Improve support for Mono<ResponseEntity<?>>

If the body class is not resolvable from the return type and there is
a body instance we now fall back on the class of the body instance.

Issue: SPR-14877

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/AbstractHandlerResultHandler.java

@@ -22,6 +22,7 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Optional;
 import java.util.Set;
+import java.util.function.Supplier;
 
 import org.springframework.core.MethodParameter;
 import org.springframework.core.Ordered;
@@ -101,13 +102,14 @@ public abstract class AbstractHandlerResultHandler implements Ordered {
 	 * Select the best media type for the current request through a content
 	 * negotiation algorithm.
 	 * @param exchange the current request
-	 * @param producibleTypes the media types that can be produced for the current request
+	 * @param producibleTypesSupplier the media types that can be produced for the current request
 	 * @return the selected media type or {@code null}
 	 */
-	protected MediaType selectMediaType(ServerWebExchange exchange, List<MediaType> producibleTypes) {
+	protected MediaType selectMediaType(ServerWebExchange exchange,
+			Supplier<List<MediaType>> producibleTypesSupplier) {
 
 		List<MediaType> acceptableTypes = getAcceptableTypes(exchange);
-		producibleTypes = getProducibleTypes(exchange, producibleTypes);
+		List<MediaType> producibleTypes = getProducibleTypes(exchange, producibleTypesSupplier);
 
 		Set<MediaType> compatibleMediaTypes = new LinkedHashSet<>();
 		for (MediaType acceptable : acceptableTypes) {
@@ -139,13 +141,15 @@ public abstract class AbstractHandlerResultHandler implements Ordered {
 	}
 
 	@SuppressWarnings("unchecked")
-	private List<MediaType> getProducibleTypes(ServerWebExchange exchange, List<MediaType> mediaTypes) {
+	private List<MediaType> getProducibleTypes(ServerWebExchange exchange,
+			Supplier<List<MediaType>> producibleTypesSupplier) {
+
 		Optional<Object> optional = exchange.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
 		if (optional.isPresent()) {
 			Set<MediaType> set = (Set<MediaType>) optional.get();
 			return new ArrayList<>(set);
 		}
-		return mediaTypes;
+		return producibleTypesSupplier.get();
 	}
 
 	private MediaType selectMoreSpecificMediaType(MediaType acceptable, MediaType producible) {

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

@@ -93,8 +93,9 @@ public abstract class AbstractMessageWriterResultHandler extends AbstractHandler
 	@SuppressWarnings("unchecked")
 	protected Mono<Void> writeBody(Object body, MethodParameter bodyParameter, ServerWebExchange exchange) {
 
-		ResolvableType bodyType = ResolvableType.forMethodParameter(bodyParameter);
-		ReactiveAdapter adapter = getAdapterRegistry().getAdapterFrom(bodyType.resolve(), body);
+		ResolvableType valueType = ResolvableType.forMethodParameter(bodyParameter);
+		Class<?> valueClass = valueType.resolve();
+		ReactiveAdapter adapter = getAdapterRegistry().getAdapterFrom(valueClass, body);
 
 		Publisher<?> publisher;
 		ResolvableType elementType;
@@ -102,11 +103,11 @@ public abstract class AbstractMessageWriterResultHandler extends AbstractHandler
 			publisher = adapter.toPublisher(body);
 			elementType = adapter.getDescriptor().isNoValue() ?
 					ResolvableType.forClass(Void.class) :
-					bodyType.getGeneric(0);
+					valueType.getGeneric(0);
 		}
 		else {
 			publisher = Mono.justOrEmpty(body);
-			elementType = bodyType;
+			elementType = (valueClass == null && body != null ? ResolvableType.forInstance(body) : valueType);
 		}
 
 		if (void.class == elementType.getRawClass() || Void.class == elementType.getRawClass()) {
@@ -114,29 +115,29 @@ public abstract class AbstractMessageWriterResultHandler extends AbstractHandler
 					.doOnSubscribe(sub -> updateResponseStatus(bodyParameter, exchange));
 		}
 
