Use scala ForkJoinPool instead (in 2.12 this just wraps Java but in 2…

….11 necessary for correct concurrency report)

core/src/main/scala/org/apache/spark/util/ThreadUtils.scala

@@ -20,6 +20,7 @@ package org.apache.spark.util
 import java.util.concurrent._
 
 import scala.concurrent.{ExecutionContext, ExecutionContextExecutor}
+import scala.concurrent.forkjoin.{ForkJoinPool => SForkJoinPool, ForkJoinWorkerThread => SForkJoinWorkerThread}
 import scala.util.control.NonFatal
 
 import com.google.common.util.concurrent.{MoreExecutors, ThreadFactoryBuilder}
@@ -158,17 +159,17 @@ private[spark] object ThreadUtils {
   }
 
   /**
-   * Construct a new Java ForkJoinPool with a specified max parallelism and name prefix.
+   * Construct a new Scala ForkJoinPool with a specified max parallelism and name prefix.
    */
-  def newForkJoinPool(prefix: String, maxThreadNumber: Int): ForkJoinPool = {
+  def newForkJoinPool(prefix: String, maxThreadNumber: Int): SForkJoinPool = {
     // Custom factory to set thread names
-    val factory = new ForkJoinPool.ForkJoinWorkerThreadFactory {
-      override def newThread(pool: ForkJoinPool) =
-        new ForkJoinWorkerThread(pool) {
+    val factory = new SForkJoinPool.ForkJoinWorkerThreadFactory {
+      override def newThread(pool: SForkJoinPool) =
+        new SForkJoinWorkerThread(pool) {
           setName(prefix + "-" + super.getName)
         }
     }
-    new ForkJoinPool(maxThreadNumber, factory,
+    new SForkJoinPool(maxThreadNumber, factory,
       null, // handler
       false // asyncMode
     )