Add Apple Silicon build workaround

README.md

@@ -251,3 +251,24 @@ Gradle project. After that, it's recommended to set these preferences (based on
   - Project -> Properties -> FindBugs -> [x] Run Automatically
   - There should 0 errors. But sometimes the plugin fails to take the
     @SuppressFBWarnings annotations into account; then use Project -> Clean. 
+
+
+## Troubleshooting
+
+### Problems building on MacOS / Apple Silicon / aarch64
+
+If you run into build problems on MacOS (e.g. `No matching toolchains found for requested specification: {languageVersion=9, vendor=any, implementation=vendor-specific} for MAC_OS on aarch64.`) that means that you don't have JDK9 / JDK16 installed on your system. 
+
+You override the java versions used for JDK9 and JDK16:
+
+```
+./gradlew -Pfreemarker.javaVersionUsedFor.9=11 -Pfreemarker.javaVersionUsedFor.16=17 jar
+```
+
+This is helpful if you don't have a JDK9 / JDK16 for MacOS installed on your system and just want to build e.g. with another replacement locally during development.
+
+You can append `--info` to get diagnostic output like this:
+
+```
+TOOLCHAIN :compileCore16Java: lang=17 vendor=Eclipse Temurin runtime=17.0.11+9 home=/Library/Java/JavaVirtualMachines/temurin-17.jdk/Contents/Home
+```

build.gradle.kts

@@ -35,6 +35,23 @@ group = "org.freemarker"
 
 val fmExt = freemarkerRoot
 
+
+/*
+* Additional diagnostic output when you want to find output
+* which Java version is used for which part.
+* enable with <i>--info</i>
+*/
+tasks.withType<JavaCompile>().configureEach {
+    doFirst {
+        val md = javaCompiler.get().metadata
+        logger.info(
+            "TOOLCHAIN ${path}: lang=${md.languageVersion.asInt()} " +
+                "vendor=${md.vendor} runtime=${md.javaRuntimeVersion} " +
+                "home=${md.installationPath.asFile}"
+        )
+    }
+}
+
 tasks.withType<JavaCompile>().configureEach {
     options.encoding = "UTF-8"
 }
@@ -61,8 +78,8 @@ freemarkerRoot {
     configureSourceSet("jython20")
     configureSourceSet("jython22")
     configureSourceSet("jython25") { enableTests() }
-    configureSourceSet("core9", "9") { enableTests() }
-    configureSourceSet("core16", "16") {
+    configureSourceSet("core9", javaVersionUsedFor(9).toString()) { enableTests() }
+    configureSourceSet("core16", javaVersionUsedFor(16).toString()) {
         enableTests();
         addDependencySourceSet("core9");
     }
@@ -733,3 +750,8 @@ dependencies {
     "testUtilsImplementation"("commons-collections:commons-collections:3.1")
     "testUtilsImplementation"("commons-lang:commons-lang:2.6")
 }
+
+fun javaVersionUsedFor(requestedVersion: Int): Int {
+    val overrideVersion = project.findProperty("freemarker.javaVersionUsedFor.$requestedVersion")?.toString()?.toIntOrNull()
+    return overrideVersion ?: requestedVersion
+}

gradle.properties

@@ -21,6 +21,10 @@
 
 # JDK version used for compiling, except for Source Sets where a specific version is set in build.gradle.kts.
 freemarker.javaVersion=8
+# JDK version used for compiling the Java 9 MR-JAR source set
+freemarker.javaVersionUsedFor.9=9
+# JDK version used for compiling the Java 16 MR-JAR source set
+freemarker.javaVersionUsedFor.16=16
 # JDK version for running javadoc
 freemarker.javadoc.javaVersion=16
 # JDK version for running JUnit tests