JSON and serialization
It is hard to think of a mobile app that doesn’t need to communicate with a web server or easily store structured data at some point. When making network-connected apps, the chances are that it needs to consume some good old JSON, sooner or later.
This guide looks into ways of using JSON with Flutter. It covers which JSON solution to use in different scenarios, and why.
Terminology: Encoding and serialization are the same thing—turning a data structure into a string. Decoding and deserialization are the opposite process—turning a string into a data structure. However, serialization also commonly refers to the entire process of translating data structures to and from a more easily readable format.
To avoid confusion, this doc uses “serialization” when referring to the overall process, and “encoding” and “decoding” when specifically referring to those processes.
Which JSON serialization method is right for me?
This article covers two general strategies for working with JSON:
- Manual serialization
- Automated serialization using code generation
Different projects come with different complexities and use cases. For smaller proof-of-concept projects or quick prototypes, using code generators might be overkill. For apps with several JSON models with more complexity, encoding by hand can quickly become tedious, repetitive, and lend itself to many small errors.
Use manual serialization for smaller projects
Manual JSON decoding refers to using the built-in JSON decoder in dart:convert
. It involves passing the raw JSON string to the jsonDecode()
function, and then looking up the values you need in the resulting Map<String, dynamic>
. It has no external dependencies or particular setup process, and it’s good for a quick proof of concept.
Manual decoding does not perform well when your project becomes bigger. Writing decoding logic by hand can become hard to manage and error-prone. If you have a typo when accessing an nonexistent JSON field, your code throws an error during runtime.
If you do not have many JSON models in your project and are looking to test a concept quickly, manual serialization might be the way you want to start. For an example of manual encoding, see Serializing JSON manually using dart:convert.
Use code generation for medium to large projects
JSON serialization with code generation means having an external library generate the encoding boilerplate for you. After some initial setup, you run a file watcher that generates the code from your model classes. For example, json_serializable
and built_value
are these kinds of libraries.
This approach scales well for a larger project. No hand-written boilerplate is needed, and typos when accessing JSON fields are caught at compile-time. The downside with code generation is that it requires some initial setup. Also, the generated source files might produce visual clutter in your project navigator.
You might want to use generated code for JSON serialization when you have a medium or a larger project. To see an example of code generation based JSON encoding, see Serializing JSON using code generation libraries.
Is there a GSON/Jackson/Moshi equivalent in Flutter?
The simple answer is no.
Such a library would require using runtime reflection, which is disabled in Flutter. Runtime reflection interferes with tree shaking, which Dart has supported for quite a long time. With tree shaking, you can “shake off” unused code from your release builds. This optimizes the app’s size significantly.
Since reflection makes all code implicitly used by default, it makes tree shaking difficult. The tools cannot know what parts are unused at runtime, so the redundant code is hard to strip away. App sizes cannot be easily optimized when using reflection.
Although you cannot use runtime reflection with Flutter, some libraries give you similarly easy-to-use APIs but are based on code generation instead. This approach is covered in more detail in the code generation libraries section.
Basic JSON serialization in Flutter is very simple. Flutter has a built-in
The following sample JSON implements a simple user model.
With
By looking at the
Unfortunately,
For example, whenever you access the
Combat the previously mentioned problems by introducing a plain model class, called
With this approach, the calling code can have type safety, autocompletion for the
user.dart
The responsibility of the decoding logic is now moved inside the model itself. With this new approach, you can decode a user easily.
To encode a user, pass the
With this approach, the calling code doesn’t have to worry about JSON serialization at all. However, the model class still definitely has to. In a production app, you would want to ensure that the serialization works properly. In practice, the
The cookbook contains a more comprehensive worked example of using JSON model classes, using an isolate to parse the JSON file on a background thread. This approach is ideal if you need your app to remain responsive while the JSON file is being decoded.
However, real-world scenarios are not always that simple. Sometimes JSON API responses are more complex, for example since they contain nested JSON objects that must be parsed through their own model class.
It would be nice if there were something that handled the JSON encoding and decoding for you. Luckily, there is!
