Flutter JSON Serialization - Not generating *.g.dart files - json

Question

I am new to flutter and the objective is to serialise complex JSON objects which contain other smaller objects.
Using the json_serializable: ^2.0.0 and the pubspec.yaml file looks something like this.
dependencies:
intl: ^0.15.7
json_annotation: ^2.0.0
built_value: ^6.7.1
flutter:
sdk: flutter
dev_dependencies:
build_runner: ^1.0.0
json_serializable: ^2.0.0
built_value_generator: ^6.7.1
flutter_test:
sdk: flutter
The user.dart look like this
import 'package:json_annotation/json_annotation.dart';
part 'user.g.dart';
#JsonSerializable(nullable: false)
class User {
final String firstName;
final String lastName;
final DateTime dateOfBirth;
User({this.firstName, this.lastName, this.dateOfBirth});
factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
Map<String, dynamic> toJson() => _$UserToJson(this);
}
I have tried flutter pub run build_runner build yet the file user.g.dart doesn't get created and I am facing issue with that.
I also have added the build.yaml file with following code
targets:
$default:
builders:
built_value_generator|built_value:
generate_for:
- model/*.dart
json_serializable|json_serializable:
generate_for:
- model/*.dart
Can anyone let me know what I am missing here. Thanks

Answer 1

Checklist
pubspec.yaml includes 3 packages:
dependencies:
json_annotation: ^4.6.0
dev_dependencies:
build_runner: ^2.2.0
json_serializable: ^6.3.1
you've run flutter pub get
if you get errors about versions at this stage, try older versions of json_serializable, json_annotation, build_runner
note the Min Dart SDK listed on those linked pages
your pubspec.yaml environment: sdk: ">=a.bb.c < x.y.z", your a.bb.c version must be >= than the Min Dart SDK listed on pub.dev for the package
if your Dart version is lower, try an older version of the json package which matches or is below your Dart version
your class files are underneath /lib or /bin
can be subdirectories under those
json_serializable won't search every directory for files to generate.
added import for json_annotation:
import 'package:json_annotation/json_annotation.dart';
you've added a part directive after import statements
your part file is named after your class filename (not the Class name itself), with a g added
e.g. for CacheItem class with ...
cache-item.dart class filename ...
part 'cache-item.g.dart'; gets corresponding part directive.
the part directive is not named after your actual Class, but the class file name.
if Android Studio Dart Analyzer continues to give errors about missing .g.dart file after you're sure the file has been generated & named correctly, shut down Android Studio and restart it (analyzer sometimes goes insane & this may fix it. Restarting just the analyzer may not work.)
you've added #JsonSerializable() above the class name
you've created a default constructor for your class.
It can be empty, have optional named parameters, or positional parameters.
As long as your class fields are accessible (either through constructor or public setters & getters), json_serializable can handle it. (i.e. don't have only _private properties and an empty constructor)
you've written two methods calling private stub methods:
a factory fromJson method
e.g.: factory CacheItem.fromJson(Map<String,dynamic> json) => _CacheItemFromJson(json)
a toJson method
e.g.: Map<String,dynamic> toJson() => _$CacheItemToJson(this)
stub methods are private (start with _ underscore)
$tub methods have the $
stub methods have proper CaSe (i.e. Pascal Case)
stub factory supplies (Map<String,dynamic> json) as argument
stub toJson() returns Map<String,dynamic>
When all that is complete, try running the generator from the command line or terminal in the project root directory...
In Flutter:
flutter pub run build_runner build
In pure Dart, this depends on your version, but one of these should work:
dart run build_runner build
pub run build_runner build
dart pub run build_runner build
If all goes well, click around in your project file explorer or Reload from disk and new files should show up such as cache-item.g.dart for the above example.
Common Errors
Bad state: Unexpected diagnostics:
Seeing this output when running the build_runner is likely a problem with flutter and json_annotation depending on an incompatible version of analyzer. This happens with json_serializable versions prior to 3.5 requiring a dependency_override of analyzer to 0.39.14 or 0.39.17.
Your first move should be to try the latest version of json_serilizable from pub.dev which apparently doesn't have this dependency problem.
If you can't upgrade json_serializable you can try placing the override lines underneath dev_dependences:
dev_dependencies:
build_runner: ^1.9.0
flutter_test:
sdk: flutter
json_serializable: 3.3.0
test: ^1.14.3
dependency_overrides:
analyzer: '0.39.14'
[SEVERE] Nothing can be built, yet a build was requested.
This error can happen when performing a flutter pub run build_runner build when we've added dependency in pubspec.yaml for json_annotation but are missing a dependency/dev_dependency for json_serializable:
dependencies:
flutter:
sdk: flutter
get:
json_annotation: ^4.3.0
some_other_packages:
