Make ShuffleDependency.aggregator explicitly optional.

It was confusing to be using

    new Aggregator[K, V, V](null, null, null, false)

to represent the absence of an aggregator.

4 files changed, 11 insertions, 7 deletions

diff --git a/core/src/main/scala/spark/Dependency.scala b/core/src/main/scala/spark/Dependency.scala
index f2b7aa33ec..19a51dd5b8 100644
--- a/core/src/main/scala/spark/Dependency.scala
+++ b/core/src/main/scala/spark/Dependency.scala
@@ -20,11 +20,15 @@ abstract class NarrowDependency[T](rdd: RDD[T]) extends Dependency(rdd) {
 
 /**
  * Represents a dependency on the output of a shuffle stage.
+ * @param shuffleId the shuffle id
+ * @param rdd the parent RDD
+ * @param aggregator optional aggregator; this allows for map-side combining
+ * @param partitioner partitioner used to partition the shuffle output
  */
 class ShuffleDependency[K, V, C](
     val shuffleId: Int,
     @transient rdd: RDD[(K, V)],
-    val aggregator: Aggregator[K, V, C],
+    val aggregator: Option[Aggregator[K, V, C]],
     val partitioner: Partitioner)
   extends Dependency(rdd)
 
diff --git a/core/src/main/scala/spark/rdd/CoGroupedRDD.scala b/core/src/main/scala/spark/rdd/CoGroupedRDD.scala
index 8fa0749184..f1defbe492 100644
--- a/core/src/main/scala/spark/rdd/CoGroupedRDD.scala
+++ b/core/src/main/scala/spark/rdd/CoGroupedRDD.scala
@@ -49,7 +49,7 @@ class CoGroupedRDD[K](@transient rdds: Seq[RDD[(_, _)]], part: Partitioner)
       } else {
         logInfo("Adding shuffle dependency with " + rdd)
         deps += new ShuffleDependency[Any, Any, ArrayBuffer[Any]](
-            context.newShuffleId, rdd, aggr, part)
+            context.newShuffleId, rdd, Some(aggr), part)
       }
     }
     deps.toList
diff --git a/core/src/main/scala/spark/rdd/ShuffledRDD.scala b/core/src/main/scala/spark/rdd/ShuffledRDD.scala
index 769ccf8caa..7577909b83 100644
--- a/core/src/main/scala/spark/rdd/ShuffledRDD.scala
+++ b/core/src/main/scala/spark/rdd/ShuffledRDD.scala
@@ -22,7 +22,7 @@ private[spark] class ShuffledRDDSplit(val idx: Int) extends Split {
  */
 abstract class ShuffledRDD[K, V, C](
     @transient parent: RDD[(K, V)],
-    aggregator: Aggregator[K, V, C],
+    aggregator: Option[Aggregator[K, V, C]],
     part: Partitioner)
   extends RDD[(K, C)](parent.context) {
 
@@ -48,7 +48,7 @@ class RepartitionShuffledRDD[K, V](
     part: Partitioner)
   extends ShuffledRDD[K, V, V](
     parent,
-    Aggregator[K, V, V](null, null, null, false),
+    None,
     part) {
 
   override def compute(split: Split): Iterator[(K, V)] = {
@@ -95,7 +95,7 @@ class ShuffledAggregatedRDD[K, V, C](
     @transient parent: RDD[(K, V)],
     aggregator: Aggregator[K, V, C],
     part : Partitioner)
-  extends ShuffledRDD[K, V, C](parent, aggregator, part) {
+  extends ShuffledRDD[K, V, C](parent, Some(aggregator), part) {
 
   override def compute(split: Split): Iterator[(K, C)] = {
     val combiners = new JHashMap[K, C]
diff --git a/core/src/main/scala/spark/scheduler/ShuffleMapTask.scala b/core/src/main/scala/spark/scheduler/ShuffleMapTask.scala
index 3e5ba10fd9..86796d3677 100644
--- a/core/src/main/scala/spark/scheduler/ShuffleMapTask.scala
+++ b/core/src/main/scala/spark/scheduler/ShuffleMapTask.scala
@@ -111,11 +111,11 @@ private[spark] class ShuffleMapTask(
 
   override def run(attemptId: Long): MapStatus = {
     val numOutputSplits = dep.partitioner.numPartitions
-    val aggregator = dep.aggregator.asInstanceOf[Aggregator[Any, Any, Any]]
     val partitioner = dep.partitioner
 
     val bucketIterators =
-      if (aggregator.mapSideCombine) {
+      if (dep.aggregator.isDefined && dep.aggregator.get.mapSideCombine) {
+        val aggregator = dep.aggregator.get.asInstanceOf[Aggregator[Any, Any, Any]]
         // Apply combiners (map-side aggregation) to the map output.
         val buckets = Array.tabulate(numOutputSplits)(_ => new JHashMap[Any, Any])
         for (elem <- rdd.iterator(split)) {