Dart JSON
last modified January 28, 2024
In this article we show how to work with JSON in Dart language. We use
the dart:convert library.
JSON
JSON (JavaScript Object Notation) is a lightweight data-interchange
format. It is easily read and written by humans and parsed and generated by
machines. The application/json is the official Internet media type
for JSON. The JSON filename extension is .json.
Dart also has JSON generation libraries
package:json_serializable and package:built_value.
In this article we focus on the built-in dart:convert library.
For each class that is going to be converted to/from JSON, we define two
methods: fromJson and toJson.
The fromJson is used for constructing a new object instance from a
map structure. The toJson method converts a class instance into a
map. These functions are used in conjunction with the
dart:convert's json.decode and
json.encode.
The json.decode function parses the JSON string and returns the
resulting Json object (a Dart map). The json.encode function
converts an object to a JSON string.
Dart JSON simple example
The following is a simple Dart example that works with JSON data.
import 'dart:convert';
class User {
final String name;
final String occupation;
User(this.name, this.occupation);
User.fromJson(Map<String, dynamic> m)
: name = m['name'],
occupation = m['occupation'];
Map<String, dynamic> toJson() => {
'name': name,
'occupation': occupation,
};
@override
String toString() {
return "$name|$occupation";
}
}
void main() {
final u1Json = '{"name": "John Doe", "occupation": "gardener"}';
final u1 = User.fromJson(json.decode(u1Json));
print(u1);
final u2 = User('Roger Roe', 'gardener');
final u2Json = json.encode(u2);
print(u2Json);
}
In the program, we convert a JSON string to a Dart object and vice versa.
import 'dart:convert';
We import the dart:convert library.
User.fromJson(Map<String, dynamic> m)
: name = m['name'],
occupation = m['occupation'];
This creates a new User instance from the map structure.
Map<String, dynamic> toJson() => {
'name': name,
'occupation': occupation,
};
The toJson method creates a map structure from an object instance.
final u1Json = '{"name": "John Doe", "occupation": "gardener"}';
We define a JSON string.
final u1 = User.fromJson(json.decode(u1Json));
We use json.decode to convert the string to a map and pass it to
the User.fromJson method to create a User object from
the generated map.
final u2 = User('Roger Roe', 'gardener');
final u2Json = json.encode(u2);
Now we have the reverse operation. We define a User object and
transform it into a JSON string using json.encode.
$ dart main.dart
John Doe|gardener
{"name":"Roger Roe","occupation":"gardener"}
We also have jsonDecode and jsonEncode shorthand
functions, which are useful if a local variable shadows the global json
constant.
import 'dart:convert';
class User {
final String name;
final String occupation;
User(this.name, this.occupation);
User.fromJson(Map<String, dynamic> m)
: name = m['name'],
occupation = m['occupation'];
Map<String, dynamic> toJson() => {
'name': name,
'occupation': occupation,
};
@override
String toString() {
return "$name|$occupation";
}
}
void main() {
final u1Json = '{"name": "John Doe", "occupation": "gardener"}';
final u1 = User.fromJson(jsonDecode(u1Json));
print(u1);
final u2 = User('Roger Roe', 'gardener');
final u2Json = jsonEncode(u2);
print(u2Json);
}
In the program we do the same convertions with jsonDecode and
jsonEncode.
Dart list of Json objects
In the next example, we convert a list of Json objects into a JSON string.
import 'dart:convert';
void main() {
final users = [
{'name': 'John Doe', 'occupation': 'gardener'},
{'name': 'Roger Roe', 'occupation': 'driver'},
{'name': 'Thomas Brown', 'occupation': 'teacher'}
];
final res = json.encode(users);
print(res);
}
We have a list of users. Each user is defined by a
Map<String, String>. We convert the list into a JSON string
with json.encode.
