Document asSubscriber stage (#28128)

* Unfortunately it seems the jdk9-only tests could not actually be compiled.
With these changes those can actually be compiled and ran again.

* Always link to jdk11 for java.* javadocs

* Update sbt-paradox-akka to fix linking to inner classes for javadoc

akka-docs/src/main/paradox/stream/operators/Source/asSubscriber.md

@@ -4,12 +4,51 @@ Integration with Reactive Streams, materializes into a `org.reactivestreams.Subs
 
 @ref[Source operators](../index.md#source-operators)
 
-@@@ div { .group-scala }
 ## Signature
 
-@@signature [Source.scala](/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala) { #asSubscriber }
+@@@ div { .group-scala }
+
+@@snip[JavaFlowSupport.scala](/akka-stream/src/main/scala-jdk-9/akka/stream/scaladsl/JavaFlowSupport.scala) { #asSubscriber }
+
+@@@
+
+@@@ div { .group-java }
+
+@@snip[JavaFlowSupport.java](/akka-stream/src/main/java-jdk-9/akka/stream/javadsl/JavaFlowSupport.java) { #asSubscriber }
+
 @@@
 
 ## Description
 
-TODO: We would welcome help on contributing descriptions and examples, see: https://github.com/akka/akka/issues/25646
+If you want to create a @apidoc[Source] that gets its elements from another library that supports
+[Reactive Streams](https://www.reactive-streams.org/), you can use `JavaFlowSupport.Source.asSubscriber`.
+Each time this @apidoc[Source] is materialized, it produces a materialized value of type
+@javadoc[java.util.concurrent.Flow.Subscriber](java.util.concurrent.Flow.Subscriber).
+This @javadoc[Subscriber](java.util.concurrent.Flow.Subscriber) can be attached to a
+[Reactive Streams](https://www.reactive-streams.org/) @javadoc[Publisher](java.util.concurrent.Flow.Publisher)
+to populate it.
+
+@@@ note
+
+For JDK 8 users: since @javadoc[java.util.concurrent.Flow](java.util.concurrent.Flow) was introduced in JDK version 9,
+if you are still on version 8 you may use the [org.reactivestreams](https://github.com/reactive-streams/reactive-streams-jvm#reactive-streams) library with `Source.asSubscriber` and `Flow.asSubscriber`.
+
+@@@
+
+## Example
+
+Suppose we use a database client that supports [Reactive Streams](https://www.reactive-streams.org/),
+we could create a @apidoc[Source] that queries the database for its rows. That @apidoc[Source] can then
+be used for further processing, for example creating a @apidoc[Source] that contains the names of the
+rows.
+
+Note that since the database is queried for each materialization, the `rowSource` can be safely re-used.
+Because both the database driver and Akka Streams support [Reactive Streams](https://www.reactive-streams.org/),
+backpressure is applied throughout the stream, preventing us from running out of memory when the database
+rows are consumed slower than they are produced by the database.
+
+Scala
+:  @@snip [AsSubscriber.scala](/akka-docs/src/test/scala-jdk9-only/docs/stream/operators/source/AsSubscriber.scala) { #imports #example }
+
+Java
+:  @@snip [AsSubscriber.java](/akka-docs/src/test/java-jdk9-only/jdocs/stream/operators/source/AsSubscriber.java) { #imports #example }

akka-docs/src/test/java-jdk9-only/jdocs/stream/operators/source/AsSubscriber.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.source;
+
+//#imports
+import java.util.concurrent.Flow.Subscriber;
+import java.util.concurrent.Flow.Publisher;
+
+import akka.NotUsed;
+import akka.stream.javadsl.Source;
+import akka.stream.javadsl.JavaFlowSupport;
+
+//#imports
+
+import org.apache.commons.lang.NotImplementedException;
+
+public interface AsSubscriber {
+    static class JavaFlowSupport {
+        public static final class Source {
+            public
+            // #api
+            static <T> akka.stream.javadsl.Source<T, Subscriber<T>> asSubscriber()
+            // #api
+            {
+                return akka.stream.javadsl.JavaFlowSupport.Source.<T>asSubscriber();
+            }
+        }
+    }
+
+    static class Row {
+        public String getField(String fieldName) {
+            throw new NotImplementedException();
+        }
+    }
+
+    static class DatabaseClient {
+        Publisher<Row> fetchRows() {
+            throw new NotImplementedException();
+        }
+    }
+
+    DatabaseClient databaseClient = null;
+
+    // #example
+    class Example {
+        Source<Row, NotUsed> rowSource =
+                JavaFlowSupport.Source.<Row>asSubscriber()
+                        .mapMaterializedValue(
+                                subscriber -> {
+                                    // For each materialization, fetch the rows from the database:
+                                    Publisher<Row> rows = databaseClient.fetchRows();
+                                    rows.subscribe(subscriber);
+
+                                    return NotUsed.getInstance();
+                                });
+
+        public Source<String, NotUsed> names() {
+            // rowSource can be re-used, since it will start a new
+            // query for each materialization, fully supporting backpressure
+            // for each materialized stream:
+            return rowSource.map(row -> row.getField("name"));
+        }
+    }
+    // #example
+}

akka-docs/src/test/scala-jdk9-only/docs/stream/operators/source/AsSubscriber.scala

