Module rustc_serialize::json [] [src]

JSON parsing 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 Json enum for more details):

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, their 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"

Rust Type-based Encoding and Decoding

Rust provides a mechanism for low boilerplate encoding & decoding of values to and from JSON via the serialization API. To be able to encode a piece of data, it must implement the rustc_serialize::Encodable trait. To be able to decode a piece of data, it must implement the rustc_serialize::Decodable trait. The Rust compiler provides an annotation to automatically generate the code for these traits: #[derive(RustcDecodable, RustcEncodable)]

The JSON API provides an enum json::Json and a trait ToJson to encode objects. The ToJson trait provides a to_json method to convert an object into a json::Json value. A json::Json value can be encoded as a string or buffer using the functions described above. You can also use the json::Encoder object, which implements the Encoder trait.

When using ToJson the Encodable trait implementation is not mandatory.

Examples of use

Using Autoserialization

Create a struct called TestStruct and serialize and deserialize it to and from JSON using the serialization API, using the derived serialization code.

extern crate rustc_serialize;
use rustc_serialize::json;

// Automatically generate `RustcDecodable` and `RustcEncodable` trait
// implementations
#[derive(RustcDecodable, RustcEncodable)]
pub struct TestStruct  {
    data_int: u8,
    data_str: String,
    data_vector: Vec<u8>,

fn main() {
    let object = TestStruct {
        data_int: 1,
        data_str: "homura".to_string(),
        data_vector: vec![2,3,4,5],

    // Serialize using `json::encode`
    let encoded = json::encode(&object).unwrap();

    // Deserialize using `json::decode`
    let decoded: TestStruct = json::decode(&encoded).unwrap();

Using the ToJson trait

The examples above use the ToJson trait to generate the JSON string, which is required for custom mappings.

Simple example of ToJson usage

extern crate rustc_serialize;
use rustc_serialize::json::{self, ToJson, Json};

// A custom data structure
struct ComplexNum {
    a: f64,
    b: f64,

// JSON value representation
impl ToJson for ComplexNum {
    fn to_json(&self) -> Json {
        Json::String(format!("{}+{}i", self.a, self.b))

// Only generate `RustcEncodable` trait implementation
pub struct ComplexNumRecord {
    uid: u8,
    dsc: String,
    val: Json,

fn main() {
    let num = ComplexNum { a: 0.0001, b: 12.539 };
    let data: String = json::encode(&ComplexNumRecord{
        uid: 1,
        dsc: "test".to_string(),
        val: num.to_json(),
    println!("data: {}", data);
    // data: {"uid":1,"dsc":"test","val":"0.0001+12.539i"};

Verbose example of ToJson usage

extern crate rustc_serialize;
use std::collections::BTreeMap;
use rustc_serialize::json::{self, Json, ToJson};

// Only generate `Decodable` trait implementation
pub struct TestStruct {
    data_int: u8,
    data_str: String,
    data_vector: Vec<u8>,

// Specify encoding method manually
impl ToJson for TestStruct {
    fn to_json(&self) -> Json {
        let mut d = BTreeMap::new();
        // All standard types implement `to_json()`, so use it
        d.insert("data_int".to_string(), self.data_int.to_json());
        d.insert("data_str".to_string(), self.data_str.to_json());
        d.insert("data_vector".to_string(), self.data_vector.to_json());

fn main() {
    // Serialize using `ToJson`
    let input_data = TestStruct {
        data_int: 1,
        data_str: "madoka".to_string(),
        data_vector: vec![2,3,4,5],
    let json_obj: Json = input_data.to_json();
    let json_str: String = json_obj.to_string();

    // Deserialize like before
    let decoded: TestStruct = json::decode(&json_str).unwrap();

Parsing a str to Json and reading the result

extern crate rustc_serialize;
use rustc_serialize::json::Json;

fn main() {
    let data = 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 {
            Json::U64(v) => format!("{} (u64)", v),
            Json::String(ref v) => format!("{} (string)", v),
            _ => format!("other")
    // bar: baz (string)
    // foo: 13 (u64)



A Builder consumes a json::Parser to create a generic Json structure.


A structure to decode JSON to values in rust.


A structure for implementing serialization to JSON.


A streaming JSON parser implemented as an iterator of JsonEvent, consuming an iterator of char.


A Stack represents the current position of the parser in the logical structure of the JSON stream. For example[3].x



The errors that can arise while parsing a JSON stream.


Represents a json value


The output of the streaming parser.


StackElements compose a Stack. For example, Key("foo"), Key("bar"), Index(3) and Key("x") are the StackElements compositing the stack that represents[3].x



A trait for converting values to JSON



Create an AsJson wrapper which can be used to print a value as JSON on-the-fly via write!


Create an AsPrettyJson wrapper which can be used to print a value as JSON on-the-fly via write!


Shortcut function to decode a JSON &str into an object


Shortcut function to encode a T into a JSON String


Returns a readable error string for a given error code.

Type Definitions