Allow customization of LinkCollector.

LinkCollector is now an interface. The actual implementation has been moved to DefaultLinkCollector. RepositoryRestConfigurer now has a customizeLinkCollector(…) callback method to tweak or even completely replace the LinkCollector instance.

Fixes #2042.

Author: odrotbohm

spring-data-rest-tests/spring-data-rest-tests-core/src/test/java/org/springframework/data/rest/tests/RepositoryTestsConfig.java

@@ -43,6 +43,7 @@
 import org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module;
 import org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module.LookupObjectSerializer;
 import org.springframework.data.rest.webmvc.mapping.Associations;
+import org.springframework.data.rest.webmvc.mapping.DefaultLinkCollector;
 import org.springframework.data.rest.webmvc.mapping.LinkCollector;
 import org.springframework.data.rest.webmvc.spi.BackendIdConverter.DefaultIdConverter;
 import org.springframework.data.rest.webmvc.support.ExcerptProjector;
@@ -120,7 +121,7 @@ public Module persistentEntityModule() {
 				repositories());
 
 		Associations associations = new Associations(mappings, config());
-		LinkCollector collector = new LinkCollector(persistentEntities(), selfLinkProvider, associations);
+		LinkCollector collector = new DefaultLinkCollector(persistentEntities(), selfLinkProvider, associations);
 
 		return new PersistentEntityJackson2Module(associations, persistentEntities(), uriToEntityConverter, collector,
 				invokerFactory, mock(LookupObjectSerializer.class),

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

@@ -43,6 +43,7 @@
 import org.springframework.data.rest.webmvc.jpa.PersonRepository;
 import org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module.LookupObjectSerializer;
 import org.springframework.data.rest.webmvc.mapping.Associations;
+import org.springframework.data.rest.webmvc.mapping.DefaultLinkCollector;
 import org.springframework.data.rest.webmvc.mapping.LinkCollector;
 import org.springframework.data.rest.webmvc.spi.BackendIdConverter.DefaultIdConverter;
 import org.springframework.data.rest.webmvc.support.ExcerptProjector;
@@ -127,7 +128,7 @@ public Module persistentEntityModule() {
 				repositories());
 
 		Associations associations = new Associations(mappings, config());
-		LinkCollector collector = new LinkCollector(persistentEntities(), selfLinkProvider, associations);
+		LinkCollector collector = new DefaultLinkCollector(persistentEntities(), selfLinkProvider, associations);
 
 		return new PersistentEntityJackson2Module(associations, persistentEntities(), uriToEntityConverter, collector,
 				invokerFactory, mock(LookupObjectSerializer.class),

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

@@ -23,6 +23,7 @@
 import org.springframework.data.auditing.AuditableBeanWrapperFactory;
 import org.springframework.data.rest.core.config.RepositoryRestConfiguration;
 import org.springframework.data.rest.core.event.ValidatingRepositoryEventListener;
+import org.springframework.data.rest.webmvc.mapping.LinkCollector;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.util.Assert;
 import org.springframework.web.servlet.config.annotation.CorsRegistry;
@@ -144,4 +145,15 @@ default void configureJacksonObjectMapper(ObjectMapper objectMapper) {}
 	default AuditableBeanWrapperFactory customizeAuditableBeanWrapperFactory(AuditableBeanWrapperFactory factory) {
 		return factory;
 	}
+
+	/**
+	 * Customize the {@link LinkCollector} to be used.
+	 *
+	 * @param collector will never be {@literal null}.
+	 * @return must not be {@literal null}.
+	 * @since 3.5
+	 */
+	default LinkCollector customizeLinkCollector(LinkCollector collector) {
+		return collector;
+	}
 }

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

@@ -21,6 +21,7 @@
 import org.springframework.data.auditing.AuditableBeanWrapperFactory;
 import org.springframework.data.rest.core.config.RepositoryRestConfiguration;
 import org.springframework.data.rest.core.event.ValidatingRepositoryEventListener;
+import org.springframework.data.rest.webmvc.mapping.LinkCollector;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.util.Assert;
 import org.springframework.web.servlet.config.annotation.CorsRegistry;
@@ -136,4 +137,18 @@ public AuditableBeanWrapperFactory customizeAuditableBeanWrapperFactory(Auditabl
 
 		return factory;
 	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.rest.webmvc.config.RepositoryRestConfigurer#customizeLinkCollector(org.springframework.data.rest.webmvc.mapping.LinkCollector)
+	 */
+	@Override
+	public LinkCollector customizeLinkCollector(LinkCollector collector) {
+
+		for (RepositoryRestConfigurer configurer : delegates) {
+			collector = configurer.customizeLinkCollector(collector);
+		}
+
+		return collector;
+	}
 }

