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The Json extractor and response type makes it easy to work with JSON data in Axum applications.

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Extracting JSON request bodies

Use the Json extractor to deserialize JSON from request bodies:

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Basic JSON extraction

1

Define your data structure

Create a struct that implements Deserialize:
use serde::Deserialize;

#[derive(Deserialize)]
struct CreateUser {
    username: String,
    email: String,
    age: u32,
}
2

Use the Json extractor

Extract JSON in your handler:
use axum::Json;

async fn create_user(Json(payload): Json<CreateUser>) {
    // payload is a CreateUser struct
    println!("Creating user: {}", payload.username);
}
3

Set up the route

use axum::{routing::post, Router};

let app = Router::new()
    .route("/users", post(create_user));
The Json extractor requires the request to have a Content-Type: application/json header. Requests without this header will be rejected with a 415 Unsupported Media Type error.

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JSON responses

Return Json to automatically serialize responses: 
use axum::Json;
use serde::Serialize;
use uuid::Uuid;

#[derive(Serialize)]
struct User {
    id: Uuid,
    username: String,
    email: String,
}

async fn get_user() -> Json<User> {
    let user = User {
        id: Uuid::new_v4(),
        username: "alice".to_string(),
        email: "alice@example.com".to_string(),
    };
    
    Json(user)
}
This automatically sets the Content-Type: application/json header and serializes the struct.

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Working with generic JSON

Use serde_json::Value for dynamic JSON: 
use serde_json::Value;

async fn handle_json(Json(payload): Json<Value>) -> String {
    // payload can be any valid JSON
    format!("Received: {}", payload)
}

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Complete CRUD example

Here’s a full REST API example: 
use axum::{
    extract::Path,
    http::StatusCode,
    response::IntoResponse,
    routing::{get, post, put, delete},
    Json, Router,
};
use serde::{Deserialize, Serialize};
use uuid::Uuid;

#[derive(Serialize, Deserialize, Clone)]
struct User {
    id: Uuid,
    username: String,
    email: String,
}

#[derive(Deserialize)]
struct CreateUser {
    username: String,
    email: String,
}

#[derive(Deserialize)]
struct UpdateUser {
    username: Option<String>,
    email: Option<String>,
}

// Create
async fn create_user(Json(payload): Json<CreateUser>) -> (StatusCode, Json<User>) {
    let user = User {
        id: Uuid::new_v4(),
        username: payload.username,
        email: payload.email,
    };
    
    (StatusCode::CREATED, Json(user))
}

// Read one
async fn get_user(Path(user_id): Path<Uuid>) -> Json<User> {
    // In a real app, fetch from database
    Json(User {
        id: user_id,
        username: "alice".to_string(),
        email: "alice@example.com".to_string(),
    })
}

// Read all
async fn list_users() -> Json<Vec<User>> {
    // In a real app, fetch from database
    Json(vec![
        User {
            id: Uuid::new_v4(),
            username: "alice".to_string(),
            email: "alice@example.com".to_string(),
        },
    ])
}

// Update
async fn update_user(
    Path(user_id): Path<Uuid>,
    Json(payload): Json<UpdateUser>,
) -> Json<User> {
    // In a real app, update in database
    Json(User {
        id: user_id,
        username: payload.username.unwrap_or_else(|| "alice".to_string()),
        email: payload.email.unwrap_or_else(|| "alice@example.com".to_string()),
    })
}

// Delete
async fn delete_user(Path(user_id): Path<Uuid>) -> StatusCode {
    // In a real app, delete from database
    StatusCode::NO_CONTENT
}

#[tokio::main]
async fn main() {
    let app = Router::new()
        .route("/users", get(list_users).post(create_user))
        .route("/users/:id", get(get_user).put(update_user).delete(delete_user));

    let listener = tokio::net::TcpListener::bind("127.0.0.1:3000")
        .await
        .unwrap();
    axum::serve(listener, app).await;
}

