serde_json 0.9.0-rc2

A JSON serialization file format

JSON and serialization

What is JSON?

JSON (JavaScript Object Notation) is a way to write data in JavaScript. Like XML, it allows to encode structured data in a text format that can be easily read by humans. Its simple syntax and native compatibility with JavaScript have made it a widely used format.

Data types that can be encoded are JavaScript types (see the serde_json:Value enum for more details):

  • Bool: equivalent to rust's bool
  • I64: equivalent to rust's i64
  • U64: equivalent to rust's u64
  • F64: equivalent to rust's f64
  • String: equivalent to rust's String
  • Array: equivalent to rust's Vec<T>, but also allowing objects of different types in the same array
  • Object: equivalent to rust's BTreeMap<String, serde_json::Value>; set the preserve_order feature to use LinkedHashMap<String, serde_json::Value> instead
  • Null

An object is a series of string keys mapping to values, in "key": value format. Arrays are enclosed in square brackets ([ ... ]) and objects in curly brackets ({ ... }). A simple JSON document encoding a person, his/her age, address and phone numbers could look like

    "FirstName": "John",
    "LastName": "Doe",
    "Age": 43,
    "Address": {
        "Street": "Downing Street 10",
        "City": "London",
        "Country": "Great Britain"
    "PhoneNumbers": [
        "+44 1234567",
        "+44 2345678"

If we assume that FirstName is optional and all other fields are mandatory, the above JSON could correspond to the following Rust structs:

#[derive(Serialize, Deserialize)]
struct Data {
    #[serde(rename="FirstName")] // to comply with Rust coding standards
    first_name: Option<String>,
    LastName: String,
    Age: u32,
    Address: Address,
    PhoneNumbers: Vec<String>,

#[derive(Serialize, Deserialize)]
struct Address {
    Street: String,
    City: String,
    Country: String,

Type-based Serialization and Deserialization

Serde provides a mechanism for low boilerplate serialization & deserialization of values to and from JSON via the serialization API. To be able to serialize a piece of data, it must implement the serde::Serialize trait. To be able to deserialize a piece of data, it must implement the serde::Deserialize trait. Serde provides provides an annotation to automatically generate the code for these traits: #[derive(Serialize, Deserialize)].

The JSON API also provides an enum serde_json::Value and a method to_value to serialize objects. A serde_json::Value value can be serialized as a string or buffer using the functions described above. You can also use the json::Serializer object, which implements the Serializer trait.

Examples of use

Parsing a str to Value and reading the result

extern crate serde_json;

use serde_json::Value;

fn main() {
    let data: Value = serde_json::from_str("{\"foo\": 13, \"bar\": \"baz\"}").unwrap();
    println!("data: {:?}", data);
    // data: {"bar":"baz","foo":13}
    println!("object? {}", data.is_object());
    // object? true

    let obj = data.as_object().unwrap();
    let foo = obj.get("foo").unwrap();

    println!("array? {:?}", foo.as_array());
    // array? None
    println!("u64? {:?}", foo.as_u64());
    // u64? Some(13u64)

    for (key, value) in obj.iter() {
        println!("{}: {}", key, match *value {
            Value::Number(ref v) => format!("{} (number)", v),
            Value::String(ref v) => format!("{} (string)", v),
            _ => format!("other")
    // bar: baz (string)
    // foo: 13 (u64)