Java CI on GitHub Actions

Every build re-downloads the same Maven or Gradle dependencies.

Why it happens

~/.m2 and ~/.gradle are empty on a fresh runner, and JVM startup plus annotation processing add fixed overhead per module.

What to change

1. Cache ~/.m2/repository or ~/.gradle/caches keyed on the build files
2. Enable the Gradle configuration cache and build cache
3. Keep the Gradle daemon alive within a job rather than forking per module
4. Give the JVM enough heap — GC thrash looks exactly like a slow build

Java on runnerhut

jobs:
  build:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-java@v4
        with: { java-version: '21', distribution: temurin, cache: gradle }
      - run: ./gradlew build --build-cache --configuration-cache

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.