GH-2227 - Introduce HalFormsHttpMessageConverter.

src/main/java/org/springframework/hateoas/mediatype/hal/forms/HalFormsHttpMessageConverter.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2024 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.hateoas.mediatype.hal.forms;
+
+import java.io.IOException;
+import java.lang.reflect.Type;
+import java.util.List;
+
+import org.springframework.beans.factory.BeanFactory;
+import org.springframework.hateoas.MediaTypes;
+import org.springframework.hateoas.RepresentationModel;
+import org.springframework.hateoas.server.mvc.TypeConstrainedMappingJackson2HttpMessageConverter;
+import org.springframework.http.HttpOutputMessage;
+import org.springframework.http.converter.HttpMessageNotWritableException;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+/**
+ * A {@link TypeConstrainedMappingJackson2HttpMessageConverter} that will inspect the returned
+ * {@link RepresentationModel} for affordances and flip the {@link org.springframework.http.MediaType} rendered back to
+ * {@link MediaTypes#HAL_JSON} if no templates have been registered.
+ *
+ * @author Oliver Drotbohm
+ * @since 2.4
+ */
+public class HalFormsHttpMessageConverter extends TypeConstrainedMappingJackson2HttpMessageConverter {
+
+	private final HalFormsTemplateBuilder builder;
+
+	/**
+	 * Creates a new {@link HalFormsHttpMessageConverter} for the given {@link BeanFactory} and {@link ObjectMapper}
+	 *
+	 * @param factory must not be {@literal null}.
+	 * @param mapper must not be {@literal null}.
+	 */
+	public HalFormsHttpMessageConverter(BeanFactory factory, ObjectMapper mapper) {
+
+		super(RepresentationModel.class, List.of(MediaTypes.HAL_FORMS_JSON), mapper);
+
+		Assert.notNull(factory, "BeanFactory must not be null!");
+		Assert.notNull(mapper, "ObjectMapper must not be null!");
+
+		this.builder = factory.getBean(HalFormsTemplateBuilder.class);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter#writeInternal(java.lang.Object, java.lang.reflect.Type, org.springframework.http.HttpOutputMessage)
+	 */
+	@Override
+	protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
+			throws IOException, HttpMessageNotWritableException {
+
+		if (!(object instanceof RepresentationModel model)) {
+			super.writeInternal(object, type, outputMessage);
+			return;
+		}
+
+		var result = builder.findTemplates(model);
+
+		// Revert back to HAL if no templates found
+		if (result.isEmpty()) {
+			var headers = outputMessage.getHeaders();
+			headers.setContentType(MediaTypes.HAL_JSON);
+		}
+
+		super.writeInternal(object, type, outputMessage);
+	}
+}

src/main/java/org/springframework/hateoas/mediatype/hal/forms/HalFormsMediaTypeConfiguration.java

@@ -83,11 +83,14 @@ LinkDiscoverer halFormsLinkDiscoverer() {
 	}
 
 	@Bean
-	HalFormsTemplatePropertyWriter halFormsTemplatePropertyWriter() {
+	HalFormsTemplateBuilder halFormsTemplateBuilder() {
 
 		HalFormsConfiguration configuration = configurationFactory.getConfiguration();
-		HalFormsTemplateBuilder builder = new HalFormsTemplateBuilder(configuration, resolver);
+		return new HalFormsTemplateBuilder(configuration, resolver);
+	}
 
+	@Bean
+	HalFormsTemplatePropertyWriter halFormsTemplatePropertyWriter(HalFormsTemplateBuilder builder) {
 		return new HalFormsTemplatePropertyWriter(builder);
 	}
 

src/main/java/org/springframework/hateoas/mediatype/hal/forms/HalFormsTemplateBuilder.java

@@ -48,8 +48,8 @@ public HalFormsTemplateBuilder(HalFormsConfiguration configuration, MessageResol
 	/**
 	 * Extract template details from a {@link RepresentationModel}'s {@link Affordance}s.
 	 *
-	 * @param resource
-	 * @return
+	 * @param resource must not be {@literal null}.
+	 * @return will never be {@literal null}.
 	 */
 	public Map<String, HalFormsTemplate> findTemplates(RepresentationModel<?> resource) {