Add MergedAnnotation.getRoot() method

Update `MergedAnnotation` with a `getRoot()` method that allows the
root direct annotation to be retrieved easily.

Closes gh-22818

Author: philwebb

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

@@ -140,6 +140,13 @@ public interface MergedAnnotation<A extends Annotation> {
 	@Nullable
 	MergedAnnotation<?> getParent();
 
+	/**
+	 * Get the root annotation, i.e. the {@link #getDepth() depth} {@code 0}
+	 * annotation as directly declared on the source.
+	 * @return the root annotation
+	 */
+	MergedAnnotation<?> getRoot();
+
 	/**
 	 * Determine if the specified attribute name has a non-default value when
 	 * compared to the annotation declaration.

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

@@ -66,6 +66,11 @@ public MergedAnnotation<?> getParent() {
 		return null;
 	}
 
+	@Override
+	public MergedAnnotation<?> getRoot() {
+		return this;
+	}
+
 	@Override
 	public int getDepth() {
 		return -1;

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

@@ -171,6 +171,16 @@ public MergedAnnotation<?> getParent() {
 				this.valueExtractor, this.aggregateIndex, this.resolvedRootMirrors);
 	}
 
+	@Override
+	public MergedAnnotation<?> getRoot() {
+		if (getDepth() == 0) {
+			return this;
+		}
+		AnnotationTypeMapping rootMapping = this.mapping.getRoot();
+		return new TypeMappedAnnotation<>(rootMapping, this.source, this.rootAttributes,
+				this.valueExtractor, this.aggregateIndex, this.resolvedRootMirrors);
+	}
+
 	@Override
 	public boolean hasDefaultValue(String attributeName) {
 		int attributeIndex = getAttributeIndex(attributeName, true);

spring-core/src/test/java/org/springframework/core/annotation/MergedAnnotationsTests.java

@@ -154,6 +154,22 @@ public void getParent() {
 				.isEqualTo(ComposedTransactionalComponent.class);
 	}
 
+	@Test
+	public void getRootWhenNotDirect() {
+		MergedAnnotations annotations = MergedAnnotations.from(ComposedTransactionalComponentClass.class);
+		MergedAnnotation<?> annotation = annotations.get(TransactionalComponent.class);
+		assertThat(annotation.getDepth()).isGreaterThan(0);
+		assertThat(annotation.getRoot().getType()).isEqualTo(ComposedTransactionalComponent.class);
+	}
+
+	@Test
+	public void getRootWhenDirect() {
+		MergedAnnotations annotations = MergedAnnotations.from(ComposedTransactionalComponentClass.class);
+		MergedAnnotation<?> annotation = annotations.get(ComposedTransactionalComponent.class);
+		assertThat(annotation.getDepth()).isEqualTo(0);
+		assertThat(annotation.getRoot()).isSameAs(annotation);
+	}
+
 	@Test
 	public void collectMultiValueMapFromNonAnnotatedClass() {
 		MultiValueMap<String, Object> map = MergedAnnotations.from(

spring-core/src/test/java/org/springframework/core/annotation/MissingMergedAnnotationTests.java

@@ -78,6 +78,11 @@ public void getParentReturnsNull() {
 		assertThat(this.missing.getParent()).isNull();
 	}
 
+	@Test
+	public void getRootReturnsEmptyAnnotation() {
+		assertThat(this.missing.getRoot()).isSameAs(this.missing);
+	}
+
 	@Test
 	public void hasNonDefaultValueThrowsNoSuchElementException() {
 		assertThatNoSuchElementException().isThrownBy(