Minor corrections on the nullability declarations in relation to Spring HATEOAS.

Related ticket #2450.

Author: odrotbohm

spring-data-rest-core/src/main/java/org/springframework/data/rest/core/support/DefaultSelfLinkProvider.java

@@ -18,7 +18,6 @@
 import java.util.List;
 
 import org.jspecify.annotations.Nullable;
-
 import org.springframework.core.convert.ConversionService;
 import org.springframework.data.mapping.PersistentEntity;
 import org.springframework.data.mapping.PersistentProperty;
@@ -74,7 +73,14 @@ public Link createSelfLinkFor(Object instance) {
 	public Link createSelfLinkFor(Class<?> type, Object reference) {
 
 		if (type.isInstance(reference)) {
-			return entityLinks.linkToItemResource(type, getResourceId(type, reference));
+
+			var identifier = getResourceId(type, reference);
+
+			if (identifier == null) {
+				throw new IllegalArgumentException("Cannot resolve identifier from reference %s".formatted(reference));
+			}
+
+			return entityLinks.linkToItemResource(type, identifier);
 		}
 
 		PersistentEntity<?, ?> entity = entities.getRequiredPersistentEntity(type);
@@ -86,19 +92,25 @@ public Link createSelfLinkFor(Class<?> type, Object reference) {
 			identifier = getResourceId(type, conversionService.convert(identifier, type));
 		}
 
+		if (identifier == null) {
+			throw new IllegalArgumentException("Cannot resolve identifier from reference %s".formatted(reference));
+		}
+
 		return entityLinks.linkToItemResource(type, identifier);
 	}
 
 	/**
 	 * Returns the identifier to be used to create the self link URI.
 	 *
-	 * @param reference must not be {@literal null}.
-	 * @return
+	 * @param reference
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	private Object getResourceId(Class<?> type, @Nullable Object reference) {
 
-		Assert.notNull(reference, "Reference must not be null");
+		if (reference == null) {
+			return null;
+		}
 
 		if (!lookups.hasPluginFor(type)) {
 			return entityIdentifierOrNull(reference);

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

@@ -15,16 +15,15 @@
  */
 package org.springframework.data.rest.webmvc.support;
 
-import java.net.URI;
 import java.util.Collections;
 import java.util.List;
 
+import org.jspecify.annotations.Nullable;
 import org.springframework.data.mapping.PersistentProperty;
 import org.springframework.data.rest.core.mapping.ResourceMetadata;
 import org.springframework.data.rest.webmvc.BaseUri;
 import org.springframework.hateoas.Affordance;
 import org.springframework.hateoas.Link;
-import org.springframework.hateoas.server.LinkBuilder;
 import org.springframework.hateoas.server.core.LinkBuilderSupport;
 import org.springframework.util.Assert;
 import org.springframework.web.util.UriComponents;
@@ -64,7 +63,7 @@ private RepositoryLinkBuilder(ResourceMetadata metadata, UriComponents component
 	}
 
 	@Override
-	public RepositoryLinkBuilder slash(Object object) {
+	public RepositoryLinkBuilder slash(@Nullable Object object) {
 
 		return PersistentProperty.class.isInstance(object) //
 				? slash((PersistentProperty<?>) object)