INT-4462: WebFluxInbound: cope with empty body

JIRA: https://jira.spring.io/browse/INT-4462

When the HTTP request body is empty, the `HttpMessageReader` ends up
with the empty `Mono` which can't be evaluated to any reasonable value.

* Add fallback to `requestParams` when `Mono` for body is empty and
also when `payloadExpression` returns null

**Cherry-pick to 5.0.x**

spring-integration-webflux/src/main/java/org/springframework/integration/webflux/inbound/WebFluxInboundEndpoint.java

@@ -150,6 +150,7 @@ public class WebFluxInboundEndpoint extends BaseHttpInboundEndpoint implements W
 	private Mono<Void> doHandle(ServerWebExchange exchange) {
 		return extractRequestBody(exchange)
 				.doOnSubscribe(s -> this.activeCount.incrementAndGet())
+				.switchIfEmpty(Mono.just(exchange.getRequest().getQueryParams()))
 				.map(body -> new HttpEntity<>(body, exchange.getRequest().getHeaders()))
 				.map(entity -> buildMessage(entity, exchange))
 				.flatMap(requestMessage -> {
@@ -267,10 +268,6 @@ public class WebFluxInboundEndpoint extends BaseHttpInboundEndpoint implements W
 		Object payload;
 		if (getPayloadExpression() != null) {
 			payload = getPayloadExpression().getValue(evaluationContext);
-			if (payload == null) {
-				throw new IllegalStateException("The payload expression '" + getPayloadExpression().getExpressionString()
-						+ "' returned null.");
-			}
 		}
 		else {
 			payload = httpEntity.getBody();
@@ -288,6 +285,10 @@ public class WebFluxInboundEndpoint extends BaseHttpInboundEndpoint implements W
 			}
 		}
 
+		if (payload == null) {
+			payload = requestParams;
+		}
+
 		AbstractIntegrationMessageBuilder<?> messageBuilder;
 
 		if (payload instanceof Message<?>) {

spring-integration-webflux/src/test/java/org/springframework/integration/webflux/inbound/WebFluxInboundEndpointTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 the original author or authors.
+ * Copyright 2017-2018 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.
@@ -25,6 +25,7 @@ import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
@@ -95,6 +96,16 @@ public class WebFluxInboundEndpointTests {
 				.is5xxServerError();
 	}
 
+	@Test
+	public void testPostWithEmptyBody() {
+		this.webTestClient
+				.post()
+				.uri("/post?foo=foo")
+				.exchange()
+				.expectStatus().isOk()
+				.expectBody(String.class).isEqualTo("{foo=[foo]}");
+	}
+
 	@Configuration
 	@EnableWebFlux
 	@EnableIntegration
@@ -156,6 +167,22 @@ public class WebFluxInboundEndpointTests {
 			return new ResponseEntity<>("<500 Internal Server Error,{}>", HttpStatus.INTERNAL_SERVER_ERROR);
 		}
 
+		@Bean
+		public WebFluxInboundEndpoint postInboundEndpoint() {
+			WebFluxInboundEndpoint endpoint = new WebFluxInboundEndpoint();
+			RequestMapping requestMapping = new RequestMapping();
+			requestMapping.setPathPatterns("/post");
+			requestMapping.setMethods(HttpMethod.POST);
+			endpoint.setRequestMapping(requestMapping);
+			endpoint.setRequestChannelName("postServiceChannel");
+			return endpoint;
+		}
+
+		@ServiceActivator(inputChannel = "postServiceChannel")
+		String service(Object payload) {
+			return payload.toString();
+		}
+
 	}
 
 

src/reference/asciidoc/webflux.adoc

@@ -6,7 +6,7 @@
 
 The WebFlux Spring Integration module (`spring-integration-webflux`) allows for the execution of HTTP requests and the processing of inbound HTTP requests in Reactive manner.
 The WebFlux support consists of the following gateway implementations: `WebFluxInboundEndpoint`, `WebFluxRequestExecutingMessageHandler`.
-The implementation is fully based on the Spring http://docs.spring.io/spring/docs/5.0.0.RC3/spring-framework-reference/web.html#web-reactive[WebFlux] and https://projectreactor.io/[Project Reactor] foundations.
+The implementation is fully based on the Spring https://docs.spring.io/spring/docs/current/spring-framework-reference/web-reactive.html#spring-webflux[WebFlux] and https://projectreactor.io/[Project Reactor] foundations.
 Also see <<http>> for more information since many options are shared between reactive and regular HTTP components.
 
 [[webflux-inbound]]
@@ -65,6 +65,8 @@ public IntegrationFlow sseFlow() {
 
 Also see <<http-request-mapping>> and <<http-cors>> for more possible configuration options.
 
+When the request body is empty, or `payloadExpression` returns `null`, the request params `MultiValueMap<String, String>` is used for a `payload` of the target message to process.
+
 [[webflux-outbound]]
 === WebFlux Outbound Components