diff --git a/compile/interface/src/main/scala/xsbt/Dependency.scala b/compile/interface/src/main/scala/xsbt/Dependency.scala
index 34c237bb3..c20974790 100644
--- a/compile/interface/src/main/scala/xsbt/Dependency.scala
+++ b/compile/interface/src/main/scala/xsbt/Dependency.scala
@@ -63,7 +63,13 @@ final class Dependency(val global: CallbackGlobal) extends LocateClassFile with
               case Some((f, className, inOutDir)) =>
                 if (inOutDir && on.isJavaDefined) registerTopLevelSym(on)
                 f match {
-                  case ze: ZipArchive#Entry => for (zip <- ze.underlyingSource; zipFile <- Option(zip.file)) binaryDependency(zipFile, className)
+                  case ze: ZipArchive#Entry =>
+                    // Scala 2.10/2.11 give back $JAVA_HOME as the underlying source for the platform classloader
+                    // See https://github.com/scala/scala/pull/6113. Treating that as not having an underlying source
+                    // is the least bad option for now. We would not trigger a recompile if the JDK is updated, but that
+                    // is better than recompiling *every* time, which seems to happen if we let the directory propagate
+                    // to `binaryDependency`.
+                    for (zip <- ze.underlyingSource; zipFile <- Option(zip.file).filterNot(_.isDirectory)) binaryDependency(zipFile, className)
                   case pf: PlainFile        => binaryDependency(pf.file, className)
                   case _                    => ()
                 }
diff --git a/compile/src/main/scala/sbt/compiler/CompilerArguments.scala b/compile/src/main/scala/sbt/compiler/CompilerArguments.scala
index df279bb37..6a2a41056 100644
--- a/compile/src/main/scala/sbt/compiler/CompilerArguments.scala
+++ b/compile/src/main/scala/sbt/compiler/CompilerArguments.scala
@@ -45,7 +45,14 @@ final class CompilerArguments(scalaInstance: xsbti.compile.ScalaInstance, cp: xs
   /** Add the correct Scala library jar to the boot classpath if `addLibrary` is true.*/
   def createBootClasspath(addLibrary: Boolean) =
     {
-      val originalBoot = System.getProperty("sun.boot.class.path", "")
+      def findBoot: String =
+        {
+          import scala.collection.JavaConverters._
+          System.getProperties.asScala.iterator.collectFirst {
+            case (k, v) if k.endsWith(".boot.class.path") => v
+          }.getOrElse("")
+        }
+      val originalBoot = Option(System.getProperty("sun.boot.class.path")).getOrElse(findBoot)
       if (addLibrary) {
         val newBootPrefix = if (originalBoot.isEmpty) "" else originalBoot + File.pathSeparator
         newBootPrefix + scalaInstance.libraryJar.getAbsolutePath
@@ -63,7 +70,7 @@ final class CompilerArguments(scalaInstance: xsbti.compile.ScalaInstance, cp: xs
   def bootClasspathFor(classpath: Seq[File]) = bootClasspath(hasLibrary(classpath))
 
   import Path._
-  def extClasspath: Seq[File] = (IO.parseClasspath(System.getProperty("java.ext.dirs", "")) * "*.jar").get
+  def extClasspath: Seq[File] = List("java.ext.dirs", "scala.ext.dirs").flatMap(k => (IO.parseClasspath(System.getProperty(k, "")) * "*.jar").get)
 }
 object CompilerArguments {
   val BootClasspathOption = "-bootclasspath"