26 Kafka message JSON file(s).
MessageGenerator: processed 44 Kafka message JSON file(s).
MessageGenerator: processed 197 Kafka message JSON file(s).
MessageGenerator: processed 5 Kafka message JSON file(s).
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::arrayBaseOffset has been called by net.openhft.hashing.UnsafeAccess (file:/home/dejan/.gradle/caches/modules-2/files-2.1/net.openhft/zero-allocation-hashing/0.16/ca252f328160ed5d027f100a4fe525d6d21daaf/zero-allocation-hashing-0.16.jar)
WARNING: Please consider reporting this to the maintainers of class net.openhft.hashing.UnsafeAccess
WARNING: sun.misc.Unsafe::arrayBaseOffset will be removed in a future release
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: /home/dejan/kafka/coordinator-common/src/test/java/org/apache/kafka/coordinator/common/runtime/CoordinatorRuntimeTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Note: /home/dejan/kafka/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/StreamsGroupTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
dejan@box:~/kafka$ sdk use java 17.0.17-tem

Using java version 17.0.17-tem in this shell.
dejan@box:~/kafka$ java -version
openjdk version "17.0.17" 2025-10-21
OpenJDK Runtime Environment Temurin-17.0.17+10 (build 17.0.17+10)
OpenJDK 64-Bit Server VM Temurin-17.0.17+10 (build 17.0.17+10, mixed mode, sharing)
dejan@box:~/kafka$ ./gradlew clean :jmh-benchmarks:shadowJar -q
Starting build with version 4.3.0-SNAPSHOT (commit id 70dac31e) using Gradle 9.2.1, Java 17 and Scala 2.13.18
Build properties: ignoreFailures=false, maxParallelForks=8, maxScalacThreads=8, maxTestRetries=0
MessageGenerator: processed 4 Kafka message JSON file(s).
MessageGenerator: processed 1 Kafka message JSON file(s).
MessageGenerator: processed 5 Kafka message JSON file(s).
MessageGenerator: processed 2 Kafka message JSON file(s).
MessageGenerator: processed 1 Kafka message JSON file(s).
MessageGenerator: processed 26 Kafka message JSON file(s).
MessageGenerator: processed 44 Kafka message JSON file(s).
MessageGenerator: processed 197 Kafka message JSON file(s).
MessageGenerator: processed 5 Kafka message JSON file(s).
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: /home/dejan/kafka/coordinator-common/src/test/java/org/apache/kafka/coordinator/common/runtime/CoordinatorRuntimeTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Note: /home/dejan/kafka/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/StreamsGroupTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
dejan@box:~/kafka$ date
Fri  9 Jan 16:38:25 CET 2026
dejan@box:~/kafka$ 

[dejan2609]

I am a bit on the fence for this change. On one hand, I don't like depending on .sourceSets.test.output in some places in the build.gradle file and on configurations/artifacts in others. On the other hand, I also don't like generating an empty folder (i.e. build/classes/java) just to satisfy a stricter expectation.

I can relate with all of this @clolov. Thing is that Kafka Gradle build is complicated due to a fact that people want Kafka to perform various complicated things.

And on top of that: dependencies versions just have to move forward: sooner or later this Shadow gradle plugin version will have to be bumped, one way or another.

All-in-all, we can either choose to:

1. put some duct tape and/or WD-40 and call it a day (like here, in this PR 🙂)
2. re-factor Gradle build (and remove some technical debt all along) - ⚠️ pitfall - it can be tricky
3. something else 🤓

Open invitation to entire community: what is your take on this ?

[dejan2609]

It may be because my expertise in Gradle is not my strong suit, but I would encourage you to give a brief overview of why all the changes in the PR are required!

Rationale section is added into description above.