Adapt Content-Type header for HAL FORMS requests based on affordances.

Whenever HAL FORMS is supposed to be rendered, we now adapt the `Content-Type` header depending on whether affordances (and thus `_templates`) are present. If none can be found, we either adapt the header to either `application/hal+json` or even `application/json` depending on the `Accept` header arrangement given. If none match, we reject the request with `406 Not Acceptable`.

Had to reintroduce a dependency on Lombok to use @SneakyThrows as otherwise we cannot throw HttpMediaTypeNotAcceptableException from a ResponseBodyAdvice. A manual implementation of the pattern does not compile on Java 8 (at least on MacOS).

Fixes #2060.

spring-data-rest-tests/spring-data-rest-tests-jpa/src/test/java/org/springframework/data/rest/webmvc/jpa/JpaWebTests.java

@@ -20,6 +20,7 @@ import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 import static org.junit.Assert.assertThat;
 import static org.springframework.data.rest.webmvc.util.TestUtils.*;
+import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.*;
 import static org.springframework.http.HttpHeaders.*;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
@@ -39,6 +40,7 @@ import org.springframework.context.ApplicationContextInitializer;
 import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.data.rest.core.mapping.ResourceMappings;
 import org.springframework.data.rest.tests.CommonWebTests;
+import org.springframework.data.rest.webmvc.RepositoryLinksResource;
 import org.springframework.data.rest.webmvc.jpa.JpaRepositoryConfig.BooksHtmlController;
 import org.springframework.data.rest.webmvc.jpa.JpaRepositoryConfig.OrdersJsonController;
 import org.springframework.hateoas.IanaLinkRelations;
@@ -47,6 +49,7 @@ import org.springframework.hateoas.LinkRelation;
 import org.springframework.hateoas.Links;
 import org.springframework.hateoas.MediaTypes;
 import org.springframework.hateoas.server.LinkRelationProvider;
+import org.springframework.hateoas.server.RepresentationModelProcessor;
 import org.springframework.http.MediaType;
 import org.springframework.mock.web.MockHttpServletResponse;
 import org.springframework.test.context.ContextConfiguration;
@@ -91,6 +94,27 @@ public class JpaWebTests extends CommonWebTests {
 			ctx.registerBean(AuthorsController.class);
 			ctx.registerBean(BooksHtmlController.class);
 			ctx.registerBean(OrdersJsonController.class);
+			ctx.registerBean(RepositoryLinkAffordanceAdder.class);
+		}
+	}
+
+	/**
+	 * Registered to add an affordance to the {@link RepositoryLinksResource} to make sure HAL FORMS can be requested in
+	 * {@link JpaWebTests#answersToHalFormsRequests()}.
+	 *
+	 * @author Oliver Drotbohm
+	 */
+	static class RepositoryLinkAffordanceAdder implements RepresentationModelProcessor<RepositoryLinksResource> {
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.hateoas.server.RepresentationModelProcessor#process(org.springframework.hateoas.RepresentationModel)
+		 */
+		@Override
+		public RepositoryLinksResource process(RepositoryLinksResource model) {
+
+			return model.mapLink(LinkRelation.of("authors"),
+					link -> link.andAffordance(afford(methodOn(AuthorsController.class).deleteAuthor(null))));
 		}
 	}
 

spring-data-rest-webmvc/pom.xml

@@ -50,6 +50,13 @@
 			<scope>provided</scope>
 		</dependency>
 
+		<dependency>
+			<groupId>org.projectlombok</groupId>
+			<artifactId>lombok</artifactId>
+			<version>${lombok}</version>
+			<optional>true</optional>
+		</dependency>
+
 		<!-- Jackson -->
 
 		<dependency>

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/config/HalFormsAdaptingResponseBodyAdvice.java