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

@@ -74,6 +74,7 @@
 import org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module.LookupObjectSerializer;
 import org.springframework.data.rest.webmvc.json.PersistentEntityToJsonSchemaConverter.ValueTypeSchemaPropertyCustomizerFactory;
 import org.springframework.data.rest.webmvc.mapping.Associations;
+import org.springframework.data.rest.webmvc.mapping.DefaultLinkCollector;
 import org.springframework.data.rest.webmvc.mapping.LinkCollector;
 import org.springframework.data.rest.webmvc.spi.BackendIdConverter;
 import org.springframework.data.rest.webmvc.spi.BackendIdConverter.DefaultIdConverter;
@@ -727,7 +728,9 @@ protected Module persistentEntityJackson2Module(LinkCollector linkCollector) {
 	@Bean
 	protected LinkCollector linkCollector(PersistentEntities persistentEntities, SelfLinkProvider selfLinkProvider,
 			Associations associationLinks) {
-		return new LinkCollector(persistentEntities, selfLinkProvider, associationLinks);
+
+		return configurerDelegate.get()
+				.customizeLinkCollector(new DefaultLinkCollector(persistentEntities, selfLinkProvider, associationLinks));
 	}
 
 	@Bean

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

@@ -0,0 +1,254 @@
+/*
+ * 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.mapping;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+import org.springframework.data.mapping.Association;
+import org.springframework.data.mapping.MappingException;
+import org.springframework.data.mapping.PersistentEntity;
+import org.springframework.data.mapping.PersistentProperty;
+import org.springframework.data.mapping.PersistentPropertyAccessor;
+import org.springframework.data.mapping.SimpleAssociationHandler;
+import org.springframework.data.mapping.context.PersistentEntities;
+import org.springframework.data.rest.core.Path;
+import org.springframework.data.rest.core.mapping.ResourceMapping;
+import org.springframework.data.rest.core.mapping.ResourceMetadata;
+import org.springframework.data.rest.core.support.SelfLinkProvider;
+import org.springframework.hateoas.IanaLinkRelations;
+import org.springframework.hateoas.Link;
+import org.springframework.hateoas.Links;
+import org.springframework.util.Assert;
+
+/**
+ * A service to collect all standard links that need to be added to a certain object.
+ *
+ * @author Oliver Drotbohm
+ * @since 3.6
+ */
+public class DefaultLinkCollector implements LinkCollector {
+
+	private final PersistentEntities entities;
+	private final Associations associationLinks;
+	private final SelfLinkProvider links;
+
+	/**
+	 * Creates a new {@link DefaultLinkCollector} for the given {@link PersistentEntities}, {@link SelfLinkProvider} and
+	 * {@link Associations}.
+	 *
+	 * @param entities must not be {@literal null}.
+	 * @param linkProvider must not be {@literal null}.
+	 * @param associationLinks must not be {@literal null}.
+	 */
+	public DefaultLinkCollector(PersistentEntities entities, SelfLinkProvider linkProvider,
+			Associations associationLinks) {
+
+		Assert.notNull(entities, "PersistentEntities must not be null!");
+		Assert.notNull(linkProvider, "SelfLinkProvider must not be null!");
+		Assert.notNull(associationLinks, "AssociationLinks must not be null!");
+
+		this.links = linkProvider;
+		this.entities = entities;
+		this.associationLinks = associationLinks;
+	}
+
+	/**
+	 * Returns all {@link Links} for the given object.
+	 *
+	 * @param object must not be {@literal null}.
+	 * @return
+	 */
+	@Override
+	public Links getLinksFor(Object object) {
+		return getLinksFor(object, Links.NONE);
+	}
+
+	/**
+	 * Returns all {@link Links} for the given object and already existing {@link Link}.
+	 *
+	 * @param object must not be {@literal null}.
+	 * @param existingLinks must not be {@literal null}.
+	 * @return
+	 */
+	@Override
+	public Links getLinksFor(Object object, Links existingLinks) {
+
+		Assert.notNull(object, "Object must not be null!");
+		Assert.notNull(existingLinks, "Existing links must not be null!");
+
+		Link selfLink = createSelfLink(object, existingLinks);
+
+		if (selfLink == null) {
+			return existingLinks;
+		}
+
+		Path path = new Path(selfLink.expand().getHref());
+
+		LinkCollectingAssociationHandler handler = new LinkCollectingAssociationHandler(path, associationLinks);
+		entities.getRequiredPersistentEntity(object.getClass()).doWithAssociations(handler);
+
+		return addSelfLinkIfNecessary(object, existingLinks.and(handler.getLinks()));
+	}
+
+	@Override
+	public Links getLinksForNested(Object object, Links existing) {
+
+		PersistentEntity<?, ?> entity = entities.getRequiredPersistentEntity(object.getClass());
+
