4.x: Add optional build/test for future JDKs

Author: akarnokd

.github/workflows/gradle_preview_jdks.yml

@@ -0,0 +1,51 @@
+# This workflow will build a Java project with Gradle
+# For more information see: https://help.github.com/actions/language-and-framework-guides/building-and-testing-java-with-gradle
+
+name: Preview-JDKs
+
+on:
+  push:
+    branches: [ '4.x' ]
+  pull_request:
+    branches: [ '4.x' ]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false                    # important: don't cancel other matrix jobs on failure
+      matrix:
+        java-version:
+          - 27-ea   # experimental/upcoming - allowed to fail
+        include:
+          - java-version: 27-ea
+            experimental: true
+
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+    - name: Set up JDK ${{ matrix.java-version }}
+      uses: actions/setup-java@be666c2fcd27ec809703dec50e508c2fdc7f6654 # v5.2.0
+      with:
+        distribution: 'zulu'
+        java-version: ${{ matrix.java-version }}
+
+    - name: Cache Gradle packages
+      uses: actions/cache@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5.0.4
+      with:
+        path: ~/.gradle/caches
+        key: ${{ runner.os }}-gradle-${{ secrets.CACHE_VERSION }}-${{ hashFiles('**/*.gradle') }}
+        restore-keys: ${{ runner.os }}-gradle-${{ secrets.CACHE_VERSION }}
+    - name: Grant execute permission for gradlew
+      run: chmod +x gradlew
+
+    - name: Run Validity Tests Upfront
+      run: ./gradlew test --tests "io.reactivex.rxjava4.validators.*" --stacktrace --no-daemon
+    - name: Build RxJava
+      run: ./gradlew build --stacktrace
+    - name: Generate Javadoc
+      run: ./gradlew javadoc --stacktrace
+
+    continue-on-error: ${{ matrix.experimental || false }}