How do I parse a JSON File?

Question

I have this so far in my goal to Parse this JSON data in Rust:

extern crate rustc_serialize;
use rustc_serialize::json::Json;
use std::fs::File;
use std::io::copy;
use std::io::stdout;

fn main() {
    let mut file = File::open("text.json").unwrap();
    let mut stdout = stdout();
    let mut str = &copy(&mut file, &mut stdout).unwrap().to_string();
    let data = Json::from_str(str).unwrap();
}

and text.json is

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

What should be my next step into parsing it? My primary goal is to get JSON data like this, and parse a key from it, like Age.

Answer 1

Serde is the preferred JSON serialization provider. You can read the JSON text from a file a number of ways. Once you have it as a string, use serde_json::from_str:

fn main() {
    let the_file = r#"{
        "FirstName": "John",
        "LastName": "Doe",
        "Age": 43,
        "Address": {
            "Street": "Downing Street 10",
            "City": "London",
            "Country": "Great Britain"
        },
        "PhoneNumbers": [
            "+44 1234567",
            "+44 2345678"
        ]
    }"#;

    let json: serde_json::Value =
        serde_json::from_str(the_file).expect("JSON was not well-formatted");
}

Cargo.toml:

[dependencies]
serde = { version = "1.0.104", features = ["derive"] }
serde_json = "1.0.48"

You could even use something like serde_json::from_reader to read directly from an opened File.

Serde can be used for formats other than JSON and it can serialize and deserialize to a custom struct instead of an arbitrary collection:

use serde::Deserialize;

#[derive(Debug, Deserialize)]
#[serde(rename_all = "PascalCase")]
struct Person {
    first_name: String,
    last_name: String,
    age: u8,
    address: Address,
    phone_numbers: Vec<String>,
}

#[derive(Debug, Deserialize)]
#[serde(rename_all = "PascalCase")]
struct Address {
    street: String,
    city: String,
    country: String,
}

fn main() {
    let the_file = /* ... */;

    let person: Person = serde_json::from_str(the_file).expect("JSON was not well-formatted");
    println!("{:?}", person)
}

Check the Serde website for more details.

Answer 2

Solved by the many helpful members of the Rust community:

extern crate rustc_serialize;
use rustc_serialize::json::Json;
use std::fs::File;
use std::io::Read;

fn main() {
    let mut file = File::open("text.json").unwrap();
    let mut data = String::new();
    file.read_to_string(&mut data).unwrap();

    let json = Json::from_str(&data).unwrap();
    println!("{}", json.find_path(&["Address", "Street"]).unwrap());
}

Answer 3

There is a brief and complete example of how to read JSON from file in serde_json::de::from_reader docs.

Here is a short snippet for:

• reading a file
• parsing its contents as a JSON
• and extracting a field with the desired key

Enjoy:

let file = fs::File::open("text.json")
    .expect("file should open read only");
let json: serde_json::Value = serde_json::from_reader(file)
    .expect("file should be proper JSON");
let first_name = json.get("FirstName")
    .expect("file should have FirstName key");

Answer 4

Upvoted the accepted answer (as it helps), but just adding my answer, using the widely used serde_json crate referenced by @FrickeFresh

Assuming your foo.json is

{
    "name": "Jane",
    "age": 11
}

Implementation would look something like

extern crate serde;
extern crate json_serde;
#[macro_use] extern crate json_derive;
use std::fs::File;
use std::io::Read;

#[derive(Serialize, Deserialize)]
struct Foo {
    name: String,
    age: u32,
}

fn main() {
   let mut file = File::open("foo.json").unwrap();
   let mut buff = String::new();
   file.read_to_string(&mut buff).unwrap();

   let foo: Foo = serde_json::from_str(&buff).unwrap();
   println!("Name: {}", foo.name);
}