Fix NPE for null values in Map and List collection attribute converters

.changes/next-release/bugfix-AmazonDynamoDBEnhancedClient-f887fc6.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "Amazon DynamoDB Enhanced Client",
+    "contributor": "",
+    "description": "Fix NullPointerException when converting null values in `Map` and `List` attributes by handling nulls in `MapAttributeConverter` and `ListAttributeConverter` before delegating to element converters. Fixes [#6639](https://github.com/aws/aws-sdk-java-v2/issues/6639)"
+}

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/converter/attribute/ListAttributeConverter.java

@@ -30,6 +30,7 @@
 import software.amazon.awssdk.enhanced.dynamodb.AttributeConverter;
 import software.amazon.awssdk.enhanced.dynamodb.AttributeValueType;
 import software.amazon.awssdk.enhanced.dynamodb.EnhancedType;
+import software.amazon.awssdk.enhanced.dynamodb.internal.AttributeValues;
 import software.amazon.awssdk.enhanced.dynamodb.internal.converter.TypeConvertingVisitor;
 import software.amazon.awssdk.services.dynamodb.model.AttributeValue;
 
@@ -145,7 +146,9 @@ public AttributeValueType attributeValueType() {
         @Override
         public AttributeValue transformFrom(T input) {
             return EnhancedAttributeValue.fromListOfAttributeValues(input.stream()
-                                                                         .map(elementConverter::transformFrom)
+                                                                         .map(e -> e == null
+                                                                             ? AttributeValues.nullAttributeValue()
+                                                                             : elementConverter.transformFrom(e))
                                                                          .collect(toList()))
                                          .toAttributeValue();
         }

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/converter/attribute/MapAttributeConverter.java

@@ -30,6 +30,7 @@
 import software.amazon.awssdk.enhanced.dynamodb.AttributeConverter;
 import software.amazon.awssdk.enhanced.dynamodb.AttributeValueType;
 import software.amazon.awssdk.enhanced.dynamodb.EnhancedType;
+import software.amazon.awssdk.enhanced.dynamodb.internal.AttributeValues;
 import software.amazon.awssdk.enhanced.dynamodb.internal.converter.StringConverter;
 import software.amazon.awssdk.enhanced.dynamodb.internal.converter.TypeConvertingVisitor;
 import software.amazon.awssdk.services.dynamodb.model.AttributeValue;
@@ -163,7 +164,9 @@ public EnhancedType<T> type() {
 
         public EnhancedAttributeValue toAttributeValue(T input) {
             Map<String, AttributeValue> result = new LinkedHashMap<>();
-            input.forEach((k, v) -> result.put(keyConverter.toString(k), valueConverter.transformFrom(v)));
+            input.forEach((k, v) -> result.put(keyConverter.toString(k),
+                                               v == null ? AttributeValues.nullAttributeValue()
+                                                         : valueConverter.transformFrom(v)));
             return EnhancedAttributeValue.fromMap(result);
         }
 

services-custom/dynamodb-enhanced/src/test/java/software/amazon/awssdk/enhanced/dynamodb/converters/attribute/CollectionNullValueConverterTest.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.enhanced.dynamodb.converters.attribute;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.math.BigDecimal;
+import java.time.Instant;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+import software.amazon.awssdk.enhanced.dynamodb.AttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.BigDecimalAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.BooleanAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.DoubleAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.FloatAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.InstantAsStringAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.IntegerAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.ListAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.LongAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.MapAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.StringAttributeConverter;
+import software.amazon.awssdk.enhanced.dynamodb.internal.converter.string.StringStringConverter;
+import software.amazon.awssdk.services.dynamodb.model.AttributeValue;
+
+/**
+ * Verifies that {@link MapAttributeConverter} and {@link ListAttributeConverter} correctly handle null
+ * values/elements by converting them to DynamoDB NULL type, regardless of the element converter type.
+ */
+public class CollectionNullValueConverterTest {
+
+    private static final AttributeValue NULL_ATTR = AttributeValue.builder().nul(true).build();
+
+    private static Stream<Arguments> elementConverters() {
+        return Stream.of(
+            Arguments.of("String", StringAttributeConverter.create()),
+            Arguments.of("Boolean", BooleanAttributeConverter.create()),
+            Arguments.of("Integer", IntegerAttributeConverter.create()),
+            Arguments.of("Long", LongAttributeConverter.create()),
+            Arguments.of("Float", FloatAttributeConverter.create()),
+            Arguments.of("Double", DoubleAttributeConverter.create()),
+            Arguments.of("BigDecimal", BigDecimalAttributeConverter.create()),
+            Arguments.of("Instant", InstantAsStringAttributeConverter.create())
+        );
+    }
+
+    @ParameterizedTest(name = "Map with null {0} value produces DynamoDB NULL")
+    @MethodSource("elementConverters")
+    void mapConverter_nullValue_producesNullAttributeValue(String name, AttributeConverter<?> elementConverter) {
+        @SuppressWarnings("unchecked")
+        AttributeConverter<Object> converter = (AttributeConverter<Object>) elementConverter;
+        MapAttributeConverter<Map<String, Object>> mapConverter =
+            MapAttributeConverter.mapConverter(StringStringConverter.create(), converter);
+
+        Map<String, Object> input = new HashMap<>();
+        input.put("key1", null);
+
+        AttributeValue result = mapConverter.transformFrom(input);
+
+        assertThat(result.hasM()).isTrue();
+        assertThat(result.m().get("key1")).isEqualTo(NULL_ATTR);
+    }
+
+    @ParameterizedTest(name = "List with null {0} element produces DynamoDB NULL")
+    @MethodSource("elementConverters")
+    void listConverter_nullElement_producesNullAttributeValue(String name, AttributeConverter<?> elementConverter) {
+        @SuppressWarnings("unchecked")
+        AttributeConverter<Object> converter = (AttributeConverter<Object>) elementConverter;
+        ListAttributeConverter<List<Object>> listConverter = ListAttributeConverter.create(converter);
+
+        List<Object> input = Arrays.asList(null, null);
+
+        AttributeValue result = listConverter.transformFrom(input);
+
+        assertThat(result.hasL()).isTrue();
+        assertThat(result.l()).hasSize(2);
+        assertThat(result.l().get(0)).isEqualTo(NULL_ATTR);
+        assertThat(result.l().get(1)).isEqualTo(NULL_ATTR);
+    }
+}