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.source;
+
+//#imports
+import java.util.concurrent.Flow.Subscriber;
+import java.util.concurrent.Flow.Publisher;
+
+import akka.NotUsed;
+import akka.stream.scaladsl.Source;
+import akka.stream.scaladsl.JavaFlowSupport;
+
+//#imports
+
+object AsSubscriber {
+    case class Row(name: String)
+
+    class DatabaseClient {
+        def fetchRows(): Publisher[Row] = ???
+    }
+
+    val databaseClient: DatabaseClient = ???;
+
+    // #example
+    val rowSource: Source[Row, NotUsed] =
+      JavaFlowSupport.Source.asSubscriber
+        .mapMaterializedValue(
+          (subscriber: Subscriber[Row]) => {
+            // For each materialization, fetch the rows from the database:
+            val rows: Publisher[Row] = databaseClient.fetchRows()
+            rows.subscribe(subscriber)
+            NotUsed
+          });
+
+    val names: Source[String, NotUsed] =
+      // rowSource can be re-used, since it will start a new
+      // query for each materialization, fully supporting backpressure
+      // for each materialized stream:
+      rowSource.map(row => row.name);
+    //#example
+}

akka-stream/src/main/java-jdk-9/akka/stream/javadsl/JavaFlowSupport.java

@@ -52,7 +52,9 @@ public final class JavaFlowSupport {
      * See also {@code Source.asSubscriber} if wanting to integrate with {@link org.reactivestreams.Subscriber} instead
      * (which carries the same semantics, however existed before RS's inclusion in Java 9).
      */
+    //#asSubscriber
     public static <T> akka.stream.javadsl.Source<T, java.util.concurrent.Flow.Subscriber<T>> asSubscriber() {
+    //#asSubscriber
       return akka.stream.javadsl.Source.<T>asSubscriber().mapMaterializedValue(JavaFlowAndRsConverters::asJava);
     }
   }

akka-stream/src/main/scala-jdk-9/akka/stream/scaladsl/JavaFlowSupport.scala

@@ -46,7 +46,9 @@ object JavaFlowSupport {
      * @see See also [[Source.asSubscriber]] if wanting to integrate with [[org.reactivestreams.Subscriber]] instead
      *      (which carries the same semantics, however existed before RS's inclusion in Java 9).
      */
-    final def asSubscriber[T]: Source[T, juc.Flow.Subscriber[T]] =
+    //#asSubscriber
+    final def asSubscriber[T]: Source[T, java.util.concurrent.Flow.Subscriber[T]] =
+    //#asSubscriber
       scaladsl.Source.asSubscriber[T].mapMaterializedValue(_.asJava)
   }
 

build.sbt

@@ -102,7 +102,8 @@ lazy val akkaScalaNightly = akkaModule("akka-scala-nightly")
   .disablePlugins(ValidatePullRequest, MimaPlugin, CopyrightHeaderInPr)
 
 lazy val benchJmh = akkaModule("akka-bench-jmh")
-  .dependsOn(Seq(actor, actorTyped, stream, streamTests, persistence, distributedData, jackson, testkit).map(
+  .enablePlugins(Jdk9)
+  .dependsOn(Seq(actor, actorTyped, stream, streamTestkit, persistence, distributedData, jackson, testkit).map(
     _ % "compile->compile;compile->test"): _*)
   .settings(Dependencies.benchJmh)
   .settings(javacOptions += "-parameters") // for Jackson
@@ -170,6 +171,7 @@ lazy val distributedData = akkaModule("akka-distributed-data")
   .enablePlugins(MultiNodeScalaTest)
 
 lazy val docs = akkaModule("akka-docs")
+  .configs(akka.Jdk9.TestJdk9)
   .dependsOn(
     actor,
     cluster,
@@ -180,6 +182,7 @@ lazy val docs = akkaModule("akka-docs")
     persistenceQuery,
     distributedData,
     stream,
+    stream % "TestJdk9->CompileJdk9",
     actorTyped,
     clusterTools % "compile->compile;test->test",
     clusterSharding % "compile->compile;test->test",
@@ -201,7 +204,8 @@ lazy val docs = akkaModule("akka-docs")
     NoPublish,
     ParadoxBrowse,
     ScaladocNoVerificationOfDiagrams,
-    StreamOperatorsIndexGenerator)
+    StreamOperatorsIndexGenerator,
+    Jdk9)
   .disablePlugins(MimaPlugin, WhiteSourcePlugin)
   .disablePlugins(ScalafixPlugin)
 