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Error handling

Handle JSON deserialization errors: 
use axum::{
    extract::rejection::JsonRejection,
    http::StatusCode,
    response::{IntoResponse, Response},
};

#[derive(Deserialize)]
struct Input {
    value: i32,
}

async fn handler(
    payload: Result<Json<Input>, JsonRejection>,
) -> Response {
    match payload {
        Ok(Json(input)) => {
            format!("Received: {}", input.value).into_response()
        }
        Err(err) => {
            // Customize error response
            (
                StatusCode::BAD_REQUEST,
                format!("Invalid JSON: {err}"),
            ).into_response()
        }
    }
}

​
JSON error types

The Json extractor can fail with these errors:
  • MissingJsonContentType: Request missing Content-Type: application/json header (HTTP 415)
  • JsonDataError: JSON is valid but doesn’t match the target type (HTTP 422)
  • JsonSyntaxError: Invalid JSON syntax (HTTP 400)
  • BytesRejection: Failed to buffer request body

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Content type variants

The Json extractor accepts various JSON content types: 
// All these content types are accepted:
// - application/json
// - application/json; charset=utf-8
// - application/cloudevents+json  (any +json suffix)

async fn flexible_json(Json(data): Json<Value>) -> String {
    format!("Received: {data}")
}

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Response customization

Customize JSON responses with status codes and headers: 
use axum::http::StatusCode;

async fn create_item(Json(item): Json<NewItem>) -> (StatusCode, Json<Item>) {
    let created_item = save_item(item).await;
    (StatusCode::CREATED, Json(created_item))
}

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Performance optimization

For large JSON payloads, consider streaming: 
use axum::{
    body::Bytes,
    extract::Request,
    http::StatusCode,
};

async fn large_json_handler(request: Request) -> Result<String, StatusCode> {
    let bytes = Bytes::from_request(request, &())
        .await
        .map_err(|_| StatusCode::BAD_REQUEST)?;
    
    // Deserialize from bytes directly
    let data: MyLargeStruct = Json::<MyLargeStruct>::from_bytes(&bytes)
        .map_err(|_| StatusCode::UNPROCESSABLE_ENTITY)?
        .0;
    
    Ok(format!("Processed {} items", data.items.len()))
}

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Testing JSON handlers

#[cfg(test)]
mod tests {
    use super::*;
    use axum::{
        body::Body,
        http::{self, Request, StatusCode},
    };
    use http_body_util::BodyExt;
    use serde_json::json;
    use tower::ServiceExt;

    #[tokio::test]
    async fn test_create_user() {
        let app = Router::new().route("/users", post(create_user));

        let response = app
            .oneshot(
                Request::post("/users")
                    .header(http::header::CONTENT_TYPE, mime::APPLICATION_JSON.as_ref())
                    .body(Body::from(
                        serde_json::to_vec(&json!({
                            "username": "alice",
                            "email": "alice@example.com"
                        }))
                        .unwrap(),
                    ))
                    .unwrap(),
            )
            .await
            .unwrap();

        assert_eq!(response.status(), StatusCode::CREATED);

        let body = response.into_body().collect().await.unwrap().to_bytes();
        let user: User = serde_json::from_slice(&body).unwrap();
        assert_eq!(user.username, "alice");
    }

    #[tokio::test]
    async fn test_invalid_json() {
        let app = Router::new().route("/users", post(create_user));

        let response = app
            .oneshot(
                Request::post("/users")
                    .header(http::header::CONTENT_TYPE, mime::APPLICATION_JSON.as_ref())
                    .body(Body::from("{invalid json}"))
                    .unwrap(),
            )
            .await
            .unwrap();

        assert_eq!(response.status(), StatusCode::BAD_REQUEST);
    }
}

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Best practices

The Json extractor must be the last extractor in a handler if you have multiple extractors, since it consumes the request body.
Always use strongly-typed structs instead of Value when possible. This provides better type safety and clearer API documentation.
Use #[serde(rename_all = "camelCase")] on your structs to automatically convert between Rust’s snake_case and JSON’s camelCase conventions.