Make sure you've got json_serializable package added as either a dependency or dev_dependency:
dependencies:
flutter:
sdk: flutter
get:
json_annotation: ^4.3.0
dev_dependencies:
build_runner: ^2.1.4
flutter_test:
sdk: flutter
json_serializable: ^6.0.1 #// ← do not forget
test:
Could not generate fromJsoncode forsomeField.
If you're json serializing a class that contains someField which is a Type for another custom class you've created, have you #JsonSerializable() that other custom class?
#JsonSerializable(explicitToJson: true)
class BuildingStatus {
final Building building; // another custom class
BuildingStatus(Building building);
factory BuildingStatus.fromJson(Map<String,dynamic> json) => _$BuildingStatusFromJson(json);
Map<String,dynamic> toJson() => _$BuildingStatusToJson(this);
}
/// This guy needs serialization too.
#JsonSerializable()
class Building {
final String name;
const Building(this.name);
factory Building.fromJson(Map<String,dynamic> json) => _$BuildingFromJson(json);
Map<String,dynamic> toJson() => _$BuildingToJson(this);
}
Without serializing the nested Building class, we'd see an error when running build_runner like:
Could not generate `fromJson` code for `building` because of type `Building`.
Instance of 'SomeNestedClass'
If we have nested serializable classes, we generally want the serialization to occur recursively. i.e. the nested classes are also serialized.
To do that we would annotate our containing class with explicitToJson: true like:
#JsonSerializable(explicitToJson: true)
So when we toJson() our BuildingStatus instance, instead of getting:
{"building": Instance of 'Building'}
... we would get:
{"building": {"name": "Empire State"}}
Notes
Subclass / Parent Class
If your class is a child of a parent class and you want to Serialize fields/properties of child only, you can annotate only the subclass. The parent class fields will automatically be found and included in the generated class files for the subclass.
If you want to be able to serialize/deserialize both parent and child separately, go ahead and annotate the base / parent classes with #JsonSerializable as well.
e.g. filename account.dart
import 'package:json_annotation/json_annotation.dart';
part 'account.g.dart';
class AccountBase {
int? created;
String? username;
String? password;
}
#JsonSerializable()
class Account extends AccountBase {
int? id;
Account();
factory Account.fromJson(Map<String,dynamic> json) => _$AccountFromJson(json);
Map<String,dynamic> toJson() => _$AccountToJson(this);
}
Produces:
// GENERATED CODE - DO NOT MODIFY BY HAND
part of 'account.dart';
// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************
Account _$AccountFromJson(Map<String, dynamic> json) => Account()
..created = json['created'] as int?
..username = json['username'] as String?
..password = json['password'] as String?
..id = json['id'] as int?;
Map<String, dynamic> _$AccountToJson(Account instance) => <String, dynamic>{
'created': instance.created,
'username': instance.username,
'password': instance.password,
'id': instance.id,
};
Reference & Docs
Example project on github, relevant files under /bin/ and packages in pubspec.yaml
Flutter & Json
Json_Serializable package
Example from the package authors
Example
import 'package:json_annotation/json_annotation.dart';
part 'cache-item.g.dart';
#JsonSerializable()
class CacheItem {
int created;
String keywords;
String response;
CacheItem(this.created, this.keywords, this.response);
factory CacheItem.fromJson(Map<String,dynamic> json) => _$CacheItemFromJson(json);
Map<String,dynamic> toJson() => _$CacheItemToJson(this);
}
Output
// GENERATED CODE - DO NOT MODIFY BY HAND
part of 'cache-item.dart';
// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************
CacheItem _$CacheItemFromJson(Map<String, dynamic> json) => CacheItem(
json['created'] as int,
json['keywords'] as String,
json['response'] as String,
);
Map<String, dynamic> _$CacheItemToJson(CacheItem instance) => <String, dynamic>{
'created': instance.created,
'keywords': instance.keywords,
'response': instance.response,
};
Example Constructor Variant
This example is the same as above except the constructor is missing some fields and has response as optional.
It's fine.
The generator will just use the public (implicit) setters after instantiating the object to assign the values.
import 'package:json_annotation/json_annotation.dart';
part 'cache-item.g.dart';
#JsonSerializable()
class CacheItem {
int? created;
String? keywords;
String? response;
CacheItem({this.response});
factory CacheItem.fromJson(Map<String,dynamic> json) => _$CacheItemFromJson(json);
Map<String,dynamic> toJson() => _$CacheItemToJson(this);
}
Output
// GENERATED CODE - DO NOT MODIFY BY HAND
part of 'cache-item.dart';
// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************
CacheItem _$CacheItemFromJson(Map<String, dynamic> json) => CacheItem(
response: json['response'] as String?,
)
..created = json['created'] as int?
..keywords = json['keywords'] as String?;
Map<String, dynamic> _$CacheItemToJson(CacheItem instance) => <String, dynamic>{
'created': instance.created,
'keywords': instance.keywords,
'response': instance.response,
};