@@ -344,9 +348,14 @@ lazy val streamTestkit = akkaModule("akka-stream-testkit")
   .disablePlugins(MimaPlugin)
 
 lazy val streamTests = akkaModule("akka-stream-tests")
-  .dependsOn(streamTestkit % "test->test", remote % "test->test", stream)
+  .configs(akka.Jdk9.TestJdk9)
+  .dependsOn(
+    streamTestkit % "test->test",
+    remote % "test->test",
+    stream % "TestJdk9->CompileJdk9"
+  )
   .settings(Dependencies.streamTests)
-  .enablePlugins(NoPublish)
+  .enablePlugins(NoPublish, Jdk9)
   .disablePlugins(MimaPlugin, WhiteSourcePlugin)
 
 lazy val streamTestsTck = akkaModule("akka-stream-tests-tck")

project/Jdk9.scala

@@ -10,7 +10,9 @@ import sbt.Keys._
 object Jdk9 extends AutoPlugin {
   import JdkOptions.notOnJdk8
 
-  lazy val CompileJdk9 = config("CompileJdk9").extend(Compile)
+  val CompileJdk9 = config("CompileJdk9").extend(Compile)
+
+  val TestJdk9 = config("TestJdk9").extend(Test)
 
   val SCALA_SOURCE_DIRECTORY = "scala-jdk-9"
   val SCALA_TEST_SOURCE_DIRECTORY = "scala-jdk9-only"
@@ -20,12 +22,26 @@ object Jdk9 extends AutoPlugin {
   val compileJdk9Settings = Seq(
     // following the scala-2.12, scala-sbt-1.0, ... convention
     unmanagedSourceDirectories := notOnJdk8(
-        Seq(
+      Seq(
-          (Compile / sourceDirectory).value / SCALA_SOURCE_DIRECTORY,
+        (Compile / sourceDirectory).value / SCALA_SOURCE_DIRECTORY,
-          (Compile / sourceDirectory).value / JAVA_SOURCE_DIRECTORY)),
+        (Compile / sourceDirectory).value / JAVA_SOURCE_DIRECTORY)),
+
     scalacOptions := AkkaBuild.DefaultScalacOptions ++ notOnJdk8(Seq("-release", "11")),
     javacOptions := AkkaBuild.DefaultJavacOptions ++ notOnJdk8(Seq("--release", "11")))
 
+  val testJdk9Settings = Seq(
+    // following the scala-2.12, scala-sbt-1.0, ... convention
+    unmanagedSourceDirectories := notOnJdk8(
+      Seq(
+        (Test / sourceDirectory).value / SCALA_TEST_SOURCE_DIRECTORY,
+        (Test / sourceDirectory).value / JAVA_TEST_SOURCE_DIRECTORY)),
+
+    scalacOptions := AkkaBuild.DefaultScalacOptions ++ notOnJdk8(Seq("-release", "11")),
+    javacOptions := AkkaBuild.DefaultJavacOptions ++ notOnJdk8(Seq("--release", "11")),
+    classpathConfiguration := TestJdk9,
+    externalDependencyClasspath := (externalDependencyClasspath in Test).value
+    )
+
   val compileSettings = Seq(
     // It might have been more 'neat' to add the jdk9 products to the jar via packageBin/mappings, but that doesn't work with the OSGi plugin,
     // so we add them to the fullClasspath instead.
@@ -38,5 +54,7 @@ object Jdk9 extends AutoPlugin {
   override lazy val projectSettings =
     inConfig(CompileJdk9)(Defaults.compileSettings) ++
     inConfig(CompileJdk9)(compileJdk9Settings) ++
-    compileSettings
+    compileSettings ++
+    inConfig(TestJdk9)(Defaults.testSettings) ++
+    inConfig(TestJdk9)(testJdk9Settings)
 }

project/Paradox.scala

@@ -25,6 +25,8 @@ object Paradox {
         "extref.ecs.base_url" -> "https://example.lightbend.com/v1/download/%s",
         "scaladoc.akka.base_url" -> "https://doc.akka.io/api/akka/2.6",
         "scaladoc.akka.http.base_url" -> "https://doc.akka.io/api/akka-http/current",
+        "javadoc.java.base_url" -> "https://docs.oracle.com/en/java/javase/11/docs/api/java.base/",
+        "javadoc.java.link_style" -> "direct",
         "javadoc.akka.base_url" -> "https://doc.akka.io/japi/akka/2.6",
         "javadoc.akka.link_style" -> "direct",
         "javadoc.akka.http.base_url" -> "https://doc.akka.io/japi/akka-http/current",