Support "Accept-Patch" for unsupported media type

This commit introduces support in both servlet and webflux for the
"Accept-Patch" header, which is sent when the client sends unsupported
data in PATCH requests.
See  section 2.2 of RFC 5789.

Closes gh-26759

spring-web/src/main/java/org/springframework/web/server/UnsupportedMediaTypeStatusException.java

 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -20,9 +20,12 @@ import java.util.Collections;
 import java.util.List;
 
 import org.springframework.core.ResolvableType;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.lang.Nullable;
+import org.springframework.util.CollectionUtils;
 
 /**
  * Exception for errors that fit response status 415 (unsupported media type).
@@ -41,6 +44,9 @@ public class UnsupportedMediaTypeStatusException extends ResponseStatusException
 	@Nullable
 	private final ResolvableType bodyType;
 
+	@Nullable
+	private final HttpMethod method;
+
 
 	/**
 	 * Constructor for when the specified Content-Type is invalid.
@@ -50,13 +56,14 @@ public class UnsupportedMediaTypeStatusException extends ResponseStatusException
 		this.contentType = null;
 		this.supportedMediaTypes = Collections.emptyList();
 		this.bodyType = null;
+		this.method = null;
 	}
 
 	/**
 	 * Constructor for when the Content-Type can be parsed but is not supported.
 	 */
 	public UnsupportedMediaTypeStatusException(@Nullable MediaType contentType, List<MediaType> supportedTypes) {
-		this(contentType, supportedTypes, null);
+		this(contentType, supportedTypes, null, null);
 	}
 
 	/**
@@ -65,11 +72,30 @@ public class UnsupportedMediaTypeStatusException extends ResponseStatusException
 	 */
 	public UnsupportedMediaTypeStatusException(@Nullable MediaType contentType, List<MediaType> supportedTypes,
 			@Nullable ResolvableType bodyType) {
+		this(contentType, supportedTypes, bodyType, null);
+	}
+
+	/**
+	 * Constructor that provides the HTTP method.
+	 * @since 5.3.6
+	 */
+	public UnsupportedMediaTypeStatusException(@Nullable MediaType contentType, List<MediaType> supportedTypes,
+			@Nullable HttpMethod method) {
+		this(contentType, supportedTypes, null, method);
+	}
+
+	/**
+	 * Constructor for when trying to encode from or decode to a specific Java type.
+	 * @since 5.3.6
+	 */
+	public UnsupportedMediaTypeStatusException(@Nullable MediaType contentType, List<MediaType> supportedTypes,
+			@Nullable ResolvableType bodyType, @Nullable HttpMethod method) {
 
 		super(HttpStatus.UNSUPPORTED_MEDIA_TYPE, initReason(contentType, bodyType));
 		this.contentType = contentType;
 		this.supportedMediaTypes = Collections.unmodifiableList(supportedTypes);
 		this.bodyType = bodyType;
+		this.method = method;
 	}
 
 	private static String initReason(@Nullable MediaType contentType, @Nullable ResolvableType bodyType) {
@@ -107,4 +133,14 @@ public class UnsupportedMediaTypeStatusException extends ResponseStatusException
 		return this.bodyType;
 	}
 
+	@Override
+	public HttpHeaders getResponseHeaders() {
+		if (HttpMethod.PATCH != this.method || CollectionUtils.isEmpty(this.supportedMediaTypes) ) {
+			return HttpHeaders.EMPTY;
+		}
+		HttpHeaders headers = new HttpHeaders();
+		headers.setAcceptPatch(this.supportedMediaTypes);
+		return headers;
+	}
+
 }

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

@@ -190,7 +190,7 @@ public abstract class RequestMappingInfoHandlerMapping extends AbstractHandlerMe
 			catch (InvalidMediaTypeException ex) {
 				throw new UnsupportedMediaTypeStatusException(ex.getMessage());
 			}
-			throw new UnsupportedMediaTypeStatusException(contentType, new ArrayList<>(mediaTypes));
+			throw new UnsupportedMediaTypeStatusException(contentType, new ArrayList<>(mediaTypes), exchange.getRequest().getMethod());
 		}
 
 		if (helper.hasProducesMismatch()) {

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

@@ -317,6 +317,26 @@ public class RequestMappingInfoHandlerMappingTests {
 		assertThat(uriVariables.get("cars")).isEqualTo("cars");
 	}
 
+	@Test
+	public void handlePatchUnsupportedMediaType() {
+		MockServerHttpRequest request = MockServerHttpRequest.patch("/qux")
+				.header("content-type", "application/xml")
+				.build();
+		ServerWebExchange exchange = MockServerWebExchange.from(request);
+		Mono<Object> mono = this.handlerMapping.getHandler(exchange);
+
+		StepVerifier.create(mono)
+				.expectErrorSatisfies(ex -> {
+					assertThat(ex).isInstanceOf(UnsupportedMediaTypeStatusException.class);
+					UnsupportedMediaTypeStatusException umtse = (UnsupportedMediaTypeStatusException) ex;
+					MediaType mediaType = new MediaType("foo", "bar");
+					assertThat(umtse.getSupportedMediaTypes()).containsExactly(mediaType);
+					assertThat(umtse.getResponseHeaders().getAcceptPatch()).containsExactly(mediaType);
+				})
+				.verify();
+
+	}
+
 
 	@SuppressWarnings("unchecked")
 	private <T> void assertError(Mono<Object> mono, final Class<T> exceptionClass, final Consumer<T> consumer) {

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseEntityExceptionHandler.java