@@ -0,0 +1,102 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.rest.webmvc.config;
+
+import lombok.SneakyThrows;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.core.MethodParameter;
+import org.springframework.hateoas.MediaTypes;
+import org.springframework.hateoas.RepresentationModel;
+import org.springframework.hateoas.server.mvc.TypeConstrainedMappingJackson2HttpMessageConverter;
+import org.springframework.http.MediaType;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.server.ServerHttpRequest;
+import org.springframework.http.server.ServerHttpResponse;
+import org.springframework.web.HttpMediaTypeNotAcceptableException;
+import org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor;
+import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;
+
+/**
+ * {@link ResponseBodyAdvice} that tweaks responses asking for HAL FORMS to potentially fall back to a non-forms
+ * {@link MediaType} in case no affordances are registered on the {@link RepresentationModel} to be rendered.
+ *
+ * @author Oliver Drotbohm
+ */
+class HalFormsAdaptingResponseBodyAdvice<T extends RepresentationModel<T>>
+		implements ResponseBodyAdvice<RepresentationModel<T>> {
+
+	private static final Logger logger = LoggerFactory.getLogger(RequestResponseBodyMethodProcessor.class);
+	private static final String MESSAGE = "HalFormsRejectingResponseBodyAdvice - Changing content type to '%s' as no affordances were registered on the representation model to be rendered!";
+	private static final List<MediaType> SUPPORTED_MEDIA_TYPES = Arrays.asList(MediaTypes.HAL_JSON,
+			MediaType.APPLICATION_JSON);
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice#supports(org.springframework.core.MethodParameter, java.lang.Class)
+	 */
+	@Override
+	public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
+		return TypeConstrainedMappingJackson2HttpMessageConverter.class.isAssignableFrom(converterType);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice#beforeBodyWrite(java.lang.Object, org.springframework.core.MethodParameter, org.springframework.http.MediaType, java.lang.Class, org.springframework.http.server.ServerHttpRequest, org.springframework.http.server.ServerHttpResponse)
+	 */
+	@Override
+	@SneakyThrows
+	public RepresentationModel<T> beforeBodyWrite(RepresentationModel<T> body, MethodParameter returnType,
+			MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType,
+			ServerHttpRequest request, ServerHttpResponse response) {
+
+		// Only step in if we are about to render HAL FORMS
+		if (!MediaTypes.HAL_FORMS_JSON.equals(selectedContentType)) {
+			return body;
+		}
+
+		List<MediaType> accept = request.getHeaders().getAccept();
+
+		boolean hasAffordances = body.getLinks().stream()
+				.anyMatch(it -> !it.getAffordances().isEmpty());
+
+		// Affordances registered -> we're fine as we will render templates
+		if (hasAffordances) {
+			return body;
+		}
+
+		// Check whether either HAL or general JSON are acceptable
+		for (MediaType candidate : accept) {
+			for (MediaType supported : SUPPORTED_MEDIA_TYPES) {
+				if (candidate.isCompatibleWith(supported)) {
+
+					// Tweak response to expose that
+					logger.debug(String.format(MESSAGE, supported));
+					response.getHeaders().setContentType(supported);
+
+					return body;
+				}
+			}
+		}
+
+		// Reject the request otherwise
+		throw new HttpMediaTypeNotAcceptableException(SUPPORTED_MEDIA_TYPES);
+	}
+}

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/config/RepositoryRestMvcConfiguration.java

@@ -123,6 +123,7 @@ import org.springframework.web.servlet.HandlerMapping;
 import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
 import org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
+import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;
 import org.springframework.web.util.pattern.PathPatternParser;
 
 import com.fasterxml.jackson.databind.DeserializationFeature;
@@ -658,10 +659,15 @@ public class RepositoryRestMvcConfiguration extends HateoasAwareSpringDataWebCon
 		handlerAdapter.setWebBindingInitializer(initializer);
 		handlerAdapter.setMessageConverters(defaultMessageConverters);
 
+		List<ResponseBodyAdvice<?>> advices = new ArrayList<>();
+		advices.add(new HalFormsAdaptingResponseBodyAdvice<>());
+
 		if (repositoryRestConfiguration.getMetadataConfiguration().alpsEnabled()) {
-			handlerAdapter.setResponseBodyAdvice(Arrays.asList(alpsJsonHttpMessageConverter));
+			advices.addAll(Arrays.asList(alpsJsonHttpMessageConverter));
 		}
 
+		handlerAdapter.setResponseBodyAdvice(advices);
+
 		return handlerAdapter;
 	}
 

