INT-4269: Add Stream & Flux Support for Splitter

JIRA: https://jira.spring.io/browse/INT-4269

* Check if `outputChannel` is `ReactiveStreamsSubscribableChannel`
to let back-pressure splitting
* Build `Flux` or `Iterator` depending in the `reactive` state
* Allow to get a size of the `iterator` if it is possible,
for example `XPathMessageSplitter`

Add tests

Fix raw type and unused import

* Add JavaDocs to the `AbstractMessageSplitter#obtainSizeIfPossible()`
* Add asserts for the `sequenceSize` populataiton in the `XPathMessageSplitter`
* Document `Stream` & `Flux` support in the splitter

Minor doc polishing

spring-integration-core/src/test/java/org/springframework/integration/splitter/DefaultSplitterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,29 +16,40 @@
 
 package org.springframework.integration.splitter;
 
-import static org.junit.Assert.assertEquals;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.nullValue;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.stream.Stream;
 
 import org.junit.Test;
-import org.springframework.messaging.Message;
+import org.reactivestreams.Subscriber;
+
 import org.springframework.integration.IntegrationMessageHeaderAccessor;
 import org.springframework.integration.channel.DirectChannel;
+import org.springframework.integration.channel.FluxMessageChannel;
 import org.springframework.integration.channel.QueueChannel;
 import org.springframework.integration.endpoint.EventDrivenConsumer;
 import org.springframework.integration.support.MessageBuilder;
+import org.springframework.integration.test.util.TestUtils;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.support.GenericMessage;
+
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
 
 /**
  * @author Mark Fisher
  * @author Iwein Fuld
  * @author Gunnar Hillert
+ * @author Artem Bilan
  */
 public class DefaultSplitterTests {
 
@@ -65,7 +76,7 @@ public class DefaultSplitterTests {
 
 	@Test
 	public void splitMessageWithCollectionPayload() throws Exception {
-		List<String> payload = Arrays.asList(new String[] { "x", "y", "z" });
+		List<String> payload = Arrays.asList("x", "y", "z");
 		Message<List<String>> message = MessageBuilder.withPayload(payload).build();
 		QueueChannel replyChannel = new QueueChannel();
 		DefaultMessageSplitter splitter = new DefaultMessageSplitter();
@@ -108,4 +119,112 @@ public class DefaultSplitterTests {
 		Message<?> output = replyChannel.receive(15);
 		assertThat(output, is(nullValue()));
 	}
+
+	@Test
+	public void splitStream() {
+		Message<?> message = new GenericMessage<>(
+				Stream.generate(Math::random)
+						.limit(10));
+		QueueChannel outputChannel = new QueueChannel();
+		DefaultMessageSplitter splitter = new DefaultMessageSplitter();
+		splitter.setOutputChannel(outputChannel);
+		splitter.handleMessage(message);
+		for (int i = 0; i < 10; i++) {
+			Message<?> reply = outputChannel.receive(0);
+			assertEquals(message.getHeaders().getId(), new IntegrationMessageHeaderAccessor(reply).getCorrelationId());
+		}
+		assertNull(outputChannel.receive(10));
+	}
+
+	@Test
+	public void splitFlux() {
+		Message<?> message = new GenericMessage<>(
+				Flux
+						.generate(() -> 0,
+								(state, sink) -> {
+									if (state == 10) {
+										sink.complete();
+									}
+									else {
+										sink.next(state);
+									}
+									return ++state;
+								}));
+		QueueChannel outputChannel = new QueueChannel();
+		DefaultMessageSplitter splitter = new DefaultMessageSplitter();
+		splitter.setOutputChannel(outputChannel);
+		splitter.handleMessage(message);
+		for (int i = 0; i < 10; i++) {
+			Message<?> reply = outputChannel.receive(0);
+			assertEquals(message.getHeaders().getId(), new IntegrationMessageHeaderAccessor(reply).getCorrelationId());
+		}
+		assertNull(outputChannel.receive(10));
+	}
+
+	@Test
+	public void splitArrayPayloadReactive() {
+		Message<?> message = new GenericMessage<>(new String[] { "x", "y", "z" });
+		FluxMessageChannel replyChannel = new FluxMessageChannel();
+		DefaultMessageSplitter splitter = new DefaultMessageSplitter();
+		splitter.setOutputChannel(replyChannel);
+
+		splitter.handleMessage(message);
+
+		Flux<String> testFlux =
+				Flux.from(replyChannel)
+						.map(Message::getPayload)
+						.cast(String.class);
+
+		StepVerifier.create(testFlux)
+				.expectNext("x", "y", "z")
+				.then(() ->
+						((Subscriber<?>) TestUtils.getPropertyValue(replyChannel, "subscribers", List.class).get(0))
+								.onComplete())
+				.verifyComplete();
+	}
+
+	@Test
+	public void splitStreamReactive() {
+		Message<?> message = new GenericMessage<>(Stream.of("x", "y", "z"));
+		FluxMessageChannel replyChannel = new FluxMessageChannel();
+		DefaultMessageSplitter splitter = new DefaultMessageSplitter();
+		splitter.setOutputChannel(replyChannel);
+
+		splitter.handleMessage(message);
+
+		Flux<String> testFlux =
+				Flux.from(replyChannel)
+						.map(Message::getPayload)
+						.cast(String.class);
+
+		StepVerifier.create(testFlux)
+				.expectNext("x", "y", "z")
+				.then(() ->
+						((Subscriber<?>) TestUtils.getPropertyValue(replyChannel, "subscribers", List.class).get(0))
+								.onComplete())
+				.verifyComplete();
+	}
+
+	@Test
+	public void splitFluxReactive() {
+		Message<?> message = new GenericMessage<>(Flux.just("x", "y", "z"));
+		FluxMessageChannel replyChannel = new FluxMessageChannel();
+		DefaultMessageSplitter splitter = new DefaultMessageSplitter();
+		splitter.setOutputChannel(replyChannel);
+
+		splitter.handleMessage(message);
+
+		Flux<String> testFlux =
+				Flux.from(replyChannel)
+						.map(Message::getPayload)
+						.cast(String.class);
+
+		StepVerifier.create(testFlux)
+				.expectNext("x", "y", "z")
+				.then(() ->
+						((Subscriber<?>) TestUtils.getPropertyValue(replyChannel, "subscribers", List.class).get(0))
+								.onComplete())
+				.verifyComplete();
+	}
+
 }