 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -231,6 +231,12 @@ public abstract class ResponseEntityExceptionHandler {
 		List<MediaType> mediaTypes = ex.getSupportedMediaTypes();
 		if (!CollectionUtils.isEmpty(mediaTypes)) {
 			headers.setAccept(mediaTypes);
+			if (request instanceof ServletWebRequest) {
+				ServletWebRequest servletWebRequest = (ServletWebRequest) request;
+				if (HttpMethod.PATCH.equals(servletWebRequest.getHttpMethod())) {
+					headers.setAcceptPatch(mediaTypes);
+				}
+			}
 		}
 
 		return handleExceptionInternal(ex, null, headers, status, request);

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/support/DefaultHandlerExceptionResolver.java

@@ -281,6 +281,9 @@ public class DefaultHandlerExceptionResolver extends AbstractHandlerExceptionRes
 		List<MediaType> mediaTypes = ex.getSupportedMediaTypes();
 		if (!CollectionUtils.isEmpty(mediaTypes)) {
 			response.setHeader("Accept", MediaType.toString(mediaTypes));
+			if (request.getMethod().equals("PATCH")) {
+				response.setHeader("Accept-Patch", MediaType.toString(mediaTypes));
+			}
 		}
 		response.sendError(HttpServletResponse.SC_UNSUPPORTED_MEDIA_TYPE);
 		return new ModelAndView();

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ResponseEntityExceptionHandlerTests.java

 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -113,6 +113,20 @@ public class ResponseEntityExceptionHandlerTests {
 
 		ResponseEntity<Object> responseEntity = testException(ex);
 		assertThat(responseEntity.getHeaders().getAccept()).isEqualTo(acceptable);
+		assertThat(responseEntity.getHeaders().getAcceptPatch()).isEmpty();
+	}
+
+	@Test
+	public void patchHttpMediaTypeNotSupported() {
+		this.servletRequest = new MockHttpServletRequest("PATCH", "/");
+		this.request = new ServletWebRequest(this.servletRequest, this.servletResponse);
+
+		List<MediaType> acceptable = Arrays.asList(MediaType.APPLICATION_ATOM_XML, MediaType.APPLICATION_XML);
+		Exception ex = new HttpMediaTypeNotSupportedException(MediaType.APPLICATION_JSON, acceptable);
+
+		ResponseEntity<Object> responseEntity = testException(ex);
+		assertThat(responseEntity.getHeaders().getAccept()).isEqualTo(acceptable);
+		assertThat(responseEntity.getHeaders().getAcceptPatch()).isEqualTo(acceptable);
 	}
 
 	@Test

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ServletAnnotationControllerHandlerMethodTests.java

 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -976,6 +976,26 @@ public class ServletAnnotationControllerHandlerMethodTests extends AbstractServl
 		assertThat(response.getHeader("Accept")).isEqualTo("text/plain");
 	}
 
+	@PathPatternsParameterizedTest
+	void unsupportedPatchBody(boolean usePathPatterns) throws Exception {
+		initDispatcherServlet(RequestResponseBodyController.class, usePathPatterns, wac -> {
+			RootBeanDefinition adapterDef = new RootBeanDefinition(RequestMappingHandlerAdapter.class);
+			StringHttpMessageConverter converter = new StringHttpMessageConverter();
+			converter.setSupportedMediaTypes(Collections.singletonList(MediaType.TEXT_PLAIN));
+			adapterDef.getPropertyValues().add("messageConverters", converter);
+			wac.registerBeanDefinition("handlerAdapter", adapterDef);
+		});
+
+		MockHttpServletRequest request = new MockHttpServletRequest("PATCH", "/something");
+		String requestBody = "Hello World";
+		request.setContent(requestBody.getBytes(StandardCharsets.UTF_8));
+		request.addHeader("Content-Type", "application/pdf");
+		MockHttpServletResponse response = new MockHttpServletResponse();
+		getServlet().service(request, response);
+		assertThat(response.getStatus()).isEqualTo(415);
+		assertThat(response.getHeader("Accept-Patch")).isEqualTo("text/plain");
+	}
+
 	@PathPatternsParameterizedTest
 	void responseBodyNoAcceptHeader(boolean usePathPatterns) throws Exception {
 		initDispatcherServlet(RequestResponseBodyController.class, usePathPatterns);

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/support/DefaultHandlerExceptionResolverTests.java

@@ -87,6 +87,18 @@ public class DefaultHandlerExceptionResolverTests {
 		assertThat(response.getHeader("Accept")).as("Invalid Accept header").isEqualTo("application/pdf");
 	}
 
+	@Test
+	public void patchHttpMediaTypeNotSupported() {
+		HttpMediaTypeNotSupportedException ex = new HttpMediaTypeNotSupportedException(new MediaType("text", "plain"),
+				Collections.singletonList(new MediaType("application", "pdf")));
+		MockHttpServletRequest request = new MockHttpServletRequest("PATCH", "/");
+		ModelAndView mav = exceptionResolver.resolveException(request, response, null, ex);
+		assertThat(mav).as("No ModelAndView returned").isNotNull();
+		assertThat(mav.isEmpty()).as("No Empty ModelAndView returned").isTrue();
+		assertThat(response.getStatus()).as("Invalid status code").isEqualTo(415);
+		assertThat(response.getHeader("Accept-Patch")).as("Invalid Accept header").isEqualTo("application/pdf");
+	}
+
 	@Test
 	public void handleMissingPathVariable() throws NoSuchMethodException {
 		Method method = getClass().getMethod("handle", String.class);