Support empty body without content type in WebFlux

Issue: SPR-15758

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractMessageReaderArgumentResolver.java

@@ -18,8 +18,10 @@ package org.springframework.web.reactive.result.method.annotation;
 
 import java.lang.annotation.Annotation;
 import java.util.Collections;
+import java.util.EnumSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.stream.Collectors;
 
 import reactor.core.publisher.Flux;
@@ -32,6 +34,8 @@ import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.core.codec.DecodingException;
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.http.HttpMethod;
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.HttpMessageReader;
 import org.springframework.http.server.reactive.ServerHttpRequest;
@@ -62,6 +66,10 @@ import org.springframework.web.server.UnsupportedMediaTypeStatusException;
  */
 public abstract class AbstractMessageReaderArgumentResolver extends HandlerMethodArgumentResolverSupport {
 
+	private static final Set<HttpMethod> SUPPORTED_METHODS =
+			EnumSet.of(HttpMethod.POST, HttpMethod.PUT, HttpMethod.PATCH);
+
+
 	private final List<HttpMessageReader<?>> messageReaders;
 
 	private final List<MediaType> supportedMediaTypes;
@@ -111,10 +119,9 @@ public abstract class AbstractMessageReaderArgumentResolver extends HandlerMetho
 
 		ServerHttpRequest request = exchange.getRequest();
 		ServerHttpResponse response = exchange.getResponse();
-		MediaType mediaType = request.getHeaders().getContentType();
-		if (mediaType == null) {
-			mediaType = MediaType.APPLICATION_OCTET_STREAM;
-		}
+
+		MediaType contentType = request.getHeaders().getContentType();
+		MediaType mediaType = (contentType != null ? contentType : MediaType.APPLICATION_OCTET_STREAM);
 
 		for (HttpMessageReader<?> reader : getMessageReaders()) {
 			if (reader.canRead(elementType, mediaType)) {
@@ -133,6 +140,7 @@ public abstract class AbstractMessageReaderArgumentResolver extends HandlerMetho
 					return Mono.just(adapter.fromPublisher(flux));
 				}
 				else {
+					// Single-value (with or without reactive type wrapper)
 					Mono<?> mono = reader.readMono(bodyType, elementType, request, response, readHints);
 					mono = mono.onErrorResume(ex -> Mono.error(handleReadError(bodyParameter, ex)));
 					if (isBodyRequired || (adapter != null && !adapter.supportsEmpty())) {
@@ -153,6 +161,20 @@ public abstract class AbstractMessageReaderArgumentResolver extends HandlerMetho
 			}
 		}
 
+		// No compatible reader but body may be empty..
+
+		HttpMethod method = request.getMethod();
+		if (contentType == null && method != null && SUPPORTED_METHODS.contains(method)) {
+			Flux<DataBuffer> body = request.getBody().doOnNext(o -> {
+				// Body not empty, back to 415..
+				throw new UnsupportedMediaTypeStatusException(mediaType, this.supportedMediaTypes);
+			});
+			if (isBodyRequired || (adapter != null && !adapter.supportsEmpty())) {
+				body = body.switchIfEmpty(Mono.error(handleMissingBody(bodyParameter)));
+			}
+			return (adapter != null ? Mono.just(adapter.fromPublisher(body)) : Mono.from(body));
+		}
+
 		return Mono.error(new UnsupportedMediaTypeStatusException(mediaType, this.supportedMediaTypes));
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/MessageReaderArgumentResolverTests.java

@@ -83,14 +83,16 @@ public class MessageReaderArgumentResolverTests {
 	}
 
 
+	@SuppressWarnings("unchecked")
 	@Test
 	public void missingContentType() throws Exception {
 		ServerWebExchange exchange = post("/path").body("{\"bar\":\"BARBAR\",\"foo\":\"FOOFOO\"}").toExchange();
 		ResolvableType type = forClassWithGenerics(Mono.class, TestBean.class);
 		MethodParameter param = this.testMethod.arg(type);
 		Mono<Object> result = this.resolver.readBody(param, true, this.bindingContext, exchange);
+		Mono<TestBean> value = (Mono<TestBean>) result.block(Duration.ofSeconds(1));
 
-		StepVerifier.create(result).expectError(UnsupportedMediaTypeStatusException.class).verify();
+		StepVerifier.create(value).expectError(UnsupportedMediaTypeStatusException.class).verify();
 	}
 
 	// More extensive "empty body" tests in RequestBody- and HttpEntityArgumentResolverTests

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestBodyArgumentResolverTests.java

@@ -19,6 +19,7 @@ package org.springframework.web.reactive.result.method.annotation;
 import java.time.Duration;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.CompletableFuture;
 
 import io.reactivex.Maybe;
@@ -106,6 +107,14 @@ public class RequestBodyArgumentResolverTests {
 		assertNull(body);
 	}
 
+	@Test // SPR-15758
+	public void emptyBodyWithoutContentType() throws Exception {
+		MethodParameter param = this.testMethod.annot(requestBody().notRequired()).arg(Map.class);
+		String body = resolveValueWithEmptyBody(param);
+
+		assertNull(body);
+	}
+
 	@Test
 	@SuppressWarnings("unchecked")
 	public void emptyBodyWithMono() throws Exception {
@@ -262,6 +271,7 @@ public class RequestBodyArgumentResolverTests {
 			@RequestBody(required = false) Observable<String> obsNotRequired,
 			@RequestBody(required = false) io.reactivex.Observable<String> rxjava2ObsNotRequired,
 			@RequestBody(required = false) CompletableFuture<String> futureNotRequired,
+			@RequestBody(required = false) Map<?, ?> mapNotRequired,
 			String notAnnotated) {}
 
 }