GH-2340: Fix WebFluxMessageHandlerSpec

Fixes https://github.com/spring-projects/spring-integration/issues/2340

* Make `WebFluxMessageHandlerSpec` to support method chain
* Add `WebFluxDslTests.testWebFluxFlowWithReplyPayloadToFlux()`

spring-integration-webflux/src/main/java/org/springframework/integration/webflux/dsl/WebFluxMessageHandlerSpec.java

@@ -35,6 +35,7 @@ import reactor.core.publisher.Mono;
  *
  * @author Shiliang Li
  * @author Artem Bilan
+ * @author Abhijit Sarkar
  *
  * @since 5.0
  *
@@ -64,22 +65,26 @@ public class WebFluxMessageHandlerSpec
 	 * Defaults to {@code false} - simple value is pushed downstream.
 	 * Makes sense when {@code expectedResponseType} is configured.
 	 * @param replyPayloadToFlux represent reply payload as a {@link Flux} or as a value from the {@link Mono}.
+	 * @return the spec
 	 * @since 5.0.1
 	 * @see WebFluxRequestExecutingMessageHandler#setReplyPayloadToFlux(boolean)
 	 */
-	public void replyPayloadToFlux(boolean replyPayloadToFlux) {
+	public WebFluxMessageHandlerSpec replyPayloadToFlux(boolean replyPayloadToFlux) {
 		this.target.setReplyPayloadToFlux(replyPayloadToFlux);
+		return this;
 	}
 
 	/**
 	 * Specify a {@link BodyExtractor} as an alternative to the {@code expectedResponseType}
 	 * to allow to get low-level access to the received {@link ClientHttpResponse}.
 	 * @param bodyExtractor the {@link BodyExtractor} to use.
+	 * @return the spec
 	 * @since 5.0.1
 	 * @see WebFluxRequestExecutingMessageHandler#setBodyExtractor(BodyExtractor)
 	 */
-	public void bodyExtractor(BodyExtractor<?, ClientHttpResponse> bodyExtractor) {
+	public WebFluxMessageHandlerSpec bodyExtractor(BodyExtractor<?, ClientHttpResponse> bodyExtractor) {
 		this.target.setBodyExtractor(bodyExtractor);
+		return this;
 	}
 
 	@Override

spring-integration-webflux/src/test/java/org/springframework/integration/webflux/dsl/WebFluxDslTests.java

@@ -26,6 +26,8 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
 
 import java.util.Collections;
 
+import javax.annotation.Resource;
+
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -33,19 +35,24 @@ import org.reactivestreams.Publisher;
 
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.ResolvableType;
+import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.client.reactive.ClientHttpConnector;
+import org.springframework.integration.channel.QueueChannel;
 import org.springframework.integration.config.EnableIntegration;
 import org.springframework.integration.dsl.IntegrationFlow;
 import org.springframework.integration.dsl.IntegrationFlows;
 import org.springframework.integration.http.dsl.Http;
+import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.webflux.outbound.WebFluxRequestExecutingMessageHandler;
 import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.PollableChannel;
 import org.springframework.security.access.AccessDecisionManager;
 import org.springframework.security.access.vote.AffirmativeBased;
@@ -76,6 +83,7 @@ import reactor.test.StepVerifier;
 /**
  * @author Artem Bilan
  * @author Shiliang Li
+ * @author Abhijit Sarkar
  *
  * @since 5.0
  */
@@ -88,7 +96,15 @@ public class WebFluxDslTests {
 	private WebApplicationContext wac;
 
 	@Autowired
-	private WebFluxRequestExecutingMessageHandler serviceInternalReactiveGatewayHandler;
+	@Qualifier("webFluxWithReplyPayloadToFlux.handler")
+	private WebFluxRequestExecutingMessageHandler webFluxWithReplyPayloadToFlux;
+
+	@Resource(name = "org.springframework.integration.webflux.outbound.WebFluxRequestExecutingMessageHandler#1")
+	private WebFluxRequestExecutingMessageHandler httpReactiveProxyFlow;
+
+	@Autowired
+	@Qualifier("webFluxFlowWithReplyPayloadToFlux.input")
+	private MessageChannel webFluxFlowWithReplyPayloadToFluxInput;
 
 	private MockMvc mockMvc;
 
@@ -106,6 +122,48 @@ public class WebFluxDslTests {
 						.build();
 	}
 
+	@Test
+	public void testWebFluxFlowWithReplyPayloadToFlux() {
+		ClientHttpConnector httpConnector = new HttpHandlerConnector((request, response) -> {
+			response.setStatusCode(HttpStatus.OK);
+			response.getHeaders().setContentType(MediaType.TEXT_PLAIN);
+
+			DataBufferFactory bufferFactory = response.bufferFactory();
+			return response.writeWith(
+					Flux.just(bufferFactory.wrap("FOO".getBytes()),
+							bufferFactory.wrap("BAR".getBytes())))
+					.then(Mono.defer(response::setComplete));
+		});
+
+		WebClient webClient = WebClient.builder()
+				.clientConnector(httpConnector)
+				.build();
+
+		new DirectFieldAccessor(this.webFluxWithReplyPayloadToFlux)
+				.setPropertyValue("webClient", webClient);
+
+		QueueChannel replyChannel = new QueueChannel();
+
+		Message<String> testMessage =
+				MessageBuilder.withPayload("test")
+						.setReplyChannel(replyChannel)
+						.build();
+
+		this.webFluxFlowWithReplyPayloadToFluxInput.send(testMessage);
+
+		Message<?> receive = replyChannel.receive(10_000);
+
+		assertNotNull(receive);
+		assertThat(receive.getPayload(), instanceOf(Flux.class));
+
+		@SuppressWarnings("unchecked")
+		Flux<String> response = (Flux<String>) receive.getPayload();
+
+		StepVerifier.create(response)
+				.expectNext("FOO", "BAR")
+				.verifyComplete();
+	}
+
 	@Test
 	public void testHttpReactiveProxyFlow() throws Exception {
 		ClientHttpConnector httpConnector = new HttpHandlerConnector((request, response) -> {
@@ -120,7 +178,7 @@ public class WebFluxDslTests {
 				.clientConnector(httpConnector)
 				.build();
 
-		new DirectFieldAccessor(this.serviceInternalReactiveGatewayHandler)
+		new DirectFieldAccessor(this.httpReactiveProxyFlow)
 				.setPropertyValue("webClient", webClient);
 
 		this.mockMvc.perform(
@@ -200,6 +258,16 @@ public class WebFluxDslTests {
 					.anonymous().disable();
 		}
 
+		@Bean
+		public IntegrationFlow webFluxFlowWithReplyPayloadToFlux() {
+			return f -> f
+					.handle(WebFlux.outboundGateway("http://www.springsource.org/spring-integration")
+									.httpMethod(HttpMethod.GET)
+									.replyPayloadToFlux(true)
+									.expectedResponseType(String.class),
+							e -> e.id("webFluxWithReplyPayloadToFlux"));
+		}
+
 		@Bean
 		public IntegrationFlow httpReactiveProxyFlow() {
 			return IntegrationFlows