Pub Package

Provides Dart Build System builders for handling JSON.

The builders generate code when they find members annotated with classes defined in package:json_annotation.

  • To generate to/from JSON code for a class, annotate it with JsonSerializable. You can provide arguments to JsonSerializable to configure the generated code. You can also customize individual fields by annotating them with JsonKey and providing custom arguments. See the table below for details on the annotation values.

  • To generate a Dart field with the contents of a file containing JSON, use the JsonLiteral annotation.


To configure your project for the latest released version of json_serializable, see the example.


Given a library example.dart with an Person class annotated with JsonSerializable:

import 'package:json_annotation/json_annotation.dart';

part 'example.g.dart';

class Person {
  /// The generated code assumes these values exist in JSON.
  final String firstName, lastName;

  /// The generated code below handles if the corresponding JSON value doesn't
  /// exist or is empty.
  final DateTime? dateOfBirth;

  Person({required this.firstName, required this.lastName, this.dateOfBirth});

  /// Connect the generated [_$PersonFromJson] function to the `fromJson`
  /// factory.
  factory Person.fromJson(Map<String, dynamic> json) => _$PersonFromJson(json);

  /// Connect the generated [_$PersonToJson] function to the `toJson` method.
  Map<String, dynamic> toJson() => _$PersonToJson(this);

Building creates the corresponding part example.g.dart:

part of 'example.dart';

Person _$PersonFromJson(Map<String, dynamic> json) => Person(
      firstName: json['firstName'] as String,
      lastName: json['lastName'] as String,
      dateOfBirth: json['dateOfBirth'] == null
          ? null
          : DateTime.parse(json['dateOfBirth'] as String),

Map<String, dynamic> _$PersonToJson(Person instance) => <String, dynamic>{
      'firstName': instance.firstName,
      'lastName': instance.lastName,
      'dateOfBirth': instance.dateOfBirth?.toIso8601String(),

Running the code generator

Once you have added the annotations to your code you then need to run the code generator to generate the missing .g.dart generated dart files.

With a Dart package, run dart run build_runner build in the package directory.

With a Flutter package, run flutter pub run build_runner build in your package directory.

Annotation values

The only annotation required to use this package is JsonSerializable. When applied to a class (in a correctly configured package), toJson and fromJson code will be generated when you build. There are three ways to control how code is generated:

  1. Setting properties on JsonKey annotating the target field.
  2. Set properties on JsonSerializable annotating the target type.
  3. Add configuration to build.yamlsee below.

Every JsonSerializable field is configurable via build.yaml. If you find you want all or most of your classes with the same configuration, it may be easier to specify values once in the YAML file. Values set explicitly on JsonSerializable take precedence over settings in build.yaml.

There is some overlap between settings on JsonKey and JsonSerializable. In these cases, the property on JsonKey takes precedence over any value set on JsonSerializable.


Annotate enum types with JsonEnum (new in json_annotation 4.2.0) to:

  1. Specify the default rename logic for each enum value using fieldRename. For instance, use fieldRename: FieldRename.kebab to encode enum value noGood as "no-good".
  2. Force the generation of the enum helpers, even if the enum is not referenced in code. This is an edge scenario, but useful for some.

Annotate enum values with JsonValue to specify the encoded value to map to target enum entries. Values can be of type String or int.

enum StatusCode {

If you are annotating an enhanced enum, you can use JsonEnum.valueField to specify the field to use for the serialized value.

@JsonEnum(valueField: 'code')
enum StatusCodeEnhanced {

  const StatusCodeEnhanced(this.code);
  final int code;

Supported types

Out of the box, json_serializable supports many common types in the dart:core library: BigInt, bool, DateTime, double, Duration, Enum, int, Iterable, List, Map, num, Object, Set, String, Uri

The collection types – Iterable, List, Map, Set – can contain values of all the above types.

For Map, the key value must be one of BigInt, DateTime, Enum, int, Object, String, Uri

Custom types and custom encoding

If you want to use types that are not supported out-of-the-box or if you want to customize the encoding/decoding of any type, you have a few options.

  1. If you own/control the desired type, add a fromJson constructor and/or a toJson() function to the type. Note: while you can use json_serializable for these types, you don't have to! The generator code only looks for these methods. It doesn't care how they were created.

     class Sample1 {
       factory Sample1.fromJson(Map<String, dynamic> json) =>
       // Sample2 is NOT annotated with @JsonSerializable(), but that's okay
       // The class has a `fromJson` constructor and a `toJson` method, which is
       // all that is required.
       final Sample2 value;
       Map<String, dynamic> toJson() => _$Sample1ToJson(this);
     class Sample2 {
       // The convention is for `fromJson` to take a single parameter of type
       // `Map<String, dynamic>` but any JSON-compatible type is allowed.
       factory Sample2.fromJson(int value) => Sample2(value);
       final int value;
       // The convention is for `toJson` to take return a type of
       // `Map<String, dynamic>` but any JSON-compatible type is allowed.
       int toJson() => value;
  2. Use the JsonKey.toJson and JsonKey.fromJson properties to specify custom conversions on the annotated field. The functions specified must be top-level or static. See the documentation of these properties for details.

     class Sample3 {
       factory Sample3.fromJson(Map<String, dynamic> json) =>
         toJson: _toJson,
         fromJson: _fromJson,
       final DateTime value;
       Map<String, dynamic> toJson() => _$Sample3ToJson(this);
       static int _toJson(DateTime value) => value.millisecondsSinceEpoch;
       static DateTime _fromJson(int value) =>
  3. Create an implementation of JsonConverter and annotate either the corresponding field or the containing class. JsonConverter is convenient if you want to use the same conversion logic on many fields. It also allows you to support a type within collections. Check out these examples.

     class Sample4 {
       factory Sample4.fromJson(Map<String, dynamic> json) =>
       final DateTime value;
       Map<String, dynamic> toJson() => _$Sample4ToJson(this);
     class EpochDateTimeConverter implements JsonConverter<DateTime, int> {
       const EpochDateTimeConverter();
       DateTime fromJson(int json) => DateTime.fromMillisecondsSinceEpoch(json);
       int toJson(DateTime object) => object.millisecondsSinceEpoch;

Build configuration

Aside from setting arguments on the associated annotation classes, you can also configure code generation by setting values in build.yaml.

          # Options configure how source code is generated for every
          # `@JsonSerializable`-annotated class in the package.
          # The default value for each is listed.
          any_map: false
          checked: false
          constructor: ""
          create_factory: true
          create_field_map: false
          create_to_json: true
          disallow_unrecognized_keys: false
          explicit_to_json: false
          field_rename: none
          generic_argument_factories: false
          ignore_unannotated: false
          include_if_null: true


Configuration for using package:build-compatible build systems.