-		List<MediaType> producibleTypes = getProducibleMediaTypes(elementType);
-		if (producibleTypes.isEmpty()) {
-			return Mono.error(new IllegalStateException(
-					"No converter for return value type: " + elementType));
-		}
-
 		ServerHttpRequest request = exchange.getRequest();
 		ServerHttpResponse response = exchange.getResponse();
-		MediaType bestMediaType = selectMediaType(exchange, producibleTypes);
+		MediaType bestMediaType = selectMediaType(exchange, () -> getProducibleMediaTypes(elementType));
 		if (bestMediaType != null) {
 			for (HttpMessageWriter<?> messageWriter : getMessageWriters()) {
 				if (messageWriter.canWrite(elementType, bestMediaType)) {
 					Mono<Void> bodyWriter = (messageWriter instanceof ServerHttpMessageWriter ?
 							((ServerHttpMessageWriter<?>) messageWriter).write((Publisher) publisher,
-									bodyType, elementType, bestMediaType, request, response, Collections.emptyMap()) :
+									valueType, elementType, bestMediaType, request, response, Collections.emptyMap()) :
 							messageWriter.write((Publisher) publisher, elementType,
 									bestMediaType, response, Collections.emptyMap()));
 					return bodyWriter.doOnSubscribe(sub -> updateResponseStatus(bodyParameter, exchange));
 				}
 			}
 		}
+		else {
+			if (getProducibleMediaTypes(elementType).isEmpty()) {
+				return Mono.error(new IllegalStateException(
+						"No converter for return value type: " + elementType));
+			}
+		}
 
-		return Mono.error(new NotAcceptableStatusException(producibleTypes));
+		return Mono.error(new NotAcceptableStatusException(getProducibleMediaTypes(elementType)));
 	}
 
 	private List<MediaType> getProducibleMediaTypes(ResolvableType elementType) {

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/view/ViewResolutionResultHandler.java

@@ -319,7 +319,7 @@ public class ViewResolutionResultHandler extends AbstractHandlerResultHandler
 					views.addAll(getDefaultViews());
 
 					List<MediaType> producibleTypes = getProducibleMediaTypes(views);
-					MediaType bestMediaType = selectMediaType(exchange, producibleTypes);
+					MediaType bestMediaType = selectMediaType(exchange, () -> producibleTypes);
 
 					if (bestMediaType != null) {
 						for (View view : views) {

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/HandlerResultHandlerTests.java

@@ -71,7 +71,7 @@ public class HandlerResultHandlerTests {
 	public void usesContentTypeResolver() throws Exception {
 		TestResultHandler resultHandler = new TestResultHandler(new FixedContentTypeResolver(IMAGE_GIF));
 		List<MediaType> mediaTypes = Arrays.asList(IMAGE_JPEG, IMAGE_GIF, IMAGE_PNG);
-		MediaType actual = resultHandler.selectMediaType(this.exchange, mediaTypes);
+		MediaType actual = resultHandler.selectMediaType(this.exchange, () -> mediaTypes);
 
 		assertEquals(IMAGE_GIF, actual);
 	}
@@ -82,7 +82,7 @@ public class HandlerResultHandlerTests {
 		this.exchange.getAttributes().put(PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE, producible);
 
 		List<MediaType> mediaTypes = Arrays.asList(IMAGE_JPEG, IMAGE_GIF, IMAGE_PNG);
-		MediaType actual = resultHandler.selectMediaType(this.exchange, mediaTypes);
+		MediaType actual = resultHandler.selectMediaType(this.exchange, () -> mediaTypes);
 
 		assertEquals(IMAGE_GIF, actual);
 	}
@@ -92,7 +92,7 @@ public class HandlerResultHandlerTests {
 		this.request.setHeader("Accept", "text/plain; q=0.5, application/json");
 
 		List<MediaType> mediaTypes = Arrays.asList(TEXT_PLAIN, APPLICATION_JSON_UTF8);
-		MediaType actual = this.resultHandler.selectMediaType(this.exchange, mediaTypes);
+		MediaType actual = this.resultHandler.selectMediaType(this.exchange, () -> mediaTypes);
 
 		assertEquals(APPLICATION_JSON_UTF8, actual);
 	}
@@ -102,7 +102,8 @@ public class HandlerResultHandlerTests {
 		MediaType text8859 = MediaType.parseMediaType("text/plain;charset=ISO-8859-1");
 		MediaType textUtf8 = MediaType.parseMediaType("text/plain;charset=UTF-8");
 		this.request.getHeaders().setAccept(Collections.singletonList(text8859));
-		MediaType actual = this.resultHandler.selectMediaType(this.exchange, Collections.singletonList(textUtf8));
+		MediaType actual = this.resultHandler.selectMediaType(this.exchange,
+				() -> Collections.singletonList(textUtf8));
 
