Support content negotiation for RFC 7807

Closes gh-28189

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

@@ -111,14 +111,25 @@ public abstract class HandlerResultHandlerSupport implements Ordered {
 	}
 
 	/**
-	 * Select the best media type for the current request through a content negotiation algorithm.
+	 * Select the best media type for the current request through a content
+	 * negotiation algorithm.
 	 * @param exchange the current request
-	 * @param producibleTypesSupplier the media types that can be produced for the current request
+	 * @param producibleTypesSupplier the media types producible for the request
 	 * @return the selected media type, or {@code null} if none
 	 */
 	@Nullable
+	protected MediaType selectMediaType(ServerWebExchange exchange, Supplier<List<MediaType>> producibleTypesSupplier) {
+		return selectMediaType(exchange, producibleTypesSupplier, getAcceptableTypes(exchange));
+	}
+
+	/**
+	 * Variant of {@link #selectMediaType(ServerWebExchange, Supplier)} with a
+	 * given list of requested (acceptable) media types.
+	 */
+	@Nullable
 	protected MediaType selectMediaType(
-			ServerWebExchange exchange, Supplier<List<MediaType>> producibleTypesSupplier) {
+			ServerWebExchange exchange, Supplier<List<MediaType>> producibleTypesSupplier,
+			List<MediaType> acceptableTypes) {
 
 		MediaType contentType = exchange.getResponse().getHeaders().getContentType();
 		if (contentType != null && contentType.isConcrete()) {
@@ -128,7 +139,6 @@ public abstract class HandlerResultHandlerSupport implements Ordered {
 			return contentType;
 		}
 
-		List<MediaType> acceptableTypes = getAcceptableTypes(exchange);
 		List<MediaType> producibleTypes = getProducibleTypes(exchange, producibleTypesSupplier);
 
 		Set<MediaType> compatibleMediaTypes = new LinkedHashSet<>();

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.method.annotation;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Set;
 
@@ -32,6 +33,7 @@ import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.Hints;
 import org.springframework.http.HttpStatusCode;
 import org.springframework.http.MediaType;
+import org.springframework.http.ProblemDetail;
 import org.springframework.http.codec.HttpMessageWriter;
 import org.springframework.http.converter.HttpMessageNotWritableException;
 import org.springframework.lang.Nullable;
@@ -57,6 +59,9 @@ public abstract class AbstractMessageWriterResultHandler extends HandlerResultHa
 
 	private final List<HttpMessageWriter<?>> messageWriters;
 
+	private final List<MediaType> problemMediaTypes =
+			Arrays.asList(MediaType.APPLICATION_PROBLEM_JSON, MediaType.APPLICATION_PROBLEM_XML);
+
 
 	/**
 	 * Constructor with {@link HttpMessageWriter HttpMessageWriters} and a
@@ -161,6 +166,12 @@ public abstract class AbstractMessageWriterResultHandler extends HandlerResultHa
 			}
 			throw ex;
 		}
+
+		// Fall back on RFC 7807 format for ProblemDetail
+		if (bestMediaType == null && elementType.toClass().equals(ProblemDetail.class)) {
+			bestMediaType = selectMediaType(exchange, () -> getMediaTypesFor(elementType), this.problemMediaTypes);
+		}
+
 		if (bestMediaType != null) {
 			String logPrefix = exchange.getLogPrefix();
 			if (logger.isDebugEnabled()) {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.result.method.annotation;
 
+import java.net.URI;
 import java.util.List;
 
 import reactor.core.publisher.Mono;
@@ -23,6 +24,8 @@ import reactor.core.publisher.Mono;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.annotation.AnnotatedElementUtils;
+import org.springframework.http.HttpStatusCode;
+import org.springframework.http.ProblemDetail;
 import org.springframework.http.codec.HttpMessageWriter;
 import org.springframework.web.bind.annotation.ResponseBody;
 import org.springframework.web.reactive.HandlerResult;
@@ -83,6 +86,13 @@ public class ResponseBodyResultHandler extends AbstractMessageWriterResultHandle
 	public Mono<Void> handleResult(ServerWebExchange exchange, HandlerResult result) {
 		Object body = result.getReturnValue();
 		MethodParameter bodyTypeParameter = result.getReturnTypeSource();
+		if (body instanceof ProblemDetail detail) {
+			exchange.getResponse().setStatusCode(HttpStatusCode.valueOf(detail.getStatus()));
+			if (detail.getInstance() == null) {
+				URI path = URI.create(exchange.getRequest().getPath().value());
+				detail.setInstance(path);
+			}
+		}
 		return writeBody(body, bodyTypeParameter, exchange);
 	}