+str #17464: mapConcat accepts immutable.Iterable

akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java

@@ -57,12 +57,12 @@ public class FlowTest extends StreamTest {
       }
     });
     final Flow<String, String, ?> flow2 = Flow.of(String.class).grouped(2
-    ).mapConcat(new Function<java.util.List<String>, java.util.List<String>>() {
+    ).mapConcat(new Function<java.util.List<String>, java.lang.Iterable<String>>() {
       public java.util.List<String> apply(java.util.List<String> elem) {
         return elem;
       }
     }).groupedWithin(100, FiniteDuration.create(50, TimeUnit.MILLISECONDS)
-    ).mapConcat(new Function<java.util.List<String>, java.util.List<String>>() {
+    ).mapConcat(new Function<java.util.List<String>, java.lang.Iterable<String>>() {
           public java.util.List<String> apply(java.util.List<String> elem) {
             return elem;
           }

akka-stream/src/main/scala/akka/stream/impl/Stages.scala

@@ -193,7 +193,7 @@ private[stream] object Stages {
     def withAttributes(attributes: OperationAttributes) = copy(attributes = attributes)
     override protected def newInstance: StageModule = this.copy()
   }
-  final case class MapConcat(f: Any ⇒ immutable.Seq[Any], attributes: OperationAttributes = mapConcat) extends StageModule {
+  final case class MapConcat(f: Any ⇒ immutable.Iterable[Any], attributes: OperationAttributes = mapConcat) extends StageModule {
     def withAttributes(attributes: OperationAttributes) = copy(attributes = attributes)
     override protected def newInstance: StageModule = this.copy()
   }

akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala

@@ -64,22 +64,30 @@ private[akka] final case class Collect[In, Out](decider: Supervision.Decider)(pf
 /**
  * INTERNAL API
  */
-private[akka] final case class MapConcat[In, Out](f: In ⇒ immutable.Seq[Out], decider: Supervision.Decider) extends PushPullStage[In, Out] {
+private[akka] final case class MapConcat[In, Out](f: In ⇒ immutable.Iterable[Out], decider: Supervision.Decider) extends PushPullStage[In, Out] {
   private var currentIterator: Iterator[Out] = Iterator.empty
 
   override def onPush(elem: In, ctx: Context[Out]): SyncDirective = {
     currentIterator = f(elem).iterator
-    if (currentIterator.isEmpty) ctx.pull()
+    if (!currentIterator.hasNext) ctx.pull()
     else ctx.push(currentIterator.next())
   }
 
   override def onPull(ctx: Context[Out]): SyncDirective =
-    if (currentIterator.hasNext) ctx.push(currentIterator.next())
+    if (ctx.isFinishing) {
-    else if (ctx.isFinishing) ctx.finish()
+      if (currentIterator.hasNext) {
-    else ctx.pull()
+        val elem = currentIterator.next()
+        if (currentIterator.hasNext) ctx.push(elem)
+        else ctx.pushAndFinish(elem)
+      } else ctx.finish()
+    } else {
+      if (currentIterator.hasNext) ctx.push(currentIterator.next())
+      else ctx.pull()
+    }
 
   override def onUpstreamFinish(ctx: Context[Out]): TerminationDirective =
-    ctx.absorbTermination()
+    if (currentIterator.hasNext) ctx.absorbTermination()
+    else ctx.finish()
 
   override def decide(t: Throwable): Supervision.Directive = decider(t)
 

akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala

@@ -9,6 +9,7 @@ import akka.japi.{ Util, Pair }
 import akka.japi.function
 import akka.stream.scaladsl
 import scala.annotation.unchecked.uncheckedVariance
+import scala.collection.immutable
 import scala.concurrent.Future
 import scala.concurrent.duration.FiniteDuration
 import akka.stream.stage.Stage
@@ -170,10 +171,14 @@ class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends Graph
     new Flow(delegate.map(f.apply))
 
