Support Void response body type in WebClient

This commit adds support for Void response body types in the WebClient,
both when using `exchange` with a response.bodyToMono(Void.class), as
well as using `retrieve` with `toEntity(Void.class)`.

Issue: SPR-15679

spring-webflux/src/main/java/org/springframework/web/reactive/function/BodyExtractors.java

@@ -19,6 +19,7 @@ package org.springframework.web.reactive.function;
 import java.util.List;
 import java.util.Optional;
 import java.util.function.Function;
+import java.util.function.Supplier;
 import java.util.stream.Collectors;
 
 import org.reactivestreams.Publisher;
@@ -101,7 +102,8 @@ public abstract class BodyExtractors {
 						return reader.readMono(elementType, inputMessage, context.hints());
 					}
 				},
-				Mono::error);
+				Mono::error,
+				Mono::empty);
 	}
 
 	/**
@@ -149,7 +151,8 @@ public abstract class BodyExtractors {
 						return reader.read(elementType, inputMessage, context.hints());
 					}
 				},
-				Flux::error);
+				Flux::error,
+				Flux::empty);
 	}
 
 	/**
@@ -221,8 +224,12 @@ public abstract class BodyExtractors {
 
 	private static <T, S extends Publisher<T>> S readWithMessageReaders(
 			ReactiveHttpInputMessage inputMessage, BodyExtractor.Context context, ResolvableType elementType,
-			Function<HttpMessageReader<T>, S> readerFunction, Function<Throwable, S> unsupportedError) {
+			Function<HttpMessageReader<T>, S> readerFunction, Function<Throwable, S> unsupportedError,
+			Supplier<S> empty) {
 
+		if (elementType.equals(ResolvableType.forClass(Void.class))) {
+			return empty.get();
+		}
 		MediaType contentType = contentType(inputMessage);
 		List<HttpMessageReader<?>> messageReaders = context.messageReaders();
 		return messageReaders.stream()

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -387,11 +387,14 @@ class DefaultWebClient implements WebClient {
 
 		@Override
 		public <T> Mono<ResponseEntity<T>> toEntity(Class<T> bodyType) {
-			return this.responseMono.flatMap(response ->
+			return this.responseMono.flatMap(response -> {
-					response.bodyToMono(bodyType).map(body -> {
 						HttpHeaders headers = response.headers().asHttpHeaders();
-						return new ResponseEntity<>(body, headers, response.statusCode());
+						HttpStatus statusCode = response.statusCode();
-					})
+						return response.bodyToMono(bodyType)
+								.map(body -> new ResponseEntity<>(body, headers, statusCode))
+								.switchIfEmpty(Mono.defer(
+										() -> Mono.just(new ResponseEntity<>(headers, statusCode))));
+					}
 			);
 		}
 

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -52,12 +52,10 @@ public class WebClientIntegrationTests {
 
 	private WebClient webClient;
 
-	private String baseUrl;
-
 	@Before
 	public void setup() {
 		this.server = new MockWebServer();
-		baseUrl = this.server.url("/").toString();
+		String baseUrl = this.server.url("/").toString();
 		this.webClient = WebClient.create(baseUrl);
 	}
 
@@ -468,6 +466,35 @@ public class WebClientIntegrationTests {
 		Assert.assertEquals(2, server.getRequestCount());
 	}
 
+	@Test
+	public void exchangeNoContent() throws Exception  {
+		this.server.enqueue(new MockResponse().setHeader("Content-Length", "0"));
+
+		Mono<ClientResponse> result = this.webClient.get()
+				.uri("/noContent")
+				.exchange();
+
+		StepVerifier.create(result).assertNext(r -> {
+			assertTrue(r.statusCode().is2xxSuccessful());
+			StepVerifier.create(r.bodyToMono(Void.class)).verifyComplete();
+		}).verifyComplete();
+	}
+
+	@Test
+	public void retrieveNoContent() throws Exception  {
+		this.server.enqueue(new MockResponse().setHeader("Content-Length", "0"));
+
+		Mono<ResponseEntity<Void>> result = this.webClient.get()
+				.uri("/noContent")
+				.retrieve()
+				.toEntity(Void.class);
+
+		StepVerifier.create(result).assertNext(r -> {
+			assertFalse(r.hasBody());
+			assertTrue(r.getStatusCode().is2xxSuccessful());
+		}).verifyComplete();
+	}
+
 
 	@SuppressWarnings("serial")
 	private static class MyException extends RuntimeException {