How to pass parameters to axum handlers?

I am trying to build a simple server that provides and decodes JWT tokens and I am missing a big part. Here is the code:

pub struct Server {
    pub host: String,
    pub port: String,
    pub public_key: String,
    pub private_key: String
}

impl Server {
    pub async fn start(&self) {
        let routes = Router::new()
            .route("/", get(check))
            .route("/auth", post(auth));

        let hostport = format!("{}:{}", self.host, self.port);
        let addr: SocketAddr = hostport.parse().expect("invalid host:port pair");
        axum::Server::bind(&addr)
            .serve(routes.into_make_service())
            .await
            .unwrap();
    }
}

async fn auth(Json(payload): Json<LoginInput>) -> impl IntoResponse {
    let token = RS384PublicKey::from_pem("id_like_to_put_Server::public_key_here")
        .sign(Claims::create(Duration::from_hours(1)))?;

    let lo = LoginOutput { token };
    (StatusCode::OK, Json(lo))
}

As you can see Server holds routing logic and applies configuration. Among configuration there is a public key I'd like to use in order to sign the JWT token (I am using jwt_simple to achieve that). Since public key is a Server's attribute, I want to pass that value to the auth handler but I can't figure out how to do that. How can I pass a parameter to an Axum handler and sign the token is generated inside?

Although you can use either Extension and State for this, State is preferred (axum docs):

You should prefer using State if possible since it’s more type safe. The downside is that its less dynamic than request extensions.

In Cargo.toml:

[dependencies]
axum = "0.6.0-rc.2"
serde = { version = "1.0.147", features = ["derive"] }
tokio = { version = "1.21.2", features = ["macros", "rt-multi-thread"] }

Using State:

You would store your Server in the Router using with_state which you can then retrieve in your handler using the State extractor:

use std::net::SocketAddr;

use axum::{
    extract::State,
    response::IntoResponse,
    routing::{get, post},
    Json, Router,
};
use serde::Deserialize;

#[derive(Clone)]
pub struct ServerConfig {
    pub host: String,
    pub port: String,
    pub public_key: String,
    pub private_key: String,
}

#[derive(Deserialize)]
pub struct LoginInput {
    username: String,
    password: String,
}

#[tokio::main]
async fn main() {
    let server_config = ServerConfig {
        host: "0.0.0.0".into(),
        port: "8080".into(),
        public_key: "public_key".into(),
        private_key: "private_key".into(),
    };

    let addr: SocketAddr = format!("{}:{}", server_config.host, server_config.port)
        .parse()
        .unwrap();

    let routes = Router::with_state(server_config) // state will be available to all the routes
        .route("/", get(check))
        .route("/auth", post(auth));

    axum::Server::bind(&addr)
        .serve(routes.into_make_service())
        .await
        .unwrap();
}

async fn check() -> &'static str {
    "check"
}

async fn auth(
    State(server_config): State<ServerConfig>, // extract state in this handler
    // `Json` supports any type that implements `serde::Deserialize`
    Json(payload): Json<LoginInput>,
) -> impl IntoResponse {
    // use server_config and payload to run the `auth` logic
    println!("host: {}", server_config.host);

    "jwt"
}

Using Extension:

You would insert an Extension holding your Server as a .layer() when building your Router. Then you would get it in your handler via the Extension extrator.

use std::net::SocketAddr;

use axum::{
    response::IntoResponse,
    routing::{get, post},
    Extension, Json, Router,
};
use serde::Deserialize;

#[derive(Clone)]
pub struct ServerConfig {
    pub host: String,
    pub port: String,
    pub public_key: String,
    pub private_key: String,
}

#[derive(Deserialize)]
pub struct LoginInput {
    username: String,
    password: String,
}

#[tokio::main]
async fn main() {
    let server_config = ServerConfig {
        host: "0.0.0.0".into(),
        port: "8080".into(),
        public_key: "public_key".into(),
        private_key: "private_key".into(),
    };

    let addr: SocketAddr = format!("{}:{}", server_config.host, server_config.port)
        .parse()
        .unwrap();

    let routes = Router::new()
        .route("/", get(check))
        .route("/auth", post(auth))
        .layer(Extension(server_config));

    axum::Server::bind(&addr)
        .serve(routes.into_make_service())
        .await
        .unwrap();
}

async fn check() -> &'static str {
    "check"
}

async fn auth(
    Extension(server_config): Extension<ServerConfig>,
    // `Json` supports any type that implements `serde::Deserialize`
    Json(payload): Json<LoginInput>,
) -> impl IntoResponse {
    // use server_config and payload to run the `auth` logic
    println!("host: {}", server_config.host);

    "jwt"
}