issue spring-projects#288

Allows links under a single rel to be represented as a multiple (i.e., serialized to an array) even if there is only one.

src/main/java/org/springframework/hateoas/Link.java

@@ -55,6 +55,7 @@ public class Link implements Serializable {
 	@XmlAttribute private String rel;
 	@XmlAttribute private String href;
 	@XmlTransient @JsonIgnore private UriTemplate template;
+	@XmlTransient @JsonIgnore private Class<? extends ResourceSupport> owningResource;
 
 	/**
 	 * Creates a new link to the given URI with the self rel.
@@ -194,10 +195,18 @@ private UriTemplate getUriTemplate() {
 		return template;
 	}
 
-	/* 
-	 * (non-Javadoc)
-	 * @see java.lang.Object#equals(java.lang.Object)
-	 */
+	public Class<? extends ResourceSupport> getOwningResource() {
+		return this.owningResource;
+	}
+
+	void setOwningResource(Class<? extends ResourceSupport> owningResource) {
+		this.owningResource = owningResource;
+	}
+
+	/*
+         * (non-Javadoc)
+         * @see java.lang.Object#equals(java.lang.Object)
+         */
 	@Override
 	public boolean equals(Object obj) {
 

src/main/java/org/springframework/hateoas/ResourceSupport.java

@@ -53,6 +53,7 @@ public Link getId() {
 	 */
 	public void add(Link link) {
 		Assert.notNull(link, "Link must not be null!");
+		link.setOwningResource(this.getClass());
 		this.links.add(link);
 	}
 
@@ -64,6 +65,7 @@ public void add(Link link) {
 	public void add(Iterable<Link> links) {
 		Assert.notNull(links, "Given links must not be null!");
 		for (Link candidate : links) {
+			candidate.setOwningResource(this.getClass());
 			add(candidate);
 		}
 	}

src/main/java/org/springframework/hateoas/hal/ForceMultipleLinksOnRels.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2012 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
+ *
+ *      http://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.hal;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Inherited;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.hateoas.core.ControllerEntityLinks;
+
+/**
+ * Annotation to force links under specified rels to be serialized as a JSON array regardless of cardinality.
+ *
+ * @author Vivin Paliath
+ */
+@Inherited
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.TYPE)
+public @interface ForceMultipleLinksOnRels {
+    public String[] value();
+}

src/main/java/org/springframework/hateoas/hal/Jackson2HalModule.java

@@ -17,21 +17,11 @@
 
 import java.io.IOException;
 import java.lang.reflect.Type;
-import java.util.ArrayList;
-import java.util.Collection;
-import java.util.HashMap;
-import java.util.Iterator;
-import java.util.LinkedHashMap;
-import java.util.List;
-import java.util.Map;
+import java.util.*;
 
 import org.springframework.beans.BeanUtils;
-import org.springframework.hateoas.Link;
-import org.springframework.hateoas.Links;
-import org.springframework.hateoas.RelProvider;
-import org.springframework.hateoas.Resource;
-import org.springframework.hateoas.ResourceSupport;
-import org.springframework.hateoas.Resources;
+import org.springframework.core.annotation.AnnotationUtils;
+import org.springframework.hateoas.*;
 import org.springframework.util.Assert;
 
 import com.fasterxml.jackson.core.JsonGenerationException;
@@ -130,15 +120,29 @@ public HalLinkListSerializer(BeanProperty property, CurieProvider curieProvider)
 		public void serialize(List<Link> value, JsonGenerator jgen, SerializerProvider provider) throws IOException,
 				JsonGenerationException {
 
+
+			// keeps track of any rels that need their links to be forced as multiple (i.e., serialized to array)
+			// regardless of cardinality
+			List<String> forcedMultipleLinkRels = new ArrayList<String>();
+			if(!value.isEmpty()) {
+                Class<? extends ResourceSupport> owningResource = value.get(0).getOwningResource();
+                ForceMultipleLinksOnRels annotation = AnnotationUtils.findAnnotation(owningResource, ForceMultipleLinksOnRels.class);
+                if(annotation != null) {
+                    forcedMultipleLinkRels.addAll(Arrays.asList(annotation.value()));
+                }
+			}
+
 			// sort links according to their relation
 			Map<String, List<Object>> sortedLinks = new LinkedHashMap<String, List<Object>>();
+
 			List<Link> links = new ArrayList<Link>();
 
 			boolean prefixingRequired = curieProvider != null;
 			boolean curiedLinkPresent = false;
 
 			for (Link link : value) {
 
+
 				String rel = prefixingRequired ? curieProvider.getNamespacedRelFrom(link) : link.getRel();
 
 				if (!link.getRel().equals(rel)) {
@@ -167,7 +171,7 @@ public void serialize(List<Link> value, JsonGenerator jgen, SerializerProvider p
 			JavaType mapType = typeFactory.constructMapType(HashMap.class, keyType, valueType);
 
