Add ReactiveMessageSourceProducer (#3254)

* Add `ReactiveMessageSourceProducer`

The `ReactiveMessageSourceProducer` wraps a provided `MessageSource`
into a `Flux` for subscription in the `subscribeToPublisher(Publisher<? extends Message<?>>)`
to make a source polling feature fully based on a reactive, on demand solution

* Introduce a `IntegrationReactiveUtils` replacing existing `MessageChannelReactiveUtils`
with more functionality
* Replace a deprecated `MessageChannelReactiveUtils` with a new `IntegrationReactiveUtils`
* Test and document the feature

* * Fix Docs typos

* * Remove unused imports from `MessageChannelReactiveUtils`

* * Fix JavaDoc copy/paste artifact

spring-integration-core/src/test/java/org/springframework/integration/channel/MessageChannelReactiveUtilsTests.java

@@ -23,6 +23,7 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.integration.util.IntegrationReactiveUtils;
 import org.springframework.messaging.support.GenericMessage;
 
 import reactor.core.Disposable;
@@ -45,7 +46,7 @@ class MessageChannelReactiveUtilsTests {
 		try {
 			DirectChannel channel = new DirectChannel();
 			int initialRequest = 10;
-			StepVerifier.create(MessageChannelReactiveUtils.toPublisher(channel), initialRequest)
+			StepVerifier.create(IntegrationReactiveUtils.messageChannelToFlux(channel), initialRequest)
 					.expectSubscription()
 					.then(() -> {
 						compositeDisposable.add(
@@ -77,7 +78,7 @@ class MessageChannelReactiveUtilsTests {
 		AtomicInteger sendCount = new AtomicInteger();
 		try {
 			int initialRequest = 10;
-			StepVerifier.create(MessageChannelReactiveUtils.toPublisher(channel), initialRequest)
+			StepVerifier.create(IntegrationReactiveUtils.messageChannelToFlux(channel), initialRequest)
 					.expectSubscription()
 					.then(() ->
 							compositeDisposable.add(

spring-integration-core/src/test/java/org/springframework/integration/channel/reactive/FluxMessageChannelTests.java

@@ -33,13 +33,13 @@ import org.springframework.context.annotation.Configuration;
 import org.springframework.integration.annotation.BridgeFrom;
 import org.springframework.integration.annotation.ServiceActivator;
 import org.springframework.integration.channel.FluxMessageChannel;
-import org.springframework.integration.channel.MessageChannelReactiveUtils;
 import org.springframework.integration.channel.QueueChannel;
 import org.springframework.integration.config.EnableIntegration;
 import org.springframework.integration.dsl.IntegrationFlow;
 import org.springframework.integration.dsl.MessageChannels;
 import org.springframework.integration.dsl.context.IntegrationFlowContext;
 import org.springframework.integration.test.util.TestUtils;
+import org.springframework.integration.util.IntegrationReactiveUtils;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessagingException;
@@ -104,7 +104,7 @@ public class FluxMessageChannelTests {
 		List<String> results = new ArrayList<>();
 
 		Disposable disposable =
-				Flux.from(MessageChannelReactiveUtils.<String>toPublisher(this.queueChannel))
+				IntegrationReactiveUtils.<String>messageChannelToFlux(this.queueChannel)
 						.map(Message::getPayload)
 						.map(String::toUpperCase)
 						.doOnNext(results::add)

spring-integration-core/src/test/java/org/springframework/integration/endpoint/ReactiveMessageSourceProducerTests.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright 2020 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.endpoint;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+import java.time.Duration;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.BeanFactory;
+import org.springframework.integration.IntegrationMessageHeaderAccessor;
+import org.springframework.integration.acks.AcknowledgmentCallback;
+import org.springframework.integration.channel.FluxMessageChannel;
+import org.springframework.integration.core.MessageSource;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.support.MessageBuilder;
+
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+/**
+ * @author Artem Bilan
+ *
+ * @since 5.3
+ */
+public class ReactiveMessageSourceProducerTests {
+
+	@Test
+	void testReactiveMessageSourceProducing() {
+		LinkedBlockingQueue<Integer> queue =
+				IntStream.range(0, 10)
+						.boxed()
+						.collect(Collectors.toCollection(LinkedBlockingQueue::new));
+
+		AtomicBoolean ackState = new AtomicBoolean();
+
+		MessageSource<Integer> messageSource =
+				() -> {
+					Integer integer = queue.poll();
+					if (integer == null) {
+						try {
+							Thread.sleep(200);
+						}
+						catch (InterruptedException e) {
+							Thread.currentThread().interrupt();
+							throw new IllegalStateException(e);
+						}
+						integer = 100;
+					}
+					return MessageBuilder.withPayload(integer)
+							.setHeader(IntegrationMessageHeaderAccessor.ACKNOWLEDGMENT_CALLBACK,
+									(AcknowledgmentCallback) status -> ackState.set(true))
+							.build();
+				};
+
+		FluxMessageChannel outputChannel = new FluxMessageChannel();
+
+		ReactiveMessageSourceProducer reactiveMessageSourceProducer = new ReactiveMessageSourceProducer(messageSource);
+		reactiveMessageSourceProducer.setDelayWhenEmpty(Duration.ofMillis(10));
+		reactiveMessageSourceProducer.setOutputChannel(outputChannel);
+		reactiveMessageSourceProducer.setBeanFactory(mock(BeanFactory.class));
+		reactiveMessageSourceProducer.afterPropertiesSet();
+
+		StepVerifier stepVerifier =
+				StepVerifier.create(
+						Flux.from(outputChannel)
+								.map(Message::getPayload)
+								.cast(Integer.class))
+						.expectNextSequence(
+								IntStream.range(0, 10)
+										.boxed()
+										.collect(Collectors.toList()))
+						.expectNoEvent(Duration.ofMillis(100))
+						.expectNext(100)
+						.thenCancel()
+						.verifyLater();
+
+		reactiveMessageSourceProducer.start();
+
+		stepVerifier.verify();
+
+		reactiveMessageSourceProducer.stop();
+
+		assertThat(ackState.get()).isTrue();
+	}
+
+}