Although there are other libraries available, this guide uses
Choosing a library: You might have noticed two Flutter Favorite packages on pub.dev that generate JSON serialization code,
Since the serialization code is not handwritten or maintained manually anymore, you minimize the risk of having JSON serialization exceptions at runtime.
To include
The latest versions of these required dependencies can be seen by following the pubspec file in the JSON serializable example.
pubspec.yaml
Run
The following shows how to convert the
user.dart
With this setup, the source code generator generates code for encoding and decoding the
If needed, it is also easy to customize the naming strategy. For example, if the API returns objects with snake_case, and you want to use lowerCamelCase in your models, you can use the
When creating
These errors are entirely normal and are simply because the generated code for the model class does not exist yet. To resolve this, run the code generator that generates the serialization boilerplate.
There are two ways of running the code generator.
By running
While this is convenient, it would be nice if you did not have to run the build manually every time you make changes in your model classes.
A watcher makes our source code generation process more convenient. It watches changes in our project files and automatically builds the necessary files when needed. Start the watcher by running
It is safe to start the watcher once and leave it running in the background.
To decode a JSON string the
The same goes for encoding. The calling API is the same as before.
With
You might have code that has nested classes within a class. If that is the case, and you have tried to pass the class in JSON format as an argument to a service (such as Firebase, for example), you might have experienced an
Consider the following
The
Running
All looks fine now, but if you do a print() on the user object:
The result is:
When what you probably want is output like the following:
To make this work, pass
For more information, see
For more information, see the following resources:
Serializing JSON manually using dart:convert
dart:convert
library that includes a straightforward JSON encoder and decoder.
{
"name": "John Smith",
"email": "john@example.com"
}
dart:convert
, you can serialize this JSON model in two ways.
Serializing JSON inline
dart:convert
documentation, you’ll see that you can decode the JSON by calling the jsonDecode()
function, with the JSON string as the method argument.
Map<String, dynamic> user = jsonDecode(jsonString);
print('Howdy, ${user['name']}!');
print('We sent the verification link to ${user['email']}.');
jsonDecode()
returns a Map<String, dynamic>
, meaning that you do not know the types of the values until runtime. With this approach, you lose most of the statically typed language features: type safety, autocompletion and most importantly, compile-time exceptions. Your code will become instantly more error-prone.name
or email
fields, you could quickly introduce a typo. A typo that the compiler doesn’t know about since the JSON lives in a map structure.
Serializing JSON inside model classes
User
in this example. Inside the User
class, you’ll find:
User.fromJson()
constructor, for constructing a new User
instance from a map structure.toJson()
method, which converts a User
instance into a map.name
and email
fields, and compile-time exceptions. If you make typos or treat the fields as int
s instead of String
s, the app won’t compile, instead of crashing at runtime.
class User {
final String name;
final String email;
User(this.name, this.email);
User.fromJson(Map<String, dynamic> json)
: name = json['name'],
email = json['email'];
Map<String, dynamic> toJson() =>
{
'name': name,
'email': email,
};
}
Map userMap = jsonDecode(jsonString);
var user = User.fromJson(userMap);
print('Howdy, ${user.name}!');
print('We sent the verification link to ${user.email}.');
User
object to the jsonEncode()
function. You don’t need to call the toJson()
method, since jsonEncode()
already does it for you.
String json = jsonEncode(user);
User.fromJson()
and User.toJson()
methods both need to have unit tests in place to verify correct behavior.
Serializing JSON using code generation libraries
json_serializable
, an automated source code generator that generates the JSON serialization boilerplate for you.
json_serializable
and built_value
. How do you choose between these packages? The json_serializable
package allows you to make regular classes serializable by using annotations, whereas the built_value
package provides a higher-level way of defining immutable value classes that can also be serialized to JSON.
Setting up json_serializable in a project
json_serializable
in your project, you need one regular dependency, and two dev dependencies. In short, dev dependencies are dependencies that are not included in our app source code—they are only used in the development environment.
dependencies:
# Your other regular dependencies here
json_annotation: <latest_version>
dev_dependencies:
# Your other dev_dependencies here
build_runner: <latest_version>
json_serializable: <latest_version>
flutter pub get
inside your project root folder (or click Packages get in your editor) to make these new dependencies available in your project.
