How do you cache dependencies in GitHub Actions to speed up builds?

jobs:
  build:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v4

      - name: Set up JDK 17
        uses: actions/setup-java@v4
        with:
          java-version: '17'
          distribution: 'temurin'

      - name: Cache Maven dependencies
        uses: actions/cache@v4
        with:
          path: ~/.m2/repository
          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
          restore-keys: |
            ${{ runner.os }}-maven-
        # key:          exact match (pom.xml unchanged → cache hit)
        # restore-keys: partial match fallback (pom.xml changed → use old cache, update after)

      - name: Build and test
        run: mvn clean verify

- name: Set up JDK 17 with Maven cache
  uses: actions/setup-java@v4
  with:
    java-version: '17'
    distribution: 'temurin'
    cache: 'maven'   # ← handles caching automatically!
    # also supports: 'gradle', 'sbt'

- name: Set up Node.js with npm cache
  uses: actions/setup-node@v4
  with:
    node-version: '20'
    cache: 'npm'       # ← built-in cache for npm
    # cache: 'yarn' or 'pnpm' also supported

- run: npm ci          # use ci (not install) for reproducible installs

# Or manual cache:
- name: Cache node_modules
  uses: actions/cache@v4
  with:
    path: ~/.npm
    key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
    restore-keys: |
      ${{ runner.os }}-node-

- name: Cache Gradle packages
  uses: actions/cache@v4
  with:
    path: |
      ~/.gradle/caches
      ~/.gradle/wrapper
    key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
    restore-keys: |
      ${{ runner.os }}-gradle-

- uses: actions/setup-python@v5
  with:
    python-version: '3.11'
    cache: 'pip'       # built-in pip cache

- run: pip install -r requirements.txt

key: ubuntu-latest-maven-abc123   ← exact match → CACHE HIT (fastest)

restore-keys:
  ubuntu-latest-maven-           ← prefix match → CACHE MISS but partial restore
  ubuntu-latest-                  ← broader fallback

On CACHE HIT:  restores files, skips download
On CACHE MISS: downloads normally, saves new cache after job

- name: Cache Maven
  id: cache-maven
  uses: actions/cache@v4
  with:
    path: ~/.m2/repository
    key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}

- name: Check cache status
  run: |
    if [ "${{ steps.cache-maven.outputs.cache-hit }}" == "true" ]; then
      echo "Cache HIT — dependencies restored from cache"
    else
      echo "Cache MISS — downloading dependencies"
    fi