$ dart main.dart
[{"name":"John Doe","occupation":"gardener"},{"name":"Roger Roe","occupation":"driver"},
{"name":"Thomas Brown","occupation":"teacher"}]
Dart list of objects
In the next example, we work with a list of User objects.
import 'dart:convert';
class User {
final String name;
final String occupation;
User(this.name, this.occupation);
User.fromJson(Map<String, dynamic> m)
: name = m['name'],
occupation = m['occupation'];
Map<String, dynamic> toJson() => {
'name': name,
'occupation': occupation,
};
@override
String toString() {
return "$name|$occupation";
}
}
void main() {
final users = <User>[
User("John Doe", "gardener"),
User("Roger Roe", "driver"),
User("Finn Bradley", "teacher")
];
String res = json.encode(users);
print(res);
print('----------------------------------');
List<dynamic> data = json.decode(res);
List<User> users2 =
List<User>.from(data.map<User>((dynamic e) => User.fromJson(e)));
print(users2);
}
We convert a list of users into a JSON string and vice versa.
final users = <User>[
User("John Doe", "gardener"),
User("Roger Roe", "driver"),
User("Finn Bradley", "teacher")
];
We define a list of user objects.
String res = json.encode(users);
The list is encoded into a JSON string with json.encode.
List<dynamic> data = json.decode(res);
We decode the JSON string into a list.
List<User> users2 = List<User>.from(data.map<User>((dynamic e) => User.fromJson(e)));
The list is transformed into a list of User instances.
$ dart main.dart
[{"name":"John Doe","occupation":"gardener"},{"name":"Roger Roe","occupation":"driver"},
{"name":"Finn Bradley","occupation":"teacher"}]
----------------------------------
[John Doe|gardener, Roger Roe|driver, Finn Bradley|teacher]
Dart fetch JSON data from URL
In the next example, we fetch JSON data from an URL resource.
import 'dart:convert';
import 'package:http/http.dart' as http;
class User {
final int id;
final String firstName;
final String lastName;
final String email;
User(this.id, this.firstName, this.lastName, this.email);
User.fromJson(Map<String, dynamic> m)
: id = m['id'],
firstName = m['first_name'],
lastName = m['last_name'],
email = m['email'];
Map<String, dynamic> toJson() => {
'id': id,
'first_name': firstName,
'last_name': lastName,
'email': email,
};
@override
String toString() {
return "$id|$firstName|$lastName|$email";
}
}
void main() async {
final url = 'webcode.me';
final path = '/users.json';
final resp = await http.get(Uri.http(url, path));
List<dynamic> data = json.decode(resp.body)['users'];
List<User> users =
List<User>.from(data.map<User>((dynamic e) => User.fromJson(e)));
print(users);
print(users[0]);
print(json.encode(users[0]));
}
In the example, we generate a GET request to a web resource. The resource is in JSON format.
class User {
final int id;
final String firstName;
final String lastName;
final String email;
User(this.id, this.firstName, this.lastName, this.email);
User.fromJson(Map<String, dynamic> m)
: id = m['id'],
firstName = m['first_name'],
lastName = m['last_name'],
email = m['email'];
Map<String, dynamic> toJson() => {
'id': id,
'first_name': firstName,
'last_name': lastName,
'email': email,
};
@override
String toString() {
return "$id|$firstName|$lastName|$email";
}
}
We have a User class. The JSON resource has the following fields:
id, first_name, last_name, and
email. We define the User.fromJson and
toJson methods.
final url = 'webcode.me'; final path = '/users.json'; final resp = await http.get(Uri.http(url, path));
We generate a GET request to the specified resource.
List<dynamic> data = json.decode(resp.body)['users'];
We decode the JSON resource into a list. The resource is a named list;
therefore, we pick the users field.
List<User> users = List<User>.from(data.map<User>((dynamic e) => User.fromJson(e)));
The list is transformed into a list of User objects.
$ dart main.dart [1|Robert|Schwartz|[email protected], 2|Lucy|Ballmer|[email protected], 3|Anna|Smith|[email protected], 4|Robert|Brown|[email protected], 5|Roger|Bacon|[email protected]] 1|Robert|Schwartz|[email protected] {"id":1,"first_name":"Robert","last_name":"Schwartz","email":"[email protected]"}
Source
In this article we have worked with JSON data in Dart. We utilized the
dart:convert library.
Author
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