Decouple JDK running Gradle from JDK building project

Gradle will by default use the same JDK for building the project as it
does for running Gradle. Newer JDKs are assumed to be more performant,
contain fewer bugs and to come with desirable features that we would
like to utilize in our build, but doing so should not endanger
backwards compatibility for the plugin itself. We can do that now by
utilizing Gradle toolchains, which allows us to use one JDK/JRE for
running Gradle and the other for building the project, i.e. executing
`javac`.

Doing this requires us to make multiple JDKs available to the Gradle
build action for each individual execution of the build. The setup-java
action doesn't support this at the moment, and the multiple build steps
configured for actions/setup-java in this commit is at best a
workaround for this, and at worst a bad assumption with respect to no
environment variables or other settings being overwritten. Also,
there's no handling of duplicate invocations of the action with the end
result being duplicate JDKs installed. Hopefully both actions/setup-java
and gradle/gradle-build-action will handle this case gracefully.

For more details, see:
- actions/setup-java#44
- gradle/gradle#14903

.github/workflows/main.yml

@@ -10,30 +10,47 @@ jobs:
     strategy:
       matrix:
         include:
-          - { os: 'macos-latest', arch: 'x64', java-version: '11', gradle-version: '7.3' }
-          - { os: 'ubuntu-latest', arch: 'x64', java-version: '11', gradle-version: '7.3' }
-          - { os: 'windows-latest', arch: 'x64', java-version: '11', gradle-version: '7.3' }
-    name: Gradle ${{ matrix.gradle-version }}, Java ${{ matrix.java-version }} on ${{ matrix.os }} (${{ matrix.arch }})
+          # Gradle is run with `gradle-jdk` and builds with `build-jdk` by utilizing Gradle toolchain support
+          - { os: 'macos-latest', arch: 'x64', build-jdk: '11', gradle-jdk: '11', gradle-version: '7.3' }
+          - { os: 'ubuntu-latest', arch: 'x64', build-jdk: '11', gradle-jdk: '11', gradle-version: '7.3' }
+          - { os: 'windows-latest', arch: 'x64', build-jdk: '11', gradle-jdk: '11', gradle-version: '7.3' }
+    name: >
+      Gradle ${{ matrix.gradle-version }} on ${{ matrix.os }} (${{ matrix.arch }})
+      w/JDK ${{ matrix.gradle-jdk }} building w/JDK ${{ matrix.build-jdk}}
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
-      - name: Configure JDK ${{ matrix.java-version }} (${{ matrix.arch }})
+      - name: Configure Gradle JDK ${{ matrix.gradle-jdk }} (${{ matrix.arch }})
+        id: configure-gradle-jdk
         uses: actions/setup-java@v2
         with:
           architecture: ${{ matrix.arch }}
           distribution: 'temurin'
-          java-version: ${{ matrix.java-version }}
-      - uses: gradle/gradle-build-action@v2
+          java-version: ${{ matrix.gradle-jdk }}
+      - name: Configure Build JDK ${{ matrix.build-jdk }} (${{ matrix.arch }})
+        id: configure-build-jdk
+        uses: actions/setup-java@v2
+        with:
+          architecture: ${{ matrix.arch }}
+          distribution: 'temurin'
+          java-version: ${{ matrix.build-jdk }}
+      - name: >
+          Build w/Gradle ${{ matrix.gradle-version }} on JDK ${{ matrix.gradle-jdk }}
+          w/JDK ${{ matrix.build-jdk }}
+        uses: gradle/gradle-build-action@v2
         with:
           # Only write to the cache on the 'main' branch
           cache-read-only: ${{ github.ref != 'refs/heads/main' }}
           gradle-version: ${{ matrix.gradle-version }}
-          arguments: build --scan
+          arguments: >
+            build --scan
+            -Porg.gradle.java.installations.auto-download=false
+            -Porg.gradle.java.installations.paths=${{ steps.configure-gradle-jdk.outputs.path }},${{ steps.configure-build-jdk.outputs.path }}
       - name: Upload build reports
         uses: actions/upload-artifact@v2
         if: always()
         with:
-          name: build-reports-${{ matrix.os }}-${{ matrix.arch }}-jdk-${{ matrix.java-version }}
+          name: build-reports-${{ matrix.os }}-${{ matrix.arch }}-jdk-${{ matrix.gradle-jdk }}
           path: |
             build/reports
             ./**/build/reports

plugin/build.gradle.kts

@@ -11,6 +11,7 @@ plugins {
   id("org.jlleitschuh.gradle.ktlint") version "10.2.0"
 }
 
+val jdkVersion: JavaLanguageVersion = JavaLanguageVersion.of(11)
 val functionalTestSourceSet: SourceSet = sourceSets.create("functionalTest")
 configurations["functionalTestImplementation"].extendsFrom(configurations["testImplementation"])
 
@@ -30,6 +31,18 @@ gradlePlugin {
   }
 }
 
+java {
+  toolchain {
+    languageVersion.set(jdkVersion)
+  }
+}
+
+kotlin {
+  jvmToolchain {
+    (this as JavaToolchainSpec).languageVersion.set(jdkVersion)
+  }
+}
+
 tasks.withType<JavaCompile>().configureEach {
   options.release.set(8)
 }