Allow @JsonKey to be used on constructor parameters

json_annotation/lib/src/json_key.dart

@@ -8,7 +8,7 @@ import 'allowed_keys_helpers.dart';
 import 'json_serializable.dart';
 
 /// An annotation used to specify how a field is serialized.
-@Target({TargetKind.field, TargetKind.getter})
+@Target({TargetKind.field, TargetKind.getter, TargetKind.parameter})
 class JsonKey {
   /// The value to use if the source JSON does not contain this key or if the
   /// value is `null`.

json_serializable/lib/src/json_key_utils.dart

@@ -24,14 +24,28 @@ KeyConfig jsonKeyForField(FieldElement field, ClassConfig classAnnotation) =>
     );
 
 KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
-  // If an annotation exists on `element` the source is a 'real' field.
-  // If the result is `null`, check the getter – it is a property.
-  // TODO: setters: github.com/google/json_serializable.dart/issues/24
   final obj = jsonKeyAnnotation(element);
+  final ctorParam = <ParameterElement?>[
+    ...classAnnotation.ctorParams,
+  ].singleWhere((e) => e!.name == element.name, orElse: () => null);
+  final ctorObj = ctorParam == null
+      ? null
+      : jsonKeyAnnotationForCtorParam(ctorParam);
+
+  ConstantReader fallbackObjRead(String field) {
+    if (ctorObj != null && !ctorObj.isNull) {
+      final ctorReadResult = ctorObj.read(field);
+      if (!ctorReadResult.isNull) return ctorReadResult;
+    }
+    if (obj.isNull) {
+      return ConstantReader(null);
+    }
+    return obj.read(field);
+  }
 
-  final ctorParamDefault = classAnnotation.ctorParamDefaults[element.name];
+  final ctorParamDefault = ctorParam?.defaultValueCode;
 
-  if (obj.isNull) {
+  if (obj.isNull && (ctorObj == null || ctorObj.isNull)) {
     return _populateJsonKey(
       classAnnotation,
       element,
@@ -121,7 +135,7 @@ KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
   /// either the annotated field is not an `enum` or `List` or if the value in
   /// [fieldName] is not an `enum` value.
   String? createAnnotationValue(String fieldName, {bool mustBeEnum = false}) {
-    final annotationValue = obj.read(fieldName);
+    final annotationValue = fallbackObjRead(fieldName);
 
     if (annotationValue.isNull) {
       return null;
@@ -228,16 +242,17 @@ KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
   }
 
   String? readValueFunctionName;
-  final readValue = obj.read('readValue');
+  final readValue = fallbackObjRead('readValue');
   if (!readValue.isNull) {
     readValueFunctionName = readValue.objectValue
         .toFunctionValue()!
         .qualifiedName;
   }
 
-  final ignore = obj.read('ignore').literalValue as bool?;
-  var includeFromJson = obj.read('includeFromJson').literalValue as bool?;
-  var includeToJson = obj.read('includeToJson').literalValue as bool?;
+  final ignore = fallbackObjRead('ignore').literalValue as bool?;
+  var includeFromJson =
+      fallbackObjRead('includeFromJson').literalValue as bool?;
+  var includeToJson = fallbackObjRead('includeToJson').literalValue as bool?;
 
   if (ignore != null) {
     if (includeFromJson != null) {
@@ -262,11 +277,12 @@ KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
     classAnnotation,
     element,
     defaultValue: defaultValue ?? ctorParamDefault,
-    disallowNullValue: obj.read('disallowNullValue').literalValue as bool?,
-    includeIfNull: obj.read('includeIfNull').literalValue as bool?,
-    name: obj.read('name').literalValue as String?,
+    disallowNullValue:
+        fallbackObjRead('disallowNullValue').literalValue as bool?,
+    includeIfNull: fallbackObjRead('includeIfNull').literalValue as bool?,
+    name: fallbackObjRead('name').literalValue as String?,
     readValueFunctionName: readValueFunctionName,
-    required: obj.read('required').literalValue as bool?,
+    required: fallbackObjRead('required').literalValue as bool?,
     unknownEnumValue: createAnnotationValue(
       'unknownEnumValue',
       mustBeEnum: true,

json_serializable/lib/src/type_helpers/config_types.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/constant/value.dart';
+import 'package:analyzer/dart/element/element.dart';
 import 'package:json_annotation/json_annotation.dart';
 
 /// Represents values from [JsonKey] when merged with local configuration.
@@ -57,7 +58,7 @@ class ClassConfig {
   final bool genericArgumentFactories;
   final bool ignoreUnannotated;
   final bool includeIfNull;
-  final Map<String, String> ctorParamDefaults;
+  final List<ParameterElement> ctorParams;
   final List<DartObject> converters;
 
   const ClassConfig({
@@ -76,7 +77,7 @@ class ClassConfig {
     required this.ignoreUnannotated,
     required this.includeIfNull,
     this.converters = const [],
-    this.ctorParamDefaults = const {},
+    this.ctorParams = const [],
   });
 
   factory ClassConfig.fromJsonSerializable(JsonSerializable config) =>

json_serializable/lib/src/utils.dart

@@ -14,6 +14,9 @@ import 'type_helpers/config_types.dart';
 
 const _jsonKeyChecker = TypeChecker.fromRuntime(JsonKey);
 
