#1220 - Backport String constants for IanalinkRelations.

This commit adds String constants to IanaLinkRelations, so that IanaLinkRelations can be leveraged in places where compile time constants are required, such as Relation#itemRelation and Relation#collectionRelation.

Original issue: #1216

src/test/java/org/springframework/hateoas/IanaLinkRelationUnitTest.java

@@ -20,9 +20,17 @@ import static org.assertj.core.api.Assertions.*;
 import lombok.Value;
 
 import org.junit.jupiter.api.Test;
+import org.springframework.util.ReflectionUtils;
+
+import java.lang.reflect.Field;
+import java.util.Arrays;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
 
 /**
  * @author Greg Turnquist
+ * @author Vedran Pavic
  */
 class IanaLinkRelationUnitTest {
 
@@ -31,7 +39,7 @@ class IanaLinkRelationUnitTest {
 	 */
 	@Test
 	void extractingValueOfIanaLinkRelationShouldWork() {
-		assertThat(IanaLinkRelations.ABOUT.value()).isEqualTo("about");
+		assertThat(IanaLinkRelations.ABOUT.value()).isEqualTo(IanaLinkRelations.ABOUT_VALUE);
 	}
 
 	/**
@@ -77,6 +85,32 @@ class IanaLinkRelationUnitTest {
 		assertThat(IanaLinkRelations.isIanaRel(new CustomLinkRelation("something-new"))).isFalse();
 	}
 
+	/**
+	 * @see #1216
+	 */
+	@Test
+	void testAllIanaLinkRelationsHaveStringConstant() {
+
+		Set<String> linkRelations = Arrays.stream(IanaLinkRelations.class.getDeclaredFields()) //
+				.filter(ReflectionUtils::isPublicStaticFinal) //
+				.filter(field -> LinkRelation.class.equals(field.getType())) //
+				.map(it -> ReflectionUtils.getField(it, null)) //
+				.filter(Objects::nonNull) //
+				.map(LinkRelation.class::cast) //
+				.map(LinkRelation::value) //
+				.collect(Collectors.toSet());
+
+		Set<String> stringConstants = Arrays.stream(IanaLinkRelations.class.getDeclaredFields()) //
+				.filter(ReflectionUtils::isPublicStaticFinal) //
+				.filter(field -> String.class.equals(field.getType()))
+				.map(it -> ReflectionUtils.getField(it, null)) //
+				.filter(Objects::nonNull) //
+				.map(String.class::cast) //
+				.collect(Collectors.toSet());
+
+		assertThat(linkRelations).containsExactlyElementsOf(stringConstants);
+	}
+
 	/**
 	 * Custom implementation of the {@link LinkRelation} interface.
 	 */

src/test/java/org/springframework/hateoas/server/core/AnnotationLinkRelationProviderUnitTest.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2020 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.server.core;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.hateoas.IanaLinkRelations;
+import org.springframework.hateoas.LinkRelation;
+import org.springframework.hateoas.server.LinkRelationProvider;
+import org.springframework.hateoas.server.LinkRelationProvider.LookupContext;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Vedran Pavic
+ */
+class AnnotationLinkRelationProviderUnitTest {
+
+	@Test
+	void ianaLinkRelationsCanBeUsedForItemAndCollectionRelations() {
+
+		LinkRelationProvider linkRelationProvider = new AnnotationLinkRelationProvider();
+
+		assertThat(linkRelationProvider.getItemResourceRelFor(Resource.class)).isEqualTo(IanaLinkRelations.ITEM);
+		assertThat(linkRelationProvider.getCollectionResourceRelFor(Resource.class)).isEqualTo(IanaLinkRelations.COLLECTION);
+	}
+
+	@Relation(itemRelation = IanaLinkRelations.ITEM_VALUE, collectionRelation = IanaLinkRelations.COLLECTION_VALUE)
+	static class Resource {}
+}