Serde is a framework for serializing and deserializing Rust data structures efficiently and generically.
[dependencies]
serde_json = "1.0"You may be looking for:
- JSON API documentation
- Serde API documentation
- Detailed documentation about Serde
- Setting up
#[derive(Serialize, Deserialize)] - Release notes
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 insert a key in a particular spot.
- 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.
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] and
from_reader for parsing from any io::Read like a File or
a TCP stream.
extern crate serde_json;
use serde_json::{Value, Error};
fn untyped_example() -> Result<(), Error> {
// Some JSON input data as a &str. Maybe this comes from the user.
let data = r#"{
"name": "John Doe",
"age": 43,
"phones": [
"+44 1234567",
"+44 2345678"
]
}"#;
// Parse the string of data into serde_json::Value.
let v: Value = serde_json::from_str(data)?;
// Access parts of the data by indexing with square brackets.
println!("Please call {} at the number {}", v["name"], v["phones"][0]);
Ok(())
}The Value representation is sufficient for very basic tasks but can be tedious
to work with for anything more significant. 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.
Serde provides a powerful way of mapping JSON data into Rust data structures largely automatically.
extern crate serde;
extern crate serde_json;
#[macro_use]
extern crate serde_derive;
use serde_json::Error;
#[derive(Serialize, Deserialize)]
struct Person {
name: String,
age: u8,
phones: Vec<String>,
}
fn typed_example() -> Result<(), Error> {
// Some JSON input data as a &str. Maybe this comes from the user.
let data = r#"{
"name": "John Doe",
"age": 43,
"phones": [
"+44 1234567",
"+44 2345678"
]
}"#;
// Parse the string of data into a Person object. This is exactly the
// same function as the one that produced serde_json::Value above, but
// now we are asking it for a Person as output.
let p: Person = serde_json::from_str(data)?;
// Do things just like with any other Rust data structure.
println!("Please call {} at the number {}", p.name, p.phones[0]);
Ok(())
}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 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.
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.
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())
]
});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.
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;
extern crate serde_json;
#[macro_use]
extern crate serde_derive;
use serde_json::Error;
#[derive(Serialize, Deserialize)]
struct Address {
street: String,
city: String,
}
fn print_an_address() -> Result<(), Error> {
// Some data structure.
let address = Address {
street: "10 Downing Street".to_owned(),
city: "London".to_owned(),
};
// Serialize it to a JSON string.
let j = serde_json::to_string(&address)?;
// Print, write to a file, or send to an HTTP server.
println!("{}", j);
Ok(())
}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)].
It is fast. You should expect in the ballpark of 500 to 1000 megabytes per second deserialization and 600 to 900 megabytes per second serialization, depending on the characteristics of your data. This is competitive with the fastest C and C++ JSON libraries or even 30% faster for many use cases. Benchmarks live in the serde-rs/json-benchmark repo.
Serde developers live in the #serde channel on
irc.mozilla.org. The #rust channel is also a
good resource with generally faster response time but less specific knowledge
about Serde. If IRC is not your thing, we are happy to respond to GitHub
issues as well.
Serde JSON is licensed under either of
- Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)
at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Serde JSON by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.