+// If an annotation exists on `element` the source is a 'real' field.
+// If the result is `null`, check the getter – it is a property.
+// TODO: setters: github.com/google/json_serializable.dart/issues/24
 DartObject? _jsonKeyAnnotation(FieldElement element) =>
     _jsonKeyChecker.firstAnnotationOf(element) ??
     (element.getter == null
@@ -27,6 +30,9 @@ ConstantReader jsonKeyAnnotation(FieldElement element) =>
 bool hasJsonKeyAnnotation(FieldElement element) =>
     _jsonKeyAnnotation(element) != null;
 
+ConstantReader jsonKeyAnnotationForCtorParam(ParameterElement element) =>
+    ConstantReader(_jsonKeyChecker.firstAnnotationOf(element));
+
 Never throwUnsupported(FieldElement element, String message) =>
     throw InvalidGenerationSourceError(
       'Error with `@JsonKey` on the `${element.name}` field. $message',
@@ -82,21 +88,15 @@ ClassConfig mergeConfig(
   required ClassElement classElement,
 }) {
   final annotation = _valueForAnnotation(reader);
-  assert(config.ctorParamDefaults.isEmpty);
+  assert(config.ctorParams.isEmpty);
 
   final constructor = annotation.constructor ?? config.constructor;
   final constructorInstance = _constructorByNameOrNull(
     classElement,
     constructor,
   );
 
-  final paramDefaultValueMap = constructorInstance == null
-      ? <String, String>{}
-      : Map<String, String>.fromEntries(
-          constructorInstance.parameters
-              .where((element) => element.hasDefaultValue)
-              .map((e) => MapEntry(e.name, e.defaultValueCode!)),
-        );
+  final ctorParams = <ParameterElement>[...?constructorInstance?.parameters];
 
   final converters = reader.read('converters');
 
@@ -120,7 +120,7 @@ ClassConfig mergeConfig(
             config.genericArgumentFactories),
     ignoreUnannotated: annotation.ignoreUnannotated ?? config.ignoreUnannotated,
     includeIfNull: annotation.includeIfNull ?? config.includeIfNull,
-    ctorParamDefaults: paramDefaultValueMap,
+    ctorParams: ctorParams,
     converters: converters.isNull ? const [] : converters.listValue,
   );
 }

json_serializable/test/json_serializable_test.dart

@@ -49,6 +49,8 @@ const _expectedAnnotatedTests = {
   'BadToFuncReturnType',
   'BadTwoRequiredPositional',
   'CtorDefaultValueAndJsonKeyDefaultValue',
+  'CtorParamJsonKey',
+  'CtorParamJsonKeyWithExtends',
   'DefaultDoubleConstants',
   'DefaultWithConstObject',
   'DefaultWithDisallowNullRequiredClass',

json_serializable/test/src/_json_serializable_test_input.dart

@@ -14,6 +14,7 @@ part 'constants_copy.dart';
 part 'core_subclass_type_input.dart';
 part 'default_value_input.dart';
 part 'field_namer_input.dart';
+part 'extends_jsonkey_override.dart';
 part 'generic_test_input.dart';
 part 'inheritance_test_input.dart';
 part 'json_converter_test_input.dart';

json_serializable/test/src/extends_jsonkey_override.dart

@@ -0,0 +1,52 @@
+// @dart=3.8
+
+part of '_json_serializable_test_input.dart';
+
+// https://github.com/google/json_serializable.dart/issues/1437
+@ShouldGenerate(r'''
+CtorParamJsonKey _$CtorParamJsonKeyFromJson(Map<String, dynamic> json) =>
+    CtorParamJsonKey(
+      attributeOne: json['first'] as String,
+      attributeTwo: json['second'] as String,
+    );
+
+Map<String, dynamic> _$CtorParamJsonKeyToJson(CtorParamJsonKey instance) =>
+    <String, dynamic>{
+      'first': instance.attributeOne,
+      'second': instance.attributeTwo,
+    };
+''')
+@JsonSerializable()
+class CtorParamJsonKey {
+  CtorParamJsonKey({
+    @JsonKey(name: 'first') required this.attributeOne,
+    @JsonKey(name: 'second') required this.attributeTwo,
+  });
+
+  @JsonKey(name: 'fake_first')
+  final String attributeOne;
+  final String attributeTwo;
+}
+
+@ShouldGenerate(r'''
+CtorParamJsonKeyWithExtends _$CtorParamJsonKeyWithExtendsFromJson(
+  Map<String, dynamic> json,
+) => CtorParamJsonKeyWithExtends(
+  attributeOne: json['fake_first'] as String,
+  attributeTwo: json['two'] as String,
+);
+
+Map<String, dynamic> _$CtorParamJsonKeyWithExtendsToJson(
+  CtorParamJsonKeyWithExtends instance,
+) => <String, dynamic>{
+  'fake_first': instance.attributeOne,
+  'two': instance.attributeTwo,
+};
+''')
+@JsonSerializable()
+class CtorParamJsonKeyWithExtends extends CtorParamJsonKey {
+  CtorParamJsonKeyWithExtends({
+    required super.attributeOne,
+    @JsonKey(name: 'two') required super.attributeTwo,
+  });
+}