vaadinBuildFrontend: redirect frontendOutputDirectory to enable Gradle build cache restore

[benstpierre]

Problem

vaadinBuildFrontend writes Vite output to build/resources/main/META-INF/VAADIN/webapp/ — the same directory owned by Gradle's processResources. Gradle's overlapping-outputs validation prevents declaring this as a task @OutputDirectory. Without declared outputs:

• Local up-to-date check works (with @CacheableTask + @Input from PR feat: BuildFrontend Incremental build #23884)
• Build cache restore does not — Gradle skips the task on a cache hit but doesn't restore the frontend files. The jar ships without index.html and the app crashes at runtime.

This is the CI use case — the one that matters most for build performance.

Discussed in #17354 — @mcollovati suggested opening a dedicated ticket.

Root cause

servletResourceOutputDirectory() in GradlePluginAdapter is hardcoded to build/resources/main/META-INF/VAADIN/ (when isBeforeProcessResources=false), and frontendOutputDirectory in VaadinFlowPluginExtension defaults to a subdirectory of it. This puts both inside processResources territory.

ProdBundleUtils.compressBundle() reads from servletResourceOutputDirectory, so redirecting frontendOutputDirectory alone breaks the bundle — both need to move together.

Proposed fix

1. Change the default frontendOutputDirectory to a non-conflicting path — e.g. build/vaadin-frontend/META-INF/VAADIN/webapp/
2. Move servletResourceOutputDirectory alongside it (or derive it from the same base)
3. Declare frontendOutputDirectory as @OutputDirectory on VaadinBuildFrontendTask
4. Wire the plugin to automatically add a from(frontendOutputDirectory) to the jar task so the files end up on the classpath

This preserves the existing behavior (files end up at META-INF/VAADIN/webapp/ in the jar) while letting Gradle's build cache do its job.

Impact

We measured the difference on our project (Vaadin 24.9.12, ~42 JS chunks):

Scenario	Time
Cache miss (Vite runs)	~26s
Cache hit (task skipped)	~5s
Saved	~21s per build

On GitHub Actions with persistent runners: build step dropped from ~116s to ~54s.

Our workaround (for anyone stuck on 24.x)

We declare src/main/bundles/ as the task output (contains prod.bundle), then zipTree() it in the jar task into META-INF/VAADIN/. We're unzipping the zip that compressBundle() created from the files we can't cache directly. Details: https://github.com/galateatech/waste-coordinator/pull/10283

Happy to contribute a PR if the team can confirm the approach direction.

Environment

• Vaadin: 24.9.12
• Gradle: 9.3.1
• Also reviewed 25.0 source — same issue persists after PR feat: BuildFrontend Incremental build #23884

[mcollovati]

I guess the linked repository is private, since it results in a 404.

The proposed change makes sense. Please go ahead with the PR so we can validate that this works as expected and moving servletResourceOutputDirectory does not have any side effects.