Github actions: Require code to be formatted with Spotless

.github/workflows/cd.yml

@@ -8,7 +8,12 @@ on:
   workflow_dispatch:
 
 jobs:
+  run_spotless_check:
+    name: Check code is formatted with Spotless
+    uses: ./.github/workflows/spotless.yml
+
   run_java_tests:
+    needs: run_spotless_check
     name: Run Java tests
     uses: ./.github/workflows/java-tests.yml
 
@@ -64,6 +69,7 @@ jobs:
 
   run_e2e_tests:
     needs:
+      - run_spotless_check
       - run_java_tests
       - push_to_registry
     name: Run E2E tests

.github/workflows/spotless.yml

@@ -0,0 +1,31 @@
+name: 'Spotless'
+on:
+  # this workflow is only called by others, won't be executed on itself
+  workflow_call:
+
+jobs:
+  spotless:
+    name: Check code is formatted with Spotless
+    runs-on: ubuntu-22.04
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          java-version: "17"
+          java-package: jdk
+          architecture: x64
+          distribution: temurin
+
+      - name: Cache Maven packages
+        uses: actions/cache@v3
+        with:
+          path: ~/.m2
+          key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-m2
+
+      - name: Run Spotless Check
+        run: mvn spotless:check