fix: restore cached Gradle frontend build outputs

[benstpierre]

Summary

Fixes #24012.

vaadinBuildFrontend previously wrote production servlet resources into build/resources/<sourceSet>/META-INF/VAADIN, which overlaps with Gradle's processResources output. That makes the task's production bundle hard to restore safely from Gradle's build cache: a cache hit can restore the task marker/token while leaving the archive without META-INF/VAADIN/webapp assets.

This change moves the default frontendOutputDirectory for Gradle production builds to a dedicated task-owned directory:

build/vaadin-build-frontend/META-INF/VAADIN/webapp

and declares the corresponding servlet resource directory as an @OutputDirectory of vaadinBuildFrontend.

Packaging tasks now explicitly consume that task-owned output directory:

• plain Jar: packaged at archive root
• War: packaged under WEB-INF/classes
• Spring Boot BootJar: packaged under BOOT-INF/classes

This keeps the archive layout unchanged while allowing Gradle to restore the production frontend bundle from the build cache.

Tests

Run from flow-plugins/flow-gradle-plugin with Java 21:

JAVA_HOME=$HOME/.sdkman/candidates/java/21.0.7-amzn ./gradlew compileKotlin compileTestKotlin
JAVA_HOME=$HOME/.sdkman/candidates/java/21.0.7-amzn ./gradlew test
JAVA_HOME=$HOME/.sdkman/candidates/java/21.0.7-amzn ./gradlew functionalTest -x test --tests 'com.vaadin.flow.gradle.VaadinSmokeTest.testBuildFrontendInProductionMode' --tests 'com.vaadin.flow.gradle.MiscSingleModuleTest.buildFrontendIncrementalBuilds_featureEnabled' --tests 'com.vaadin.flow.gradle.MiscSingleModuleTest.testBuildFrontendPropagatesBuildInfo' --tests 'com.vaadin.flow.gradle.MiscSingleModuleTest.buildFrontendBuildCacheRestoresProductionBundleForWar'
JAVA_HOME=$HOME/.sdkman/candidates/java/21.0.7-amzn ./gradlew functionalTest -x test --tests 'com.vaadin.flow.gradle.MiscSingleModuleTest.buildFrontendBuildCacheRestoresProductionBundleForWar' --tests 'com.vaadin.flow.gradle.MiscSingleModuleTest.testSpringProjectProductionMode' --tests 'com.vaadin.flow.gradle.MiscSingleModuleTest.testSpringProjectAutoProductionMode'

The new regression test verifies that after deleting the local production frontend output, cached token, and WAR, the next --build-cache -Pvaadin.productionMode build restores :vaadinBuildFrontend FROM-CACHE and the rebuilt WAR still contains the Vaadin production bundle.

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[mshabarov]

Thanks for your contribution @benstpierre ! I've started the blocked validation, let's see how gradle tests end up.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

 1 405 files  ±0   1 405 suites  ±0   1h 20m 40s ⏱️ - 2m 33s
10 137 tests ±0  10 067 ✅ ±0  70 💤 ±0  0 ❌ ±0 
10 612 runs  ±0  10 533 ✅ ±0  79 💤 ±0  0 ❌ ±0 

Results for commit 62c56fe. ± Comparison against base commit ba7d3cb.

[benstpierre]

So anything left for me to do? This case has been insanely aggravating for years.

[mcollovati]

@benstpierre I'm going to review and test this PR today. I'll keep you posted.

[mcollovati]

@benstpierre could you please sign the CLA? Otherwise we cannot merge your patch as is, but we need to make a new PR on our own based on this one.

[mcollovati]

Added a first bunch of comments; in the meanwhile I'll make some manual test

• BuildFrontendTokenService still references build/resources/main/, but since this PR changes servletResourceOutputDirectory it should most likely mention build/vaadin-build-frontend/.

[mcollovati]

Why is this condition needed? Is this to avoid the configuration for sources and Javadoc JARs?
Wouldn't this check potentially break projects that define custom-named JAR tasks or using shadow/fatJAR plugins?

[mcollovati]

We should probably add a test for a plain JAR, since the plugin code allows it.

[mcollovati]

I set up a repository as an attempt to have some tests for the Gradle build cache. I hope this correctly covers the usage of build cache.

Here are two builds:

• Flow main: all projects run vaadinBuildFrontend but the content of the build cache is not restored properly, resulting in artifact without build resources https://github.com/mcollovati/vaadin-gradle-build-cache-tests/actions/runs/25740505264
• this PR: the build cache is restored, and the production artifacts contain the necessary resources, but several project configurations are broken, not running vaadinBuildFrontend anymore: https://github.com/mcollovati/vaadin-gradle-build-cache-tests/actions/runs/25739966965

I plan to add a scenario also for the custom frontendOutputDirectory.