Consistent exposure of empty attribute arrays in AnnotationMetadata

Issue: SPR-17347

Author: jhoeller

spring-core/src/main/java/org/springframework/core/type/classreading/RecursiveAnnotationArrayVisitor.java

@@ -16,6 +16,7 @@
 
 package org.springframework.core.type.classreading;
 
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Array;
 import java.util.ArrayList;
 import java.util.List;
@@ -82,6 +83,20 @@ public void visitEnd() {
 		if (!this.allNestedAttributes.isEmpty()) {
 			this.attributes.put(this.attributeName, this.allNestedAttributes.toArray(new AnnotationAttributes[0]));
 		}
+		else if (!this.attributes.containsKey(this.attributeName)) {
+			Class<? extends Annotation> annotationType = this.attributes.annotationType();
+			if (annotationType != null) {
+				try {
+					Class<?> attributeType = annotationType.getMethod(this.attributeName).getReturnType();
+					if (attributeType.isArray()) {
+						this.attributes.put(this.attributeName, Array.newInstance(attributeType.getComponentType(), 0));
+					}
+				}
+				catch (NoSuchMethodException ex) {
+					// Corresponding attribute method not found: cannot expose empty array.
+				}
+			}
+		}
 	}
 
 }

spring-core/src/test/java/org/springframework/core/type/AnnotationMetadataTests.java

@@ -338,7 +338,9 @@ private void doTestAnnotationInfo(AnnotationMetadata metadata) {
 			allMeta = metadata.getAllAnnotationAttributes(DirectAnnotation.class.getName()).get("value");
 			assertThat(new HashSet<>(allMeta), is(equalTo(new HashSet<Object>(Arrays.asList("direct", "meta")))));
 			allMeta = metadata.getAllAnnotationAttributes(DirectAnnotation.class.getName()).get("additional");
-			assertThat(new HashSet<>(allMeta), is(equalTo(new HashSet<Object>(Arrays.asList("direct")))));
+			assertThat(new HashSet<>(allMeta), is(equalTo(new HashSet<Object>(Arrays.asList("direct", "")))));
+			assertEquals("", metadata.getAnnotationAttributes(DirectAnnotation.class.getName()).get("additional"));
+			assertEquals(0, ((String[]) metadata.getAnnotationAttributes(DirectAnnotation.class.getName()).get("additionalArray")).length);
 		}
 		{ // perform tests with classValuesAsString = true
 			AnnotationAttributes specialAttrs = (AnnotationAttributes) metadata.getAnnotationAttributes(
@@ -425,6 +427,8 @@ public static enum SomeEnum {
 		String myValue() default "";
 
 		String additional() default "direct";
+
+		String[] additionalArray() default "direct";
 	}
 
 	@Target(ElementType.TYPE)
@@ -470,7 +474,7 @@ public enum SubclassEnum {
 			nestedAnno = @NestedAnno(value = "na", anEnum = SomeEnum.LABEL1, classArray = {String.class}),
 			nestedAnnoArray = {@NestedAnno, @NestedAnno(value = "na1", anEnum = SomeEnum.LABEL2, classArray = {Number.class})})
 	@SuppressWarnings({"serial", "unused"})
-	@DirectAnnotation("direct")
+	@DirectAnnotation(value = "direct", additional = "", additionalArray = {})
 	@MetaMetaAnnotation
 	@EnumSubclasses({SubclassEnum.FOO, SubclassEnum.BAR})
 	private static class AnnotatedComponent implements Serializable {