Answer 2

The constructor's argument shouldn't be optional
User({this.firstName, this.lastName, this.dateOfBirth});
They should be obligatory:
User(this.firstName, this.lastName, this.dateOfBirth);
And the part
'user.g.dart';
should be matching the Uppercase User class:
part 'User.g.dart';

Answer 3

try running
>flutter packages pub run build_runner build --delete-conflicting-outputs

Answer 4

The file name, class, and part 'Book.g.dart'; should all match.

Answer 5

I got to know that , file has to be saved before running the command
flutter packages pub run build_runner build
I know answer seems so simple.

Answer 6

I had this following error
error
[WARNING] json_serializable:json_serializable on lib/day10/models/sentence.dart:
Missing "part 'sentence.g.dart';".
I noticed that I had to change on the model file this part 'Sentence.g.dart'; to this part 'sentence.g.dart'; in other words, I had to lowercase it.

Answer 7

in my case I had to add hive_generator package, along with the build_runner package.
If your model class name is for example transaction.dart, then it would be like this-
part 'transaction.g.dart'
Run the command-
flutter packages pub run build_runner build it will generate the transaction model for you.

Answer 8

adding
json_serializable: ^6.0.0
to dev_dependencies solve it for me

Answer 9

If you are trying to generate .g files by just running the command flutter packages pub run build_runner build and you are not getting your desired result then just try:
flutter clean
'flutter pub cache repair'
flutter packages pub run build_runner build --delete-conflicting-outputs
delete all the gradle temp files
then run flutter pub get
flutter packages pub run build_runner build.

Answer 10

Some times we forgot to save the files, please save the file then run the command.
flutter packages pub run build_runner build

Answer 11

Step 1: Add "part 'your_class_name.g.dart'"
Step 2: Run this command on Terminal: flutter pub run build_runner build
Example:
part 'student.g.dart'
#JsonSerializable()
class Student{
....
}

Answer 12

Do not forget to save the file that has the class even if it still has errors
Then run the command on any terminal (o integrated terminal)...
But for Dart:
pub run build_runner build
And for Flutter:
flutter pub run build_runner build
Official reference link:
https://pub.dev/packages/json_serializable
https://github.com/google/json_serializable.dart/tree/master/example