Fix com-lihaoyi#370 Use ExtClassLoader instead of null as parent

main/api/src/mill/api/ClassLoader.scala

@@ -58,8 +58,8 @@ object ClassLoader {
    *  mill could be compiled only with jdk 9 or above. We don't want to introduce this restriction now.
    */
   private def refinePlatformParent(parent: java.lang.ClassLoader): ClassLoader = {
-    if (!java9OrAbove || parent != null) parent
-    else {
+    if (parent != null) parent
+    else if (java9OrAbove) {
       // Make sure when `parent == null`, we only delegate java.* classes
       // to the parent getPlatformClassLoader. This is necessary because
       // in Java 9+, somehow the getPlatformClassLoader ends up with all
@@ -69,6 +69,19 @@ object ClassLoader {
         .getMethod("getPlatformClassLoader")
         .invoke(null)
         .asInstanceOf[ClassLoader]
+    } else {
+      // With Java 8 we want a clean classloader that still contains classes
+      // coming from com.sun.* etc.
+      // We get the application classloader parent which happens to be of
+      // type sun.misc.Launcher$ExtClassLoader
+      // We can't call the method directly since it doesn't exist in Java 9
+      // So we load it via reflection.
+      val launcherClass = getClass.getClassLoader().loadClass("sun.misc.Launcher")
+      val getLauncherMethod = launcherClass.getMethod("getLauncher")
+      val launcher = getLauncherMethod.invoke(null)
+      val getClassLoaderMethod = launcher.getClass().getMethod("getClassLoader")
+      val appClassLoader = getClassLoaderMethod.invoke(launcher).asInstanceOf[ClassLoader]
+      appClassLoader.getParent()
     }
   }
 

scalalib/test/resources/classloader-test/test/src/ClassLoaderTests.scala

@@ -0,0 +1,13 @@
+package mill.scalalib
+
+import utest._
+
+object ClassLoaderTests extends TestSuite {
+  try {
+    getClass().getClassLoader().loadClass("com.sun.nio.zipfs.ZipFileSystemProvider")
+  } catch {
+    case _: ClassNotFoundException if !System.getProperty("java.specification.version").startsWith("1.") =>
+      // Don't fail on Java 9+
+  }
+  val tests = Tests {}
+}

scalalib/test/src/TestClassLoaderTests.scala

@@ -0,0 +1,50 @@
+package mill.scalalib
+
+import mill.{Agg, T}
+
+import scala.util.Success
+import mill.testrunner.TestRunner.TestArgs
+import mill.util.{TestEvaluator, TestUtil}
+import org.scalacheck.Prop.forAll
+import utest._
+import utest.framework.TestPath
+
+object TestClassLoaderTests extends TestSuite {
+  object testclassloader extends TestUtil.BaseModule with ScalaModule {
+    override def millSourcePath = TestUtil.getSrcPathBase() / millOuterCtx.enclosing.split('.')
+
+    def scalaVersion = sys.props.getOrElse("TEST_SCALA_2_12_VERSION", ???)
+
+    object test extends super.Tests with TestModule.Utest {
+      override def ivyDeps = T {
+        super.ivyDeps() ++ Agg(
+          ivy"com.lihaoyi::utest:${sys.props.getOrElse("TEST_UTEST_VERSION", ???)}"
+        )
+      }
+    }
+  }
+
+  val resourcePath = os.pwd / "scalalib" / "test" / "resources" / "classloader-test"
+
+  def workspaceTest[T](
+      m: TestUtil.BaseModule,
+      resourcePath: os.Path = resourcePath
+  )(t: TestEvaluator => T)(
+      implicit tp: TestPath
+  ): T = {
+    val eval = new TestEvaluator(m)
+    os.remove.all(m.millSourcePath)
+    os.remove.all(eval.outPath)
+    os.makeDir.all(m.millSourcePath / os.up)
+    os.copy(resourcePath, m.millSourcePath)
+    t(eval)
+  }
+
+  override def tests: Tests = Tests {
+    test("classloader can load com.sun.* on Java 8") {
+      workspaceTest(testclassloader) { eval =>
+        assert(eval.apply(testclassloader.test.test()).isRight)
+      }
+    }
+  }
+}