+		NestedLinkCollectingAssociationHandler handler = new NestedLinkCollectingAssociationHandler(links,
+				entity.getPropertyAccessor(object), associationLinks);
+		entity.doWithAssociations(handler);
+
+		return existing.and(handler.getLinks());
+	}
+
+	private Links addSelfLinkIfNecessary(Object object, Links existing) {
+		return existing.andIf(!existing.hasLink(IanaLinkRelations.SELF),
+				() -> links.createSelfLinkFor(object).withSelfRel());
+	}
+
+	private Link createSelfLink(Object object, Links existing) {
+
+		return existing.getLink(IanaLinkRelations.SELF) //
+				.orElseGet(() -> links.createSelfLinkFor(object).withSelfRel());
+	}
+
+	/**
+	 * {@link SimpleAssociationHandler} that will collect {@link Link}s for all linkable associations.
+	 *
+	 * @author Oliver Gierke
+	 * @since 2.1
+	 */
+	private static class LinkCollectingAssociationHandler implements SimpleAssociationHandler {
+
+		private static final String AMBIGUOUS_ASSOCIATIONS = "Detected multiple association links with same relation type! Disambiguate association %s using @RestResource!";
+
+		private final Path basePath;
+		private final Associations associationLinks;
+		private final List<Link> links = new ArrayList<Link>();
+
+		public LinkCollectingAssociationHandler(Path basePath, Associations associationLinks) {
+
+			Assert.notNull(basePath, "Base Path must not be null!");
+			Assert.notNull(associationLinks, "Associations must not be null!");
+
+			this.basePath = basePath;
+			this.associationLinks = associationLinks;
+		}
+
+		/**
+		 * Returns the links collected after the {@link Association} has been traversed.
+		 *
+		 * @return the links
+		 */
+		public Links getLinks() {
+			return Links.of(links);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mapping.SimpleAssociationHandler#doWithAssociation(org.springframework.data.mapping.Association)
+		 */
+		@Override
+		public void doWithAssociation(final Association<? extends PersistentProperty<?>> association) {
+
+			if (associationLinks.isLinkableAssociation(association)) {
+
+				PersistentProperty<?> property = association.getInverse();
+				Links existingLinks = Links.of(links);
+
+				for (Link link : associationLinks.getLinksFor(association, basePath)) {
+					if (existingLinks.hasLink(link.getRel())) {
+						throw new MappingException(String.format(AMBIGUOUS_ASSOCIATIONS, property.toString()));
+					} else {
+						links.add(link);
+					}
+				}
+			}
+		}
+	}
+
+	private static class NestedLinkCollectingAssociationHandler implements SimpleAssociationHandler {
+
+		private final SelfLinkProvider selfLinks;
+		private final PersistentPropertyAccessor<?> accessor;
+		private final Associations associations;
+		private Links links = Links.NONE;
+
+		public NestedLinkCollectingAssociationHandler(SelfLinkProvider selfLinks,
+				PersistentPropertyAccessor<?> accessor, Associations associations) {
+
+			Assert.notNull(selfLinks, "SelfLinkProvider must not be null!");
+			Assert.notNull(accessor, "PersistentPropertyAccessor must not be null!");
+			Assert.notNull(associations, "Associations must not be null!");
+
+			this.selfLinks = selfLinks;
+			this.accessor = accessor;
+			this.associations = associations;
+		}
+
+		public List<Link> getLinks() {
+			return this.links.toList();
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mapping.SimpleAssociationHandler#doWithAssociation(org.springframework.data.mapping.Association)
+		 */
+		@Override
+		public void doWithAssociation(Association<? extends PersistentProperty<?>> association) {
+
+			if (!associations.isLinkableAssociation(association)) {
+				return;
+			}
+
+			PersistentProperty<?> property = association.getInverse();
+			Object value = accessor.getProperty(property);
+
+			if (value == null) {
+				return;
+			}
+
+			ResourceMetadata metadata = associations.getMappings().getMetadataFor(property.getOwner().getType());
+			ResourceMapping propertyMapping = metadata.getMappingFor(property);
+
+			for (Object element : asCollection(value)) {
+
+				links = links.andIf(element != null,
+						() -> selfLinks.createSelfLinkFor(property.getAssociationTargetType(), element)
+								.withRel(propertyMapping.getRel()));
+			}
+		}
+
+		/**
+		 * Returns the given object as {@link Collection}, i.e. the object as is if it's a collection already or wrapped
+		 * into a single-element collection otherwise.
+		 *
+		 * @param object can be {@literal null}.
+		 * @return
+		 */
+		@SuppressWarnings("unchecked")
+		private static Collection<Object> asCollection(Object object) {
+
+			return object instanceof Collection //
+					? (Collection<Object>) object //
+					: Collections.singleton(object);
+		}
+	}
+}