pub trait Handler<T, S>: Sized + Clone + Send + 'static {
type Future: Future<Output = Response<Body>> + Send + 'static;
// Required method
fn call(self, req: Request<Body>, state: S) -> Self::Future;
// Provided methods
fn layer<L>(self, layer: L) -> Layered<L, Self, T, S>
where L: Layer<HandlerService<Self, T, S>> + Clone,
<L as Layer<HandlerService<Self, T, S>>>::Service: Service<Request<Body>> { ... }
fn with_state(self, state: S) -> HandlerService<Self, T, S> { ... }
}Expand description
Trait for async functions that can be used to handle requests.
You shouldn’t need to depend on this trait directly. It is automatically implemented to closures of the right types.
See the module docs for more details.
§Converting Handlers into Services
To convert Handlers into Services you have to call either
HandlerWithoutStateExt::into_service or Handler::with_state:
use tower::Service;
use axum::{
extract::{State, Request},
body::Body,
handler::{HandlerWithoutStateExt, Handler},
};
// this handler doesn't require any state
async fn one() {}
// so it can be converted to a service with `HandlerWithoutStateExt::into_service`
assert_service(one.into_service());
// this handler requires state
async fn two(_: State<String>) {}
// so we have to provide it
let handler_with_state = two.with_state(String::new());
// which gives us a `Service`
assert_service(handler_with_state);
// helper to check that a value implements `Service`
fn assert_service<S>(service: S)
where
S: Service<Request>,
{}§Debugging handler type errors
For a function to be used as a handler it must implement the Handler trait.
axum provides blanket implementations for functions that:
- Are
async fns. - Take no more than 16 arguments that all implement
Send.- All except the last argument implement
FromRequestParts. - The last argument implements
FromRequest.
- All except the last argument implement
- Returns something that implements
IntoResponse. - If a closure is used it must implement
Clone + Sendand be'static. - Returns a future that is
Send. The most common way to accidentally make a future!Sendis to hold a!Sendtype across an await.
Unfortunately Rust gives poor error messages if you try to use a function
that doesn’t quite match what’s required by Handler.
You might get an error like this:
error[E0277]: the trait bound `fn(bool) -> impl Future {handler}: Handler<_, _>` is not satisfied
--> src/main.rs:13:44
|
13 | let app = Router::new().route("/", get(handler));
| ^^^^^^^ the trait `Handler<_, _>` is not implemented for `fn(bool) -> impl Future {handler}`
|
::: axum/src/handler/mod.rs:116:8
|
116 | H: Handler<T, B>,
| ------------- required by this bound in `axum::routing::get`
This error doesn’t tell you why your function doesn’t implement
Handler. It’s possible to improve the error with the debug_handler
proc-macro from the axum-macros crate.
§Handlers that aren’t functions
The Handler trait is also implemented for T: IntoResponse. That allows easily returning
fixed data for routes:
use axum::{
Router,
routing::{get, post},
Json,
http::StatusCode,
};
use serde_json::json;
let app = Router::new()
// respond with a fixed string
.route("/", get("Hello, World!"))
// or return some mock data
.route("/users", post((
StatusCode::CREATED,
Json(json!({ "id": 1, "username": "alice" })),
)));Required Associated Types§
Required Methods§
Provided Methods§
fn layer<L>(self, layer: L) -> Layered<L, Self, T, S>where
L: Layer<HandlerService<Self, T, S>> + Clone,
<L as Layer<HandlerService<Self, T, S>>>::Service: Service<Request<Body>>,
fn layer<L>(self, layer: L) -> Layered<L, Self, T, S>where
L: Layer<HandlerService<Self, T, S>> + Clone,
<L as Layer<HandlerService<Self, T, S>>>::Service: Service<Request<Body>>,
Apply a tower::Layer to the handler.
All requests to the handler will be processed by the layer’s corresponding middleware.
This can be used to add additional processing to a request for a single handler.
Note this differs from routing::Router::layer
which adds a middleware to a group of routes.
If you’re applying middleware that produces errors you have to handle the errors so they’re converted into responses. You can learn more about doing that here.
§Example
Adding the [tower::limit::ConcurrencyLimit] middleware to a handler
can be done like so:
use axum::{
routing::get,
handler::Handler,
Router,
};
use tower::limit::{ConcurrencyLimitLayer, ConcurrencyLimit};
async fn handler() { /* ... */ }
let layered_handler = handler.layer(ConcurrencyLimitLayer::new(64));
let app = Router::new().route("/", get(layered_handler));fn with_state(self, state: S) -> HandlerService<Self, T, S>
fn with_state(self, state: S) -> HandlerService<Self, T, S>
Convert the handler into a Service by providing the state
Object Safety§
This trait is not object safe.