Improve Object return type support in WebFlux

Issue: SPR-15731

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

@@ -96,7 +96,8 @@ public abstract class AbstractMessageWriterResultHandler extends HandlerResultHa
 		ResolvableType elementType;
 		if (adapter != null) {
 			publisher = adapter.toPublisher(body);
-			elementType = adapter.isNoValue() ? ResolvableType.forClass(Void.class) : bodyType.getGeneric(0);
+			ResolvableType genericType = bodyType.getGeneric(0);
+			elementType = getElementType(adapter, genericType);
 		}
 		else {
 			publisher = Mono.justOrEmpty(body);
@@ -127,6 +128,18 @@ public abstract class AbstractMessageWriterResultHandler extends HandlerResultHa
 		return Mono.error(new NotAcceptableStatusException(getProducibleMediaTypes(elementType)));
 	}
 
+	private ResolvableType getElementType(ReactiveAdapter adapter, ResolvableType genericType) {
+		if (adapter.isNoValue()) {
+			return ResolvableType.forClass(Void.class);
+		}
+		else if (genericType != ResolvableType.NONE) {
+			return genericType;
+		}
+		else {
+			return ResolvableType.forClass(Object.class);
+		}
+	}
+
 	private List<MediaType> getProducibleMediaTypes(ResolvableType elementType) {
 		return getMessageWriters().stream()
 				.filter(converter -> converter.canWrite(elementType, null))

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

@@ -137,6 +137,12 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 		assertEquals(expected, performGet("/person-response/mono", JSON, Person.class).getBody());
 	}
 
+	@Test
+	public void personResponseBodyWithMonoDeclaredAsObject() throws Exception {
+		Person expected = new Person("Robert");
+		assertEquals(expected, performGet("/person-response/mono-declared-as-object", JSON, Person.class).getBody());
+	}
+
 	@Test
 	public void personResponseBodyWithSingle() throws Exception {
 		Person expected = new Person("Robert");
@@ -442,6 +448,11 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 			return Mono.just(new Person("Robert"));
 		}
 
+		@GetMapping("/mono-declared-as-object")
+		public Object getMonoDeclaredAsObject() {
+			return Mono.just(new Person("Robert"));
+		}
+
 		@GetMapping("/single")
 		public Single<Person> getSingle() {
 			return Single.just(new Person("Robert"));