actix-web/guide/src/qs_4.md

Handler

A request handler can by any object that implements Handler trait. Request handling happen in two stages. First handler object get called. Handle can return any object that implements Responder trait. Then respond_to() get called on returned object. And finally result of the respond_to() call get converted to Reply object.

By default actix provides Responder implementations for some standard types, like &'static str, String, etc. For complete list of implementations check Responder documentation.

Examples of valid handlers:

fn index(req: HttpRequest) -> &'static str {
    "Hello world!"
}

fn index(req: HttpRequest) -> String {
    "Hello world!".to_owned()
}

fn index(req: HttpRequest) -> Bytes {
    Bytes::from_static("Hello world!")
}

fn index(req: HttpRequest) -> Box<Future<Item=HttpResponse, Error=Error>> {
    ...
}

Some notes on shared application state and handler state. If you noticed Handler trait is generic over S, which defines application state type. So application state is accessible from handler with HttpRequest::state() method. But state is accessible as a read-only reference, if you need mutable access to state you have to implement it yourself. On other hand handler can mutable access it's own state as handle method takes mutable reference to self. Beware, actix creates multiple copies of application state and handlers, unique for each thread, so if you run your application in several threads actix will create same amount as number of threads of application state objects and handler objects.

Here is example of handler that stores number of processed requests:

# extern crate actix;
# extern crate actix_web;
use actix_web::*;
use actix_web::dev::Handler;

struct MyHandler(usize);

impl<S> Handler<S> for MyHandler {
    type Result = HttpResponse;

    /// Handle request
    fn handle(&mut self, req: HttpRequest<S>) -> Self::Result {
        self.0 += 1;
        httpcodes::HTTPOk.response()
    }
}
# fn main() {}

This handler will work, but self.0 value will be different depends on number of threads and number of requests processed per thread. Proper implementation would use Arc and AtomicUsize

# extern crate actix;
# extern crate actix_web;
use actix_web::*;
use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};

struct MyHandler(Arc<AtomicUsize>);

impl<S> Handler<S> for MyHandler {
    type Result = HttpResponse;

    /// Handle request
    fn handle(&mut self, req: HttpRequest<S>) -> Self::Result {
        let num = self.0.load(Ordering::Relaxed) + 1;
        self.0.store(num, Ordering::Relaxed);
        httpcodes::HTTPOk.response()
    }
}

fn main() {
    let sys = actix::System::new("example");

    let inc = Arc::new(AtomicUsize::new(0));

    HttpServer::new(
        move || { 
            let cloned = inc.clone();
            Application::new()
            .resource("/", move |r| r.h(MyHandler(cloned)))
        })
        .bind("127.0.0.1:8088").unwrap()
        .start();

    println!("Started http server: 127.0.0.1:8088");
#    actix::Arbiter::system().send(actix::msgs::SystemExit(0));
    let _ = sys.run();
}

Response with custom type

To return custom type directly from handler function type needs to implement Responder trait. Let's create response for custom type that serializes to application/json response:

# extern crate actix;
# extern crate actix_web;
extern crate serde;
extern crate serde_json;
#[macro_use] extern crate serde_derive;
use actix_web::*;

#[derive(Serialize)]
struct MyObj {
    name: &'static str,
}

/// Responder
impl Responder for MyObj {
    type Item = HttpResponse;
    type Error = Error;

    fn respond_to(self, req: HttpRequest) -> Result<HttpResponse> {
        let body = serde_json::to_string(&self)?;

        // Create response and set content type
        Ok(HttpResponse::Ok()
            .content_type("application/json")
            .body(body)?)
    }
}

/// Because `MyObj` implements `Responder`, it is possible to return it directly
fn index(req: HttpRequest) -> MyObj {
    MyObj{name: "user"}
}

fn main() {
    let sys = actix::System::new("example");

    HttpServer::new(
        || Application::new()
            .resource("/", |r| r.method(Method::GET).f(index)))
        .bind("127.0.0.1:8088").unwrap()
        .start();

    println!("Started http server: 127.0.0.1:8088");
#    actix::Arbiter::system().send(actix::msgs::SystemExit(0));
    let _ = sys.run();
}

Async handlers

There are two different types of async handlers.

Response object could be generated asynchronously or more precisely, any type that implements Responder trait. In this case handle must return Future object that resolves to Responder type, i.e:

# extern crate actix_web;
# extern crate futures;
# extern crate bytes;
# use actix_web::*;
# use bytes::Bytes;
# use futures::stream::once;
# use futures::future::{FutureResult, result};
fn index(req: HttpRequest) -> FutureResult<HttpResponse, Error> {

    result(HttpResponse::Ok()
           .content_type("text/html")
           .body(format!("Hello!"))
           .map_err(|e| e.into()))
}

fn index2(req: HttpRequest) -> FutureResult<&'static str, Error> {
    result(Ok("Welcome!"))
}

fn main() {
    Application::new()
        .resource("/async", |r| r.route().a(index))
        .resource("/", |r| r.route().a(index2))
        .finish();
}

Or response body can be generated asynchronously. In this case body must implement stream trait Stream<Item=Bytes, Error=Error>, i.e:

# extern crate actix_web;
# extern crate futures;
# extern crate bytes;
# use actix_web::*;
# use bytes::Bytes;
# use futures::stream::once;
fn index(req: HttpRequest) -> HttpResponse {
    let body = once(Ok(Bytes::from_static(b"test")));

    HttpResponse::Ok()
       .content_type("application/json")
       .body(Body::Streaming(Box::new(body))).unwrap()
}

fn main() {
    Application::new()
        .resource("/async", |r| r.f(index))
        .finish();
}

Both methods could be combined. (i.e Async response with streaming body)