diff --git a/modules/openapi-generator/src/main/resources/dart2/class.mustache b/modules/openapi-generator/src/main/resources/dart2/class.mustache
index 636aab61b89..f65bce93991 100644
--- a/modules/openapi-generator/src/main/resources/dart2/class.mustache
+++ b/modules/openapi-generator/src/main/resources/dart2/class.mustache
@@ -50,7 +50,7 @@ class {{{classname}}} {
   {{/isEnum}}
   {{^isBinary}}
   @JsonKey(
-    nullable: {{#nullable}}true{{/nullable}}{{^nullable}}false{{/nullable}},
+    {{#defaultValue}}defaultValue: {{{defaultValue}}},{{/defaultValue}}{{^defaultValue}}nullable: {{isNullable}},{{/defaultValue}}
     name: r'{{{baseName}}}',
     required: {{#required}}true{{/required}}{{^required}}false{{/required}},
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/additional_properties_class.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/additional_properties_class.dart
index 85a663f6b10..558e7f204a3 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/additional_properties_class.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/additional_properties_class.dart
@@ -23,14 +23,14 @@ class AdditionalPropertiesClass {
   });
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'map_property',
     required: false,
   )
   Map<String, String> mapProperty;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'map_of_map_property',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/animal.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/animal.dart
index f8d223ed50e..b73140cc066 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/animal.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/animal.dart
@@ -30,7 +30,7 @@ class Animal {
   String className;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: 'red',
     name: r'color',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_array_of_number_only.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_array_of_number_only.dart
index 969f1f788e6..e8d735fde05 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_array_of_number_only.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_array_of_number_only.dart
@@ -22,7 +22,7 @@ class ArrayOfArrayOfNumberOnly {
   });
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'ArrayArrayNumber',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_number_only.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_number_only.dart
index 18c3e29792c..bf821233a35 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_number_only.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_of_number_only.dart
@@ -22,7 +22,7 @@ class ArrayOfNumberOnly {
   });
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'ArrayNumber',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_test.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_test.dart
index f96fca86296..e155c201c7c 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_test.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/array_test.dart
@@ -24,21 +24,21 @@ class ArrayTest {
   });
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'array_of_string',
     required: false,
   )
   List<String> arrayOfString;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'array_array_of_integer',
     required: false,
   )
   List<List<int>> arrayArrayOfInteger;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'array_array_of_model',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/cat.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/cat.dart
index fe9ff5a5edd..6fc7a1fe3f0 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/cat.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/cat.dart
@@ -31,7 +31,7 @@ class Cat {
   String className;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: 'red',
     name: r'color',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/category.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/category.dart
index 7725d8f4598..1943448babd 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/category.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/category.dart
@@ -30,7 +30,7 @@ class Category {
   int id;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: 'default-name',
     name: r'name',
     required: true,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/dog.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/dog.dart
index 920424afa64..dc7b2bd0386 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/dog.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/dog.dart
@@ -31,7 +31,7 @@ class Dog {
   String className;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: 'red',
     name: r'color',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_arrays.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_arrays.dart
index 1ca0a07a3fb..d39f69699d8 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_arrays.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_arrays.dart
@@ -30,7 +30,7 @@ class EnumArrays {
   EnumArraysJustSymbolEnum justSymbol;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'array_enum',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_test.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_test.dart
index a6159e20830..7389cf1246b 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_test.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/enum_test.dart
@@ -57,7 +57,7 @@ class EnumTest {
   EnumTestEnumNumberEnum enumNumber;
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'outerEnum',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/file_schema_test_class.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/file_schema_test_class.dart
index 432ed75edb1..d3c07377d13 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/file_schema_test_class.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/file_schema_test_class.dart
@@ -30,7 +30,7 @@ class FileSchemaTestClass {
   ModelFile file;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'files',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/foo.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/foo.dart
index 90cecc2a17d..47375d9f05a 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/foo.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/foo.dart
@@ -22,7 +22,7 @@ class Foo {
   });
 
   @JsonKey(
-    nullable: false,
+    defaultValue: 'bar',
     name: r'bar',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/health_check_result.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/health_check_result.dart
index 3150d9df5a5..506c7312ea0 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/health_check_result.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/health_check_result.dart
@@ -22,7 +22,7 @@ class HealthCheckResult {
   });
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'NullableMessage',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/map_test.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/map_test.dart
index 2080fe0929a..034e4c6ab16 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/map_test.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/map_test.dart
@@ -25,28 +25,28 @@ class MapTest {
   });
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'map_map_of_string',
     required: false,
   )
   Map<String, Map<String, String>> mapMapOfString;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'map_of_enum_string',
     required: false,
   )
   Map<String, MapTestMapOfEnumStringEnum> mapOfEnumString;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'direct_map',
     required: false,
   )
   Map<String, bool> directMap;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'indirect_map',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/mixed_properties_and_additional_properties_class.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/mixed_properties_and_additional_properties_class.dart
index 634753e657e..7f6305f2195 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/mixed_properties_and_additional_properties_class.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/mixed_properties_and_additional_properties_class.dart
@@ -38,7 +38,7 @@ class MixedPropertiesAndAdditionalPropertiesClass {
   DateTime dateTime;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'map',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/nullable_class.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/nullable_class.dart
index 42c21d5c590..acbda5d7199 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/nullable_class.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/nullable_class.dart
@@ -33,84 +33,84 @@ class NullableClass {
   });
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'integer_prop',
     required: false,
   )
   int integerProp;
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'number_prop',
     required: false,
   )
   num numberProp;
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'boolean_prop',
     required: false,
   )
   bool booleanProp;
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'string_prop',
     required: false,
   )
   String stringProp;
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'date_prop',
     required: false,
   )
   DateTime dateProp;
 
   @JsonKey(
-    nullable: false,
+    nullable: true,
     name: r'datetime_prop',
     required: false,
   )
   DateTime datetimeProp;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'array_nullable_prop',
     required: false,
   )
   List<Object> arrayNullableProp;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'array_and_items_nullable_prop',
     required: false,
   )
   List<Object> arrayAndItemsNullableProp;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'array_items_nullable',
     required: false,
   )
   List<Object> arrayItemsNullable;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'object_nullable_prop',
     required: false,
   )
   Map<String, Object> objectNullableProp;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'object_and_items_nullable_prop',
     required: false,
   )
   Map<String, Object> objectAndItemsNullableProp;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'object_items_nullable',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/order.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/order.dart
index e42c82c5cf1..6286697b172 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/order.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/order.dart
@@ -63,7 +63,7 @@ class Order {
   OrderStatusEnum status;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: false,
     name: r'complete',
     required: false,
   )
diff --git a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/pet.dart b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/pet.dart
index b93a9a1100f..032d8ba77e6 100644
--- a/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/pet.dart
+++ b/samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/model/pet.dart
@@ -48,14 +48,14 @@ class Pet {
   String name;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const {},
     name: r'photoUrls',
     required: true,
   )
   Set<String> photoUrls;
 
   @JsonKey(
-    nullable: false,
+    defaultValue: const [],
     name: r'tags',
     required: false,
   )