Enable rendering as HAL FORMS.

We're now registering an HttpMessageConverter to render requests for HAL FORMS using the HalFormsConfiguration present in the application.

We're currently not adding any affordances yet. The change solely enables user code to add affordances explicitly, e.g. via RepresentationModelProcessor implementations.

Fixes: #1991.

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

@@ -45,6 +45,7 @@ import org.springframework.hateoas.IanaLinkRelations;
 import org.springframework.hateoas.Link;
 import org.springframework.hateoas.LinkRelation;
 import org.springframework.hateoas.Links;
+import org.springframework.hateoas.MediaTypes;
 import org.springframework.hateoas.server.LinkRelationProvider;
 import org.springframework.http.MediaType;
 import org.springframework.mock.web.MockHttpServletResponse;
@@ -731,6 +732,14 @@ public class JpaWebTests extends CommonWebTests {
 		assertThat(content.read("$.calculatedName", String.class)).isEqualTo("calculated-put");
 	}
 
+	@Test // #1991
+	public void answersToHalFormsRequests() throws Exception {
+
+		mvc.perform(get("/")
+				.accept(MediaTypes.HAL_FORMS_JSON))
+				.andExpect(status().isOk());
+	}
+
 	private List<Link> preparePersonResources(Person primary, Person... persons) throws Exception {
 
 		Link peopleLink = client.discoverUnique(LinkRelation.of("people"));

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

@@ -34,6 +34,7 @@ import org.springframework.data.rest.core.mapping.ResourceMetadata;
 import org.springframework.data.rest.webmvc.support.JpaHelper;
 import org.springframework.data.util.ProxyUtils;
 import org.springframework.data.util.Streamable;
+import org.springframework.hateoas.MediaTypes;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.MediaType;
 import org.springframework.orm.jpa.support.OpenEntityManagerInViewInterceptor;
@@ -217,6 +218,7 @@ public class RepositoryRestHandlerMapping extends BasePathAwareHandlerMapping {
 		Set<String> mediaTypes = new LinkedHashSet<String>();
 		mediaTypes.add(configuration.getDefaultMediaType().toString());
 		mediaTypes.add(MediaType.APPLICATION_JSON_VALUE);
+		mediaTypes.add(MediaTypes.HAL_FORMS_JSON_VALUE);
 
 		return new ProducesRequestCondition(mediaTypes.toArray(new String[mediaTypes.size()]));
 	}

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

@@ -104,6 +104,8 @@ import org.springframework.hateoas.mediatype.hal.DefaultCurieProvider;
 import org.springframework.hateoas.mediatype.hal.HalConfiguration;
 import org.springframework.hateoas.mediatype.hal.Jackson2HalModule;
 import org.springframework.hateoas.mediatype.hal.Jackson2HalModule.HalHandlerInstantiator;
+import org.springframework.hateoas.mediatype.hal.forms.HalFormsConfiguration;
+import org.springframework.hateoas.mediatype.hal.forms.Jackson2HalFormsModule;
 import org.springframework.hateoas.server.LinkRelationProvider;
 import org.springframework.hateoas.server.core.EvoInflectorLinkRelationProvider;
 import org.springframework.hateoas.server.mvc.RepresentationModelProcessorInvoker;
@@ -143,7 +145,7 @@ import com.fasterxml.jackson.datatype.jdk8.Jdk8Module;
  * @author Christoph Strobl
  */
 @Configuration(proxyBeanMethods = false)
-@EnableHypermediaSupport(type = HypermediaType.HAL)
+@EnableHypermediaSupport(type = { HypermediaType.HAL, HypermediaType.HAL_FORMS })
 @ImportResource("classpath*:META-INF/spring-data-rest/**/*.xml")
 @Import({ RestControllerImportSelector.class, //
 		SpringDataJacksonConfiguration.class, //
@@ -573,13 +575,45 @@ public class RepositoryRestMvcConfiguration extends HateoasAwareSpringDataWebCon
 		return converter;
 	}
 
