JSON

The Json<T> extractor deserializes request bodies as JSON and serializes response values as JSON.

Extracting JSON Bodies

#![allow(unused)]
fn main() {
use serde::Deserialize;
use volter::*;

#[derive(Deserialize)]
struct CreateUser {
    name: String,
    age: u8,
}

async fn create_user(Json(payload): Json<CreateUser>) -> String {
    format!("Created {} (age {})", payload.name, payload.age)
}
}

Json<T> requires:

• The request’s Content-Type header to be application/json or application/*+json
• The body to be valid JSON that deserializes to T
• T: DeserializeOwned + Send + 'static

Returning JSON Responses

#![allow(unused)]
fn main() {
use serde::Serialize;

#[derive(Serialize)]
struct User {
    id: u64,
    name: String,
}

async fn get_user(Path(id): Path<u64>) -> Json<User> {
    Json(User { id, name: "Alice".into() })
}
}

The response has status 200 OK and Content-Type: application/json.

Error Handling

Json<T> can fail in several ways:

Handling specific rejections:

#![allow(unused)]
fn main() {
async fn create(Json(payload): Json<CreateUser>) -> Result<String, JsonRejection> {
    // JsonRejection implements IntoResponse, so return it directly
    let user = payload?;
    Ok(format!("Created {}", user.name))
}
}

JSON Echo Example

#![allow(unused)]
fn main() {
use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
struct Echo {
    message: String,
}

async fn echo(Json(payload): Json<Echo>) -> Json<Echo> {
    Json(payload)
}
}

Performance Notes

• JSON deserialization uses serde_json under the hood
• The body is fully buffered before deserialization (streaming JSON parsing is not supported)
• For large payloads, consider using RequestBodyLimitLayer to reject oversized bodies early