 		assertEquals(text8859, actual);
 	}
@@ -110,7 +111,7 @@ public class HandlerResultHandlerTests {
 	@Test // SPR-12894
 	public void noConcreteMediaType() throws Exception {
 		List<MediaType> producible = Collections.singletonList(ALL);
-		MediaType actual = this.resultHandler.selectMediaType(this.exchange, producible);
+		MediaType actual = this.resultHandler.selectMediaType(this.exchange, () -> producible);
 
 		assertEquals(APPLICATION_OCTET_STREAM, actual);
 	}

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

@@ -43,7 +43,6 @@ import org.springframework.core.codec.ByteBufferEncoder;
 import org.springframework.core.codec.CharSequenceEncoder;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
-import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.codec.EncoderHttpMessageWriter;
@@ -51,9 +50,9 @@ import org.springframework.http.codec.HttpMessageWriter;
 import org.springframework.http.codec.ResourceHttpMessageWriter;
 import org.springframework.http.codec.json.Jackson2JsonEncoder;
 import org.springframework.http.codec.xml.Jaxb2XmlEncoder;
+import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpRequest;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse;
-import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.util.ObjectUtils;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolverBuilder;
@@ -211,7 +210,7 @@ public class MessageWriterResultHandlerTests {
 		public ParentClass() {
 		}
 
-		public ParentClass(String parentProperty) {
+		ParentClass(String parentProperty) {
 			this.parentProperty = parentProperty;
 		}
 
@@ -235,7 +234,7 @@ public class MessageWriterResultHandlerTests {
 	@JsonTypeName("bar")
 	private static class Bar extends ParentClass {
 
-		public Bar(String parentProperty) {
+		Bar(String parentProperty) {
 			super(parentProperty);
 		}
 	}
@@ -253,7 +252,7 @@ public class MessageWriterResultHandlerTests {
 
 		private String name;
 
-		public SimpleBean(Long id, String name) {
+		SimpleBean(Long id, String name) {
 			this.id = id;
 			this.name = name;
 		}

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

@@ -23,6 +23,7 @@ import java.time.Instant;
 import java.time.temporal.ChronoUnit;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
 
@@ -37,11 +38,11 @@ import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.ByteBufferEncoder;
 import org.springframework.core.codec.CharSequenceEncoder;
-import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.codec.EncoderHttpMessageWriter;
 import org.springframework.http.codec.HttpMessageWriter;
@@ -51,6 +52,7 @@ import org.springframework.http.codec.xml.Jaxb2XmlEncoder;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpRequest;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse;
 import org.springframework.util.ObjectUtils;
+import org.springframework.web.reactive.HandlerMapping;
 import org.springframework.web.reactive.HandlerResult;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolverBuilder;
@@ -270,6 +272,23 @@ public class ResponseEntityResultHandlerTests {
 		assertConditionalResponse(HttpStatus.OK, "body", newEtag, oneMinAgo);
 	}
 
+	@Test // SPR-14877
+	public void handleMonoWithWildcardBodyType() throws Exception {
+
+		this.exchange.getAttributes().put(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE,
+				Collections.singleton(MediaType.APPLICATION_JSON));
+
+		HandlerResult result = new HandlerResult(new TestController(), Mono.just(ok().body("body")),
+				ResolvableMethod.onClass(TestController.class)
+						.name("monoResponseEntityWildcard")
+						.resolveReturnType());
+
+		this.resultHandler.handleResult(this.exchange, result).block(Duration.ofSeconds(5));
+
+		assertEquals(HttpStatus.OK, this.response.getStatusCode());
+		assertResponseBody("\"body\"");
+	}
+
 
 	private void testHandle(Object returnValue, ResolvableType type) {
 		HandlerResult result = handlerResult(returnValue, type);
@@ -333,6 +352,9 @@ public class ResponseEntityResultHandlerTests {
 		String string() { return null; }
 
 		Completable completable() { return null; }
+
+		Mono<ResponseEntity<?>> monoResponseEntityWildcard() { return null; }
+
 	}
 
 }