4.x: Add optional build/test for future JDKs (#8077)

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

* try temurin builds

* use toolchain override

* Run Gradle on 26, invoke toolchain for 27

* Minor log fix

* hope gradle runs on 26

* Don't run jacoco on EA as it breaks its assembly parser

* another attempt at running Gradle on 26 and code on 27

* Fix jacoco again

* fix java home

* No jacoco for experimentals

Author: akarnokd

.github/workflows/gradle_preview_jdks.yml

@@ -0,0 +1,63 @@
+# 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:
+    runs-on: ubuntu-latest
+    continue-on-error: true
+    strategy:
+      fail-fast: false
+      matrix:
+        jdk: [27]   # add 28-ea etc. later
+
+    steps:
+    - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+    - name: Set up JDK 26
+      uses: actions/setup-java@be666c2fcd27ec809703dec50e508c2fdc7f6654 # v5.2.0
+      with:
+        distribution: 'zulu'
+        java-version: '26'
+        cache: gradle
+
+    - name: Set up JDK ${{ matrix.jdk }}-ea
+      uses: actions/setup-java@be666c2fcd27ec809703dec50e508c2fdc7f6654 # v5.2.0
+      with:
+        distribution: 'temurin'
+        java-version: '${{ matrix.jdk }}-ea'
+
+    - 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
+      env:
+        JDK_EXPERIMENTAL: ${{ matrix.jdk }}
+        JAVA_HOME: ${{ env.JAVA_HOME_26_X64 }}
+      run: ./gradlew test --tests "io.reactivex.rxjava4.validators.*" --stacktrace --no-daemon
+    - name: Build RxJava
+      env:
+        JDK_EXPERIMENTAL: ${{ matrix.jdk }}
+        JAVA_HOME: ${{ env.JAVA_HOME_26_X64 }}
+      run: ./gradlew build --stacktrace
+    - name: Generate Javadoc
+      env:
+        JDK_EXPERIMENTAL: ${{ matrix.jdk }}
+        JAVA_HOME: ${{ env.JAVA_HOME_26_X64 }}
+      run: ./gradlew javadoc --stacktrace
+

build.gradle

@@ -3,7 +3,6 @@ plugins {
     id("java-library")
     id("checkstyle")
     id("eclipse")
-    id("jacoco")
     id("maven-publish")
     id("me.champeau.jmh") version "0.7.3"
     id("com.github.hierynomus.license") version "0.16.1"
@@ -54,10 +53,17 @@ dependencies {
     testRuntimeOnly "org.junit.platform:junit-platform-launcher:$jupiterLauncherVersion"  // match your JUnit version family
 }
 
+// === Experimental JDK handling for Outreach Program ===
+def experimental = System.getenv("JDK_EXPERIMENTAL")
+def toolchainJdk = "26"   // default / baseline
+
+if (experimental != null && !experimental.trim().isEmpty()) {
+    toolchainJdk = experimental  // e.g. "27"
+}
+
 java {
     toolchain {
-        // vendor = JvmVendorSpec.ADOPTIUM <-- for now
-        languageVersion = JavaLanguageVersion.of(26)
+        languageVersion = JavaLanguageVersion.of(toolchainJdk)
     }
     sourceCompatibility = JavaVersion.VERSION_26
     targetCompatibility = JavaVersion.VERSION_26
@@ -256,18 +262,23 @@ tasks.withType(Test) {
     }
 }
 
-jacocoTestReport {
-    dependsOn test
-    dependsOn testNG
-
-    reports {
-        xml.required.set(true)
-        csv.required.set(false)
-        html.required.set(true)
+if (experimental == null || "".equals(experimental)) {
+    apply plugin: "jacoco"
+
+    jacocoTestReport {
+        dependsOn test
+        dependsOn testNG
+    
+        reports {
+            xml.required.set(true)
+            csv.required.set(false)
+            html.required.set(true)
+        }
     }
+    
+    check.dependsOn jacocoTestReport
 }
 
-check.dependsOn jacocoTestReport
 
 checkstyle {
     configFile = project.file("config/checkstyle/checkstyle.xml")