Document getAttributeAliasMap() in AnnotationUtils

This commit also introduces a cache for attribute alias metadata.

Issue: SPR-11512

spring-core/src/main/java/org/springframework/core/annotation/AnnotationUtils.java

@@ -118,6 +118,9 @@ public abstract class AnnotationUtils {
 	private static final Map<Class<? extends Annotation>, Boolean> synthesizableCache =
 			new ConcurrentReferenceHashMap<Class<? extends Annotation>, Boolean>(256);
 
+	private static final Map<Class<? extends Annotation>, Map<String, String>> attributeAliasCache =
+			new ConcurrentReferenceHashMap<Class<? extends Annotation>, Map<String, String>>(256);
+
 	private static transient Log logger;
 
 
@@ -1058,20 +1061,38 @@ public abstract class AnnotationUtils {
 			return annotation;
 		}
 
-		InvocationHandler handler = new SynthesizedAnnotationInvocationHandler(annotatedElement, annotation, getAliasMap(annotationType));
+		InvocationHandler handler = new SynthesizedAnnotationInvocationHandler(annotatedElement, annotation,
+			getAttributeAliasMap(annotationType));
 		A synthesizedAnnotation = (A) Proxy.newProxyInstance(ClassUtils.getDefaultClassLoader(), new Class<?>[] {
 			(Class<A>) annotationType, SynthesizedAnnotation.class }, handler);
 
 		return synthesizedAnnotation;
 	}
 
+
 	/**
-	 * TODO Document getAliasMap().
+	 * Get a map of all attribute alias pairs, declared via {@code @AliasFor}
+	 * in the supplied annotation type.
+	 *
+	 * <p>The map is keyed by attribute name with each value representing
+	 * the name of the aliased attribute. For each entry {@code [x, y]} in
+	 * the map there will be a corresponding {@code [y, x]} entry in the map.
+	 *
+	 * <p>An empty return value implies that the annotation does not declare
+	 * any attribute aliases.
+	 *
+	 * @param annotationType the annotation type to find attribute aliases in
+	 * @return a map containing attribute alias pairs; never {@code null}
 	 * @since 4.2
 	 */
-	private static Map<String, String> getAliasMap(Class<? extends Annotation> annotationType) {
+	private static Map<String, String> getAttributeAliasMap(Class<? extends Annotation> annotationType) {
 		if (annotationType == null) {
-			return null;
+			return Collections.emptyMap();
+		}
+
+		Map<String, String> cachedMap = attributeAliasCache.get(annotationType);
+		if (cachedMap != null) {
+			return cachedMap;
 		}
 
 		Map<String, String> map = new HashMap<String, String>();
@@ -1082,6 +1103,9 @@ public abstract class AnnotationUtils {
 				map.put(attributeName, aliasedAttributeName);
 			}
 		}
+
+		attributeAliasCache.put(annotationType, map);
+
 		return map;
 	}
 
@@ -1283,9 +1307,9 @@ public abstract class AnnotationUtils {
 	/**
 	 * TODO Document postProcessAnnotationAttributes().
 	 *
-	 * @param annotatedElement the element that is annotated with the supplied
+	 * @param element the element that is annotated with the supplied annotation,
-	 * annotation, used for contextual logging; may be {@code null} if unknown
+	 * used for contextual logging; may be {@code null} if unknown
-	 * @param attributes the annotation attributes to validate
+	 * @param attributes the annotation attributes to post-process
 	 * @since 4.2
 	 */
 	static void postProcessAnnotationAttributes(AnnotatedElement element, AnnotationAttributes attributes,
@@ -1297,12 +1321,10 @@ public abstract class AnnotationUtils {
 		}
 
 		Class<? extends Annotation> annotationType = attributes.annotationType();
-		Map<String, String> aliasMap = getAliasMap(annotationType);
 
 		// Validate @AliasFor configuration
-		if (aliasMap != null) {
+		Map<String, String> aliasMap = getAttributeAliasMap(annotationType);
 		Set<String> validated = new HashSet<String>();
-
 		for (String attributeName : aliasMap.keySet()) {
 			String aliasedAttributeName = aliasMap.get(attributeName);
 
@@ -1332,7 +1354,6 @@ public abstract class AnnotationUtils {
 				}
 			}
 		}
-		}
 
 		for (String attributeName : attributes.keySet()) {
 			Object value = attributes.get(attributeName);