   /**
-   * Transform each input element into a sequence of output elements that is
+   * Transform each input element into an `Iterable` of output elements that is
    * then flattened into the output stream.
    *
-   * The returned list MUST NOT contain `null` values,
+   * Make sure that the `Iterable` is immutable or at least not modified after
+   * being used as an output sequence. Otherwise the stream may fail with
+   * `oncurrentModificationException` or other more subtle errors may occur.
+   *
+   * The returned `Iterable` MUST NOT contain `null` values,
    * as they are illegal as stream elements - according to the Reactive Streams specification.
    *
    * '''Emits when''' the mapping function returns an element or there are still remaining elements
@@ -186,8 +191,14 @@ class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends Graph
    *
    * '''Cancels when''' downstream cancels
    */
-  def mapConcat[T](f: function.Function[Out, java.util.List[T]]): javadsl.Flow[In, T, Mat] =
+  def mapConcat[T](f: function.Function[Out, java.lang.Iterable[T]]): javadsl.Flow[In, T, Mat] =
-    new Flow(delegate.mapConcat(elem ⇒ Util.immutableSeq(f.apply(elem))))
+    new Flow(delegate.mapConcat { elem ⇒
+      val scalaIterable = new immutable.Iterable[T] {
+        import collection.JavaConverters._
+        override def iterator: Iterator[T] = f(elem).iterator().asScala
+      }
+      scalaIterable
+    })
 
   /**
    * Transform this stream by applying the given function to each of the elements

akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala

@@ -361,10 +361,10 @@ trait FlowOps[+Out, +Mat] {
   def map[T](f: Out ⇒ T): Repr[T, Mat] = andThen(Map(f.asInstanceOf[Any ⇒ Any]))
 
   /**
-   * Transform each input element into a sequence of output elements that is
+   * Transform each input element into an `Iterable` of output elements that is
    * then flattened into the output stream.
    *
-   * The returned sequence MUST NOT contain `null` values,
+   * The returned `Iterable` MUST NOT contain `null` values,
    * as they are illegal as stream elements - according to the Reactive Streams specification.
    *
    * '''Emits when''' the mapping function returns an element or there are still remaining elements
@@ -378,7 +378,7 @@ trait FlowOps[+Out, +Mat] {
    * '''Cancels when''' downstream cancels
    *
    */
-  def mapConcat[T](f: Out ⇒ immutable.Seq[T]): Repr[T, Mat] = andThen(MapConcat(f.asInstanceOf[Any ⇒ immutable.Seq[Any]]))
+  def mapConcat[T](f: Out ⇒ immutable.Iterable[T]): Repr[T, Mat] = andThen(MapConcat(f.asInstanceOf[Any ⇒ immutable.Iterable[Any]]))
 
   /**
    * Transform this stream by applying the given function to each of the elements

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -215,36 +215,7 @@ object Source extends SourceApply {
    * beginning) regardless of when they subscribed.
    */
   def apply[T](iterable: immutable.Iterable[T]): Source[T, Unit] = {
-    Source.empty.transform(() ⇒ {
+    Source.single(()).mapConcat((_: Unit) ⇒ iterable).withAttributes(DefaultAttributes.iterableSource)
-      new PushPullStage[Nothing, T] {
-        var iterator: Iterator[T] = null
-
-        // Delayed init so we onError instead of failing during construction of the Source
-        def initIterator(): Unit = if (iterator eq null) iterator = iterable.iterator
-
-        // Upstream is guaranteed to be empty
-        override def onPush(elem: Nothing, ctx: Context[T]): SyncDirective =
-          throw new UnsupportedOperationException("The IterableSource stage cannot be pushed")
-
-        override def onUpstreamFinish(ctx: Context[T]): TerminationDirective = {
-          initIterator()
-          if (iterator.hasNext) ctx.absorbTermination()
-          else ctx.finish()
-        }
-
-        override def onPull(ctx: Context[T]): SyncDirective = {
-          if (!ctx.isFinishing) {
-            initIterator()
-            ctx.pull()
-          } else {
-            val elem = iterator.next()
-            if (iterator.hasNext) ctx.push(elem)
-            else ctx.pushAndFinish(elem)
-          }
-        }
-      }
-
-    }).withAttributes(DefaultAttributes.iterableSource)
   }
 
   /**