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/config/HalFormsAdaptingResponseBodyAdviceTests.java

@@ -0,0 +1,109 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.rest.webmvc.config;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.util.Arrays;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.core.MethodParameter;
+import org.springframework.hateoas.Link;
+import org.springframework.hateoas.MediaTypes;
+import org.springframework.hateoas.RepresentationModel;
+import org.springframework.hateoas.mediatype.Affordances;
+import org.springframework.hateoas.server.mvc.TypeConstrainedMappingJackson2HttpMessageConverter;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
+import org.springframework.http.MediaType;
+import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
+import org.springframework.http.server.ServletServerHttpRequest;
+import org.springframework.http.server.ServletServerHttpResponse;
+import org.springframework.mock.web.MockHttpServletRequest;
+import org.springframework.mock.web.MockHttpServletResponse;
+import org.springframework.web.HttpMediaTypeNotAcceptableException;
+
+/**
+ * Unit tests for {@link HalFormsAdaptingResponseBodyAdvice}.
+ *
+ * @author Oliver Drotbohm
+ */
+@ExtendWith(MockitoExtension.class)
+public class HalFormsAdaptingResponseBodyAdviceTests<T extends RepresentationModel<T>> {
+
+	HalFormsAdaptingResponseBodyAdvice<T> advice = new HalFormsAdaptingResponseBodyAdvice<>();
+
+	@Mock MethodParameter parameter;
+	MockHttpServletRequest request = new MockHttpServletRequest();
+	MockHttpServletResponse response = new MockHttpServletResponse();
+
+	@Test // #2060
+	void supportsTypeConstraintedHttpMessageConverterOnly() {
+
+		assertThat(advice.supports(parameter, TypeConstrainedMappingJackson2HttpMessageConverter.class)).isTrue();
+		assertThat(advice.supports(parameter, MappingJackson2HttpMessageConverter.class)).isFalse();
+	}
+
+	@Test // #2060
+	void usesHalJsonContentTypeIfNoAffordancesSet() {
+
+		request.addHeader(HttpHeaders.ACCEPT,
+				MediaType.toString(Arrays.asList(MediaTypes.HAL_FORMS_JSON, MediaTypes.HAL_JSON)));
+
+		RepresentationModel<T> model = new RepresentationModel<>();
+
+		assertResponseContentType(model, MediaTypes.HAL_JSON);
+	}
+
+	@Test // #2060
+	void usesHalFormsContentTypeIfAffordancesPresent() {
+
+		request.addHeader(HttpHeaders.ACCEPT,
+				MediaType.toString(Arrays.asList(MediaTypes.HAL_FORMS_JSON, MediaTypes.HAL_JSON)));
+
+		RepresentationModel<T> model = new RepresentationModel<>();
+		model.add(Affordances.of(Link.of("localhost")).afford(HttpMethod.GET).build().toLink());
+
+		assertResponseContentType(model, MediaTypes.HAL_FORMS_JSON);
+	}
+
+	@Test // #2060
+	void issues415IfNoCompatibleMediaTypeWasRequested() {
+
+		request.addHeader(HttpHeaders.ACCEPT,
+				MediaType.toString(Arrays.asList(MediaTypes.HAL_FORMS_JSON)));
+
+		RepresentationModel<T> model = new RepresentationModel<>();
+
+		assertThatExceptionOfType(HttpMediaTypeNotAcceptableException.class)
+				.isThrownBy(() -> assertResponseContentType(model, MediaTypes.HAL_JSON));
+	}
+
+	private void assertResponseContentType(RepresentationModel<T> model, MediaType mediaType) {
+
+		this.response.addHeader(HttpHeaders.CONTENT_TYPE, MediaTypes.HAL_FORMS_JSON_VALUE);
+		ServletServerHttpResponse response = new ServletServerHttpResponse(this.response);
+
+		advice.beforeBodyWrite(model, parameter, MediaTypes.HAL_FORMS_JSON,
+				TypeConstrainedMappingJackson2HttpMessageConverter.class,
+				new ServletServerHttpRequest(request), response);
+
+		assertThat(response.getHeaders().getContentType()).isEqualTo(mediaType);
+	}
+}