Creating model classes the json_serializable way
User
class to a json_serializable
class. For the sake of simplicity, this code uses the simplified JSON model from the previous samples.
import 'package:json_annotation/json_annotation.dart';
/// This allows the `User` class to access private members in
/// the generated file. The value for this is *.g.dart, where
/// the star denotes the source file name.
part 'user.g.dart';
/// An annotation for the code generator to know that this class needs the
/// JSON serialization logic to be generated.
@JsonSerializable()
class User {
User(this.name, this.email);
String name;
String email;
/// A necessary factory constructor for creating a new User instance
/// from a map. Pass the map to the generated `_$UserFromJson()` constructor.
/// The constructor is named after the source class, in this case, User.
factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
/// `toJson` is the convention for a class to declare support for serialization
/// to JSON. The implementation simply calls the private, generated
/// helper method `_$UserToJson`.
Map<String, dynamic> toJson() => _$UserToJson(this);
}
name
and email
fields from JSON.@JsonKey
annotation with a name parameter:
/// Tell json_serializable that "registration_date_millis" should be
/// mapped to this property.
@JsonKey(name: 'registration_date_millis')
final int registrationDateMillis;
Running the code generation utility
json_serializable
classes the first time, you’ll get errors similar to what is shown in the image below.
One-time code generation
flutter pub run build_runner build
in the project root, you generate JSON serialization code for your models whenever they are needed. This triggers a one-time build that goes through the source files, picks the relevant ones, and generates the necessary serialization code for them.
Generating code continuously
flutter pub run build_runner watch
in the project root.
Consuming json_serializable models
json_serializable
way, you do not have actually to make any changes to our previous code.
Map userMap = jsonDecode(jsonString);
var user = User.fromJson(userMap);
String json = jsonEncode(user);
json_serializable
, you can forget any manual JSON serialization in the User
class. The source code generator creates a file called user.g.dart
, that has all the necessary serialization logic. You no longer have to write automated tests to ensure that the serialization works—it’s now the library’s responsibility to make sure the serialization works appropriately.
Generating code for nested classes
Invalid argument
error.Address
class:
import 'package:json_annotation/json_annotation.dart';
part 'address.g.dart';
@JsonSerializable()
class Address {
String street;
String city;
Address(this.street, this.city);
factory Address.fromJson(Map<String, dynamic> json) => _$AddressFromJson(json);
Map<String, dynamic> toJson() => _$AddressToJson(this);
}
Address
class is nested inside the User
class:
import 'address.dart';
import 'package:json_annotation/json_annotation.dart';
part 'user.g.dart';
@JsonSerializable()
class User {
String firstName;
Address address;
User(this.firstName, this.address);
factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
Map<String, dynamic> toJson() => _$UserToJson(this);
}
flutter pub run build_runner build
in the terminal creates the *.g.dart
file, but the private _$UserToJson()
function looks something like the following:
(
Map<String, dynamic> _$UserToJson(User instance) => <String, dynamic>{
'firstName': instance.firstName,
'address': instance.address,
};
Address address = Address("My st.", "New York");
User user = User("John", address);
print(user.toJson());
{name: John, address: Instance of 'address'}
{name: John, address: {street: My st., city: New York}}
explicitToJson: true
in the @JsonSerializable()
annotation over the class declaration. The User
class now looks as follows:
import 'address.dart';
import 'package:json_annotation/json_annotation.dart';
part 'user.g.dart';
@JsonSerializable(explicitToJson: true)
class User {
String firstName;
Address address;
User(this.firstName, this.address);
factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
Map<String, dynamic> toJson() => _$UserToJson(this);
}
explicitToJson
in the JsonSerializable
class for the json_annotation
package.
Further references
dart:convert
and JsonCodec
documentationjson_serializable
package on pub.devjson_serializable
examples on GitHub