Try to get newer JDKs to work

.github/workflows/ci.yml

@@ -92,6 +92,14 @@ jobs:
       with:
         java-version: ${{ matrix.java.version }}
         distribution: 'temurin'
+    - name: Set up JDK 21 on an experimental platform
+      if: matrix.java.experimental
+      uses: actions/setup-java@v4
+      with:
+        # Install JDK 21 second, to make it the default on which gradle runs.
+        # This unfortunately also means that all tests run on JDK 21 instead of the newer version.
+        java-version: 21
+        distribution: 'temurin'
 
     # Configure Gradle for optimal use in GitHub Actions, including caching of downloaded dependencies.
     # See: https://github.com/gradle/actions/blob/main/setup-gradle/README.md
@@ -100,6 +108,9 @@ jobs:
 
     - name: Run test script checker/bin-devel/test-${{ matrix.script }}
       run: ./checker/bin-devel/test-${{ matrix.script }}.sh
+      # Set the compiler version to use, allowing us to e.g. run Java 23 while gradle does not work
+      # on Java 23 yet. This only tests the compiler, it does not use that version to run the tests.
+      env: ORG_GRADLE_PROJECT_useJdkCompiler=${{ matrix.java.version }}
 
   # Sanity tests on Windows and MacOS.
   otheros:

build.gradle

@@ -387,15 +387,17 @@ allprojects {
         // Add standard javac options
         tasks.withType(JavaCompile) { compilationTask ->
             dependsOn(':installGitHooks')
-            boolean jdk21Compiler = project.getProperties().getOrDefault('useJdk21Compiler', false)
-            if (!isJava8 && jdk21Compiler) {
-                // This uses the Java 21 compiler to compile all code, like we do for a release.
+            int useJdkCompiler = project.getProperties().getOrDefault('useJdkCompiler', 21)
+            boolean useToolchains = (JavaVersion.current() != useJdkCompiler)
+            if (!isJava8 && useToolchains) {
+                // This uses the requested Java compiler to compile all code.
+                // If no version is requested, we use Java 21, like we do for a release.
                 // CI test test-cftests-junit-jdk21 then runs the JUnit tests on the different JDK versions,
                 // to ensure there is no version mismatch between compiled-against javac API and runtime API.
                 //   https://docs.gradle.org/current/userguide/toolchains.html
                 // This property is final on Java 8, so don't set it then.
                 javaCompiler = javaToolchains.compilerFor {
-                    languageVersion = JavaLanguageVersion.of(21)
+                    languageVersion = JavaLanguageVersion.of(useJdkCompiler)
                 }
             }
 
@@ -459,7 +461,7 @@ allprojects {
             //   warnings are suppressible with a "// fall through" comment.
             // -classfile: classgraph jar file and https://bugs.openjdk.org/browse/JDK-8190452
             String lint = '-Xlint:-options,-fallthrough,-classfile'
-            if (isJava21plus || jdk21Compiler) {
+            if (useJdkCompiler >= 21) {
                 // TODO: Ignore this-escape for now, we may want to review and suppress each one later.
                 lint +=',-this-escape'
             }

checker/bin-devel/test-cftests-junit-jdk21.sh

@@ -8,7 +8,7 @@ echo "SHELLOPTS=${SHELLOPTS}"
 
 SCRIPTDIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
 # shellcheck disable=SC1090# In newer shellcheck than 0.6.0, pass: "-P SCRIPTDIR" (literally)
-export ORG_GRADLE_PROJECT_useJdk21Compiler=true
+export ORG_GRADLE_PROJECT_useJdkCompiler=21
 source "$SCRIPTDIR"/clone-related.sh