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-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);
+	}
+}