+	/**
+	 * {@link HttpMessageConverter} to support rendering HAL FORMS.
+	 *
+	 * @param linkCollector
+	 * @return
+	 * @since 3.5
+	 */
+	@Bean
+	TypeConstrainedMappingJackson2HttpMessageConverter halFormsJacksonHttpMessageConverter(LinkCollector linkCollector) {
+
+		LinkRelationProvider defaultedRelProvider = this.relProvider.getIfUnique(EvoInflectorLinkRelationProvider::new);
+		HalFormsConfiguration configuration = new HalFormsConfiguration(
+				halConfiguration.getIfUnique(() -> new HalConfiguration()));
+		CurieProvider curieProvider = this.curieProvider
+				.getIfUnique(() -> new DefaultCurieProvider(Collections.emptyMap()));
+		ObjectMapper mapper = basicObjectMapper();
+
+		mapper.registerModule(persistentEntityJackson2Module(linkCollector));
+		mapper.registerModule(new Jackson2HalFormsModule());
+		mapper.setHandlerInstantiator(new Jackson2HalModule.HalHandlerInstantiator(
+				defaultedRelProvider, curieProvider, resolver.getObject(), configuration.getHalConfiguration(),
+				applicationContext.getAutowireCapableBeanFactory()));
+
+		TypeConstrainedMappingJackson2HttpMessageConverter converter = new TypeConstrainedMappingJackson2HttpMessageConverter(
+				RepresentationModel.class);
+		converter.setSupportedMediaTypes(Collections.singletonList(MediaTypes.HAL_FORMS_JSON));
+		converter.setObjectMapper(mapper);
+
+		return converter;
+	}
+
 	public ObjectMapper halObjectMapper(LinkCollector linkCollector) {
 
 		LinkRelationProvider defaultedRelProvider = this.relProvider.getIfUnique(EvoInflectorLinkRelationProvider::new);
 		HalConfiguration halConfiguration = this.halConfiguration.getIfUnique(HalConfiguration::new);
-		HalHandlerInstantiator instantiator = new HalHandlerInstantiator(defaultedRelProvider,
-				curieProvider.getIfUnique(() -> new DefaultCurieProvider(Collections.emptyMap())), resolver.getObject(),
-				halConfiguration, applicationContext.getAutowireCapableBeanFactory());
+		CurieProvider curieProvider = this.curieProvider
+				.getIfUnique(() -> new DefaultCurieProvider(Collections.emptyMap()));
+		HalHandlerInstantiator instantiator = new HalHandlerInstantiator(defaultedRelProvider, curieProvider,
+				resolver.getObject(), halConfiguration, applicationContext.getAutowireCapableBeanFactory());
 
 		ObjectMapper mapper = basicObjectMapper();
 		mapper.registerModule(persistentEntityJackson2Module(linkCollector));
@@ -739,6 +773,7 @@ public class RepositoryRestMvcConfiguration extends HateoasAwareSpringDataWebCon
 	public List<HttpMessageConverter<?>> defaultMessageConverters(
 			@Qualifier("jacksonHttpMessageConverter") TypeConstrainedMappingJackson2HttpMessageConverter jacksonHttpMessageConverter,
 			@Qualifier("halJacksonHttpMessageConverter") TypeConstrainedMappingJackson2HttpMessageConverter halJacksonHttpMessageConverter,
+			@Qualifier("halFormsJacksonHttpMessageConverter") TypeConstrainedMappingJackson2HttpMessageConverter halFormsJacksonHttpMessageConverter,
 			AlpsJsonHttpMessageConverter alpsJsonHttpMessageConverter,
 			UriListHttpMessageConverter uriListHttpMessageConverter, RepositoryRestConfigurerDelegate configurerDelegate,
 			RepositoryRestConfiguration repositoryRestConfiguration) {
@@ -757,6 +792,8 @@ public class RepositoryRestMvcConfiguration extends HateoasAwareSpringDataWebCon
 			messageConverters.add(halJacksonHttpMessageConverter);
 		}
 
+		messageConverters.add(halFormsJacksonHttpMessageConverter);
+
 		MappingJackson2HttpMessageConverter fallbackJsonConverter = new MappingJackson2HttpMessageConverter();
 		fallbackJsonConverter.setObjectMapper(basicObjectMapper());