Crate serde_json
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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 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.
Operating on untyped JSON values
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.
use serde_json::{Result, Value};
fn untyped_example() -> Result<()> {
// 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 result of square bracket indexing like v["name"] is a borrow of the
data at that index, so the type is &Value. A JSON map can be indexed with
string keys, while a JSON array can be indexed with integer keys. If the
type of the data is not right for the type with which it is being indexed,
or if a map does not contain the key being indexed, or if the index into a
vector is out of bounds, the returned element is Value::Null.
When a Value is printed, it is printed as a JSON string. So in the code
above, the output looks like Please call "John Doe" at the number "+44 1234567". The quotation marks appear because v["name"] is a &Value
containing a JSON string and its JSON representation is "John Doe".
Printing as a plain string without quotation marks involves converting from
a JSON string to a Rust string with as_str() or avoiding the use of
Value as described in the following section.
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.
Parsing JSON as strongly typed data structures
Serde provides a powerful way of mapping JSON data into Rust data structures largely automatically.
use serde::{Deserialize, Serialize};
use serde_json::Result;
#[derive(Serialize, Deserialize)]
struct Person {
name: String,
age: u8,
phones: Vec<String>,
}
fn typed_example() -> Result<()> {
// 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.
Constructing JSON values
Serde JSON provides a json! macro to build serde_json::Value
objects with very natural JSON syntax.
use serde_json::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: 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.
Creating JSON by 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.
use serde::{Deserialize, Serialize};
use serde_json::Result;
#[derive(Serialize, Deserialize)]
struct Address {
street: String,
city: String,
}
fn print_an_address() -> Result<()> {
// 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)].
No-std support
As long as there is a memory allocator, it is possible to use serde_json without the rest of the Rust standard library. Disable the default “std” feature and enable the “alloc” feature:
[dependencies]
serde_json = { version = "1.0", default-features = false, features = ["alloc"] }For JSON support in Serde without a memory allocator, please see the
serde-json-core crate.
Modules
Macros
serde_json::Value from a JSON literal.Structs
Enums
Functions
T from an I/O stream of JSON.T from bytes of JSON text.T from a string of JSON text.serde_json::Value as an instance of type T.T into serde_json::Value which is an enum that can represent
any valid JSON data.Type Definitions
Result with the error type serde_json::Error.