 			MapSerializer serializer = MapSerializer.construct(new String[] {}, mapType, true, null,
-					provider.findKeySerializer(keyType, null), new OptionalListJackson2Serializer(property), null);
+					provider.findKeySerializer(keyType, null), new OptionalListJackson2Serializer(property, forcedMultipleLinkRels), null);
 
 			serializer.serialize(sortedLinks, jgen, provider);
 		}
@@ -320,6 +324,7 @@ public static class OptionalListJackson2Serializer extends ContainerSerializer<O
 
 		private final BeanProperty property;
 		private final Map<Class<?>, JsonSerializer<Object>> serializers;
+		private final List<String> forcedMultipleLinkRels;
 
 		public OptionalListJackson2Serializer() {
 			this(null);
@@ -331,10 +336,24 @@ public OptionalListJackson2Serializer() {
 		 * @param property
 		 */
 		public OptionalListJackson2Serializer(BeanProperty property) {
+			this(property, new ArrayList<String>());
+		}
+
+		/**
+		 * Private constructor that creates a new instance using the given {@link BeanProperty}
+		 * and a list of rels whose links need to be forced into an array representation regardless of
+		 * cardinality.
+		 *
+		 * @param property
+		 * @param forcedMultipleLinkRels
+		 */
+		private OptionalListJackson2Serializer(BeanProperty property, List<String> forcedMultipleLinkRels) {
 
 			super(List.class, false);
+
 			this.property = property;
 			this.serializers = new HashMap<Class<?>, JsonSerializer<Object>>();
+			this.forcedMultipleLinkRels = forcedMultipleLinkRels;
 		}
 
 		/*
@@ -360,14 +379,13 @@ public void serialize(Object value, JsonGenerator jgen, SerializerProvider provi
 				return;
 			}
 
-			if (list.size() == 1) {
-				serializeContents(list.iterator(), jgen, provider);
-				return;
+			if(list.size() > 1 || ((list.get(0) instanceof Link) && forcedMultipleLinkRels.contains(((Link) list.get(0)).getRel()))) {
+                jgen.writeStartArray();
+                serializeContents(list.iterator(), jgen, provider);
+                jgen.writeEndArray();
+			} else {
+                serializeContents(list.iterator(), jgen, provider);
 			}
-
-			jgen.writeStartArray();
-			serializeContents(list.iterator(), jgen, provider);
-			jgen.writeEndArray();
 		}
 
 		private void serializeContents(Iterator<?> value, JsonGenerator jgen, SerializerProvider provider)

src/test/java/org/springframework/hateoas/hal/Jackson2HalIntegrationTest.java

@@ -26,15 +26,8 @@
 
 import org.junit.Before;
 import org.junit.Test;
-import org.springframework.hateoas.AbstractJackson2MarshallingIntegrationTest;
-import org.springframework.hateoas.Link;
-import org.springframework.hateoas.Links;
-import org.springframework.hateoas.PagedResources;
+import org.springframework.hateoas.*;
 import org.springframework.hateoas.PagedResources.PageMetadata;
-import org.springframework.hateoas.Resource;
-import org.springframework.hateoas.ResourceSupport;
-import org.springframework.hateoas.Resources;
-import org.springframework.hateoas.UriTemplate;
 import org.springframework.hateoas.core.AnnotationRelProvider;
 import org.springframework.hateoas.hal.Jackson2HalModule.HalHandlerInstantiator;
 
@@ -49,6 +42,7 @@
 public class Jackson2HalIntegrationTest extends AbstractJackson2MarshallingIntegrationTest {
 
 	static final String SINGLE_LINK_REFERENCE = "{\"_links\":{\"self\":{\"href\":\"localhost\"}}}";
+	static final String SINGLE_LINK_REFERENCE_AS_MULTIPLE = "{\"_links\":{\"multiple\":[{\"href\":\"localhost\"}]}}";
 	static final String LIST_LINK_REFERENCE = "{\"_links\":{\"self\":[{\"href\":\"localhost\"},{\"href\":\"localhost2\"}]}}";
 
 	static final String SIMPLE_EMBEDDED_RESOURCE_REFERENCE = "{\"_links\":{\"self\":{\"href\":\"localhost\"}},\"_embedded\":{\"content\":[\"first\",\"second\"]}}";
@@ -95,6 +89,13 @@ public void deserializeSingleLink() throws Exception {
 		assertThat(read(SINGLE_LINK_REFERENCE, ResourceSupport.class), is(expected));
 	}
 
+	@Test
+	public void rendersSingleLinkAsArrayWhenForced() throws Exception {
+		ResourceWithForcedMultipleLink expected = new ResourceWithForcedMultipleLink();
+		expected.add(new Link("localhost").withRel("multiple"));
+		assertThat(read(SINGLE_LINK_REFERENCE_AS_MULTIPLE, ResourceWithForcedMultipleLink.class), is(expected));
+	}
+
 	/**
 	 * @see #29
 	 */
@@ -384,4 +385,8 @@ private static ObjectMapper getCuriedObjectMapper(CurieProvider provider) {
 
 		return mapper;
 	}
+
+	@ForceMultipleLinksOnRels({"multiple"})
+	private static class ResourceWithForcedMultipleLink extends ResourceSupport {
+	}
 }