serde_json 0.9.1

A JSON serialization file format

Serde JSON

JSON is a ubiquitous open-standard format that uses human-readable text to transmit data objects consisting of key-value pairs.

{
  "name": "John Doe",
  "age": 43,
  "address": {
    "street": "10 Downing Street",
    "city": "London"
  },
  "phones": [
    "+44 1234567",
    "+44 2345678"
  ]
}

There are three common ways that you might find yourself needing to work with JSON data in Rust.

  • As text data. An unprocessed string of JSON data that you receive on an HTTP endpoint, read from a file, or prepare to send to a remote server.
  • As an untyped or loosely typed representation. Maybe you want to check that some JSON data is valid before passing it on, but without knowing the structure of what it contains. Or you want to do very basic manipulations like add a level of nesting.
  • As a strongly typed Rust data structure. When you expect all or most of your data to conform to a particular structure and want to get real work done without JSON's loosey-goosey nature tripping you up.

Serde JSON provides efficient, flexible, safe ways of converting data between each of these representations.

JSON to the Value enum

Any valid JSON data can be manipulated in the following recursive enum representation. This data structure is serde_json::Value.

enum Value {
    Null,
    Bool(bool),
    Number(Number),
    String(String),
    Array(Vec<Value>),
    Object(Map<String, Value>),
}

A string of JSON data can be parsed into a serde_json::Value by the serde_json::from_str function. There is also from_slice for parsing from a byte slice &[u8], from_iter for parsing from an iterator of bytes, and from_reader for parsing from any io::Read like a File or a TCP stream.

# extern crate serde_json;
# use serde_json::Error;
# pub fn example() -> Result<(), Error> {
use serde_json::Value;

let data = r#" { "name": "John Doe", "age": 43, ... } "#;
let v: Value = serde_json::from_str(data)?;
println!("Please call {} at the number {}", v["name"], v["phones"][0]);
# Ok(()) }
# fn main() {}

The Value representation is sufficient for very basic tasks but is brittle and tedious to work with. Error handling is verbose to implement correctly, for example imagine trying to detect the presence of unrecognized fields in the input data. The compiler is powerless to help you when you make a mistake, for example imagine typoing v["name"] as v["nmae"] in one of the dozens of places it is used in your code.

JSON to strongly typed data structures

Serde provides a powerful way of mapping JSON data into Rust data structures largely automatically.

# extern crate serde_json;
# #[macro_use] extern crate serde_derive;
# use serde_json::Error;
# pub fn example() -> Result<(), Error> {
#[derive(Serialize, Deserialize)]
struct Person {
    name: String,
    age: u8,
    address: Address,
    phones: Vec<String>,
}

#[derive(Serialize, Deserialize)]
struct Address {
    street: String,
    city: String,
}

let data = r#" { "name": "John Doe", "age": 43, ... } "#;
let p: Person = serde_json::from_str(data)?;
println!("Please call {} at the number {}", p.name, p.phones[0]);
# Ok(()) }
# fn main() {}

This is the same serde_json::from_str function as before, but this time we assign the return value to a variable of type Person so Serde JSON will automatically interpret the input data as a Person and produce informative error messages if the layout does not conform to what a Person is expected to look like.

Any type that implements Serde's Deserialize trait can be deserialized this way. This includes built-in Rust standard library types like Vec<T> and HashMap<K, V>, as well as any structs or enums annotated with #[derive(Deserialize)].

Once we have p of type Person, our IDE and the Rust compiler can help us use it correctly like they do for any other Rust code. The IDE can autocomplete field names to prevent typos, which was impossible in the serde_json::Value representation. And the Rust compiler can check that when we write p.phones[0], then p.phones is guaranteed to be a Vec<String> so indexing into it makes sense and produces a String.

Constructing JSON

Serde JSON provides a json! macro to build serde_json::Value objects with very natural JSON syntax. In order to use this macro, serde_json needs to be imported with the #[macro_use] attribute.

#[macro_use]
extern crate serde_json;

fn main() {
    // The type of `john` is `serde_json::Value`
    let john = json!({
      "name": "John Doe",
      "age": 43,
      "phones": [
        "+44 1234567",
        "+44 2345678"
      ]
    });

    println!("first phone number: {}", john["phones"][0]);

    // Convert to a string of JSON and print it out
    println!("{}", john.to_string());
}

The Value::to_string() function converts a serde_json::Value into a String of JSON text.

One neat thing about the json! macro is that variables and expressions can be interpolated directly into the JSON value as you are building it. Serde will check at compile time that the value you are interpolating is able to be represented as JSON.

# #[macro_use] extern crate serde_json;
# fn random_phone() -> u16 { 0 }
# fn main() {
let full_name = "John Doe";
let age_last_year = 42;

// The type of `john` is `serde_json::Value`
let john = json!({
  "name": full_name,
  "age": age_last_year + 1,
  "phones": [
    format!("+44 {}", random_phone())
  ]
});
# let _ = john;
# }

This is amazingly convenient but we have the problem we had before with Value which is that the IDE and Rust compiler cannot help us if we get it wrong. Serde JSON provides a better way of serializing strongly-typed data structures into JSON text.

Serializing data structures

A data structure can be converted to a JSON string by serde_json::to_string. There is also serde_json::to_vec which serializes to a Vec<u8> and serde_json::to_writer which serializes to any io::Write such as a File or a TCP stream.

# extern crate serde_json;
# #[macro_use] extern crate serde_derive;
# use serde_json::Error;
# pub fn example() -> Result<String, Error> {
#[derive(Serialize, Deserialize)]
struct Address {
    street: String,
    city: String,
}

let address = Address {
    street: "10 Downing Street".to_owned(),
    city: "London".to_owned(),
};

let j = serde_json::to_string(&address)?;
# Ok(j) }
# fn main() {}

Any type that implements Serde's Serialize trait can be serialized this way. This includes built-in Rust standard library types like Vec<T> and HashMap<K, V>, as well as any structs or enums annotated with #[derive(Serialize)].