Add Apple Silicon build workaround

Document a local workaround for building on MacOS Apple Silicon and add a Gradle flag to enable it. README.md shows running ./gradlew -Pfreemarker.skipJavaMRJSourceSets=true jar to avoid MR-JAR toolchain errors when JDK9/16 are unavailable. build.gradle.kts adds the skipJavaMRJSourceSets property, conditionally omits configuring the core9/core16 MR-JAR source sets, and updates the Eclipse classpath assembly to avoid referencing those configurations when skipped.

Author: chrisrueger

README.md

@@ -125,6 +125,15 @@ To build `freemarker.jar`, just issue `./gradlew jar` (`gradlew.bat jar` on Wind
 project root directory, and it should download all dependencies automatically, and build
 `freemarker.jar`.
 
+**Note if you have trouble 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.`) try this as a local-only-developer-hack:
+
+`./gradlew -Pfreemarker.skipJavaMRJSourceSets=true jar`
+
+This is helpful if you have trouble finding a JDK9 / JDK16 for MacOS.
+
+
 To run all JUnit tests and some other checks, issue `./gradlew check`. (Avoid the
 `test` task, as that will only run the tests of the `core` source set.)
 

build.gradle.kts

@@ -35,6 +35,12 @@ group = "org.freemarker"
 
 val fmExt = freemarkerRoot
 
+// HACK: disable MR-JAR source sets locally on Apple Silicon without JDK 9 / 16
+val skipJavaMRJSourceSets = providers
+    .gradleProperty("freemarker.skipJavaMRJSourceSets")
+    .map(String::toBoolean)
+    .getOrElse(false)
+
 tasks.withType<JavaCompile>().configureEach {
     options.encoding = "UTF-8"
 }
@@ -61,10 +67,14 @@ freemarkerRoot {
     configureSourceSet("jython20")
     configureSourceSet("jython22")
     configureSourceSet("jython25") { enableTests() }
-    configureSourceSet("core9", "9") { enableTests() }
-    configureSourceSet("core16", "16") {
+
+    
+    if (!skipJavaMRJSourceSets) {
+        configureSourceSet("core9", "9") { enableTests() }
+        configureSourceSet("core16", "16") {
         enableTests();
-        addDependencySourceSet("core9");
+            addDependencySourceSet("core9");
+        }
     }
 
     configureGeneratedSourceSet("jakartaServlet") {
@@ -592,15 +602,15 @@ registerDistRatTask("ratDistSrc", file("rat-excludes"), distSrc)
 
 eclipse {
     classpath {
-        // Eclipse sees only a single classpath,
-        // so make a best effort for a combined classpath.
-        plusConfigurations = listOf(
-            configurations["combinedClasspath"],
-            configurations["core9CompileClasspath"],
-            configurations["core16CompileClasspath"],
-            configurations["testUtilsCompileClasspath"],
-            configurations["javaxServletTestCompileClasspath"]
-        )
+        plusConfigurations = buildList {
+            add(configurations["combinedClasspath"])
+            if (!skipJavaMRJSourceSets) {
+                add(configurations["core9CompileClasspath"])
+                add(configurations["core16CompileClasspath"])
+            }
+            add(configurations["testUtilsCompileClasspath"])
+            add(configurations["javaxServletTestCompileClasspath"])
+        }
     }
 }