Polishing.

Add Override annotations and comment why we optimize.

See #4612
Original pull request: #4613

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/ReferenceLoader.java

@@ -130,7 +130,7 @@ public interface ReferenceLoader {
 
 		/**
 		 * @return a {@link DocumentReferenceQuery} that will not match any documents.
-		 * @since 4.3
+		 * @since 4.2.5
 		 */
 		static DocumentReferenceQuery forNoResult() {
 			return NoResultsFilter.INSTANCE;
@@ -139,8 +139,8 @@ public interface ReferenceLoader {
 
 	/**
 	 * A dedicated {@link DocumentReferenceQuery} that will not match any documents.
-	 * 
-	 * @since 4.3
+	 *
+	 * @since 4.2.5
 	 */
 	enum NoResultsFilter implements DocumentReferenceQuery {
 		INSTANCE;

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/ReferenceLookupDelegate.java

@@ -240,7 +240,8 @@ public final class ReferenceLookupDelegate {
 
 	EvaluationContext evaluationContextFor(MongoPersistentProperty property, Object source, SpELContext spELContext) {
 
-		Object target = source instanceof DocumentReferenceSource documentReferenceSource ? documentReferenceSource.getTargetSource()
+		Object target = source instanceof DocumentReferenceSource documentReferenceSource
+				? documentReferenceSource.getTargetSource()
 				: source;
 
 		if (target == null) {
@@ -285,6 +286,7 @@ public final class ReferenceLookupDelegate {
 
 			Collection<Object> objects = (Collection<Object>) value;
 
+			// optimization: bypass query if the collection pointing to the references is empty
 			if (objects.isEmpty()) {
 				return DocumentReferenceQuery.forNoResult();
 			}
@@ -301,7 +303,7 @@ public final class ReferenceLookupDelegate {
 
 		if (property.isMap() && value instanceof Map) {
 
-			if(ObjectUtils.isEmpty(value)) {
+			if (ObjectUtils.isEmpty(value)) {
 				return DocumentReferenceQuery.forNoResult();
 			}
 
@@ -461,6 +463,7 @@ public final class ReferenceLookupDelegate {
 			return target.stream().sorted((o1, o2) -> compareAgainstReferenceIndex(ors, o1, o2)).collect(Collectors.toList());
 		}
 
+		@Override
 		public Document getQuery() {
 			return query;
 		}