Handle [] leniently in constructor binding

See gh-34305

Author: rstoyanchev

spring-context/src/main/java/org/springframework/validation/DataBinder.java

@@ -142,6 +142,10 @@ public class DataBinder implements PropertyEditorRegistry, TypeConverter {
 	 */
 	protected static final Log logger = LogFactory.getLog(DataBinder.class);
 
+	/** Internal constant for constructor binding via "[]". */
+	private static final int NO_INDEX = -1;
+
+
 	@Nullable
 	private Object target;
 
@@ -1056,15 +1060,17 @@ private List<?> createList(
 			return null;
 		}
 
-		int size = (indexes.last() < this.autoGrowCollectionLimit ? indexes.last() + 1 : 0);
+		int lastIndex = Math.max(indexes.last(), 0);
+		int size = (lastIndex < this.autoGrowCollectionLimit ? lastIndex + 1 : 0);
 		List<?> list = (List<?>) CollectionFactory.createCollection(paramType, size);
 		for (int i = 0; i < size; i++) {
 			list.add(null);
 		}
 
 		for (int index : indexes) {
-			String indexedPath = paramPath + "[" + index + "]";
-			list.set(index, createIndexedValue(paramPath, paramType, elementType, indexedPath, valueResolver));
+			String indexedPath = paramPath + "[" + (index != NO_INDEX ? index : "") + "]";
+			list.set(Math.max(index, 0),
+					createIndexedValue(paramPath, paramType, elementType, indexedPath, valueResolver));
 		}
 
 		return list;
@@ -1108,12 +1114,14 @@ private <V> V[] createArray(
 			return null;
 		}
 
-		int size = (indexes.last() < this.autoGrowCollectionLimit ? indexes.last() + 1: 0);
+		int lastIndex = Math.max(indexes.last(), 0);
+		int size = (lastIndex < this.autoGrowCollectionLimit ? lastIndex + 1: 0);
 		V[] array = (V[]) Array.newInstance(elementType.resolve(), size);
 
 		for (int index : indexes) {
-			String indexedPath = paramPath + "[" + index + "]";
-			array[index] = createIndexedValue(paramPath, paramType, elementType, indexedPath, valueResolver);
+			String indexedPath = paramPath + "[" + (index != NO_INDEX ? index : "") + "]";
+			array[Math.max(index, 0)] =
+					createIndexedValue(paramPath, paramType, elementType, indexedPath, valueResolver);
 		}
 
 		return array;
@@ -1124,13 +1132,20 @@ private static SortedSet<Integer> getIndexes(String paramPath, ValueResolver val
 		SortedSet<Integer> indexes = null;
 		for (String name : valueResolver.getNames()) {
 			if (name.startsWith(paramPath + "[")) {
-				int endIndex = name.indexOf(']', paramPath.length() + 2);
-				String rawIndex = name.substring(paramPath.length() + 1, endIndex);
-				if (StringUtils.hasLength(rawIndex)) {
-					int index = Integer.parseInt(rawIndex);
-					indexes = (indexes != null ? indexes : new TreeSet<>());
-					indexes.add(index);
+				int index;
+				if (paramPath.length() + 2 == name.length()) {
+					if (!name.endsWith("[]")) {
+						continue;
+					}
+					index = NO_INDEX;
+				}
+				else {
+					int endIndex = name.indexOf(']', paramPath.length() + 2);
+					String indexValue = name.substring(paramPath.length() + 1, endIndex);
+					index = Integer.parseInt(indexValue);
 				}
+				indexes = (indexes != null ? indexes : new TreeSet<>());
+				indexes.add(index);
 			}
 		}
 		return indexes;

spring-context/src/test/java/org/springframework/validation/DataBinderConstructTests.java

@@ -188,6 +188,17 @@ void simpleListBinding() {
 		assertThat(target.integerList()).containsExactly(1, 2);
 	}
 
+	@Test
+	void simpleListBindingEmptyBrackets() {
+		MapValueResolver valueResolver = new MapValueResolver(Map.of("integerList[]", "1"));
+
+		DataBinder binder = initDataBinder(IntegerListRecord.class);
+		binder.construct(valueResolver);
+
+		IntegerListRecord target = getTarget(binder);
+		assertThat(target.integerList()).containsExactly(1);
+	}
+
 	@Test
 	void simpleMapBinding() {
 		MapValueResolver valueResolver = new MapValueResolver(Map.of("integerMap[a]", "1", "integerMap[b]", "2"));