已应用：HTTP 服务器

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2023-12-01

让我们用async/.await建立一个回声服务器！

开始之前，运行rustup update stable，以确保你有 stable Rust 1.39 或更新的版本。一旦完成，就cargo new async-await-echo创建新项目，并打开输出的async-await-echo文件夹。

让我们将一些依赖项，添加到Cargo.toml文件：

[dependencies]
# The latest version of the "futures" library, which has lots of utilities
# for writing async code. Enable the "compat" feature to include the
# functions for using futures 0.3 and async/await with the Hyper library,
# which use futures 0.1.
futures = { version = "0.3", features = ["compat"] }

# Hyper is an asynchronous HTTP library. We'll use it to power our HTTP
# server and to make HTTP requests.
hyper = "0.12.9"

既然我们已经摆脱了依赖关系，让我们开始编写一些代码。我们有一些 imports 要添加：

use {
    hyper::{
        // Miscellaneous types from Hyper for working with HTTP.
        Body, Client, Request, Response, Server, Uri,

        // This function turns a closure which returns a future into an
        // implementation of the the Hyper `Service` trait, which is an
        // asynchronous function from a generic `Request` to a `Response`.
        service::service_fn,

        // A function which runs a future to completion using the Hyper runtime.
        rt::run,
    },
    futures::{
        // Extension trait for futures 0.1 futures, adding the `.compat()` method
        // which allows us to use `.await` on 0.1 futures.
        compat::Future01CompatExt,
        // Extension traits providing additional methods on futures.
        // `FutureExt` adds methods that work for all futures, whereas
        // `TryFutureExt` adds methods to futures that return `Result` types.
        future::{FutureExt, TryFutureExt},
    },
    std::net::SocketAddr,
};

一旦搞定这些 imports，我们就可以开始整理样板文件，以便满足以下要求：

async fn serve_req(_req: Request<Body>) -> Result<Response<Body>, hyper::Error> {
    // Always return successfully with a response containing a body with
    // a friendly greeting ;)
    Ok(Response::new(Body::from("hello, world!")))
}

async fn run_server(addr: SocketAddr) {
    println!("Listening on http://{}", addr);

    // Create a server bound on the provided address
    let serve_future = Server::bind(&addr)
        // Serve requests using our `async serve_req` function.
        // `serve` takes a closure which returns a type implementing the
        // `Service` trait. `service_fn` returns a value implementing the
        // `Service` trait, and accepts a closure which goes from request
        // to a future of the response. To use our `serve_req` function with
        // Hyper, we have to box it and put it in a compatability
        // wrapper to go from a futures 0.3 future (the kind returned by
        // `async fn`) to a futures 0.1 future (the kind used by Hyper).
        .serve(|| service_fn(|req| serve_req(req).boxed().compat()));

    // Wait for the server to complete serving or exit with an error.
    // If an error occurred, print it to stderr.
    if let Err(e) = serve_future.compat().await {
        eprintln!("server error: {}", e);
    }
}

fn main() {
    // Set the address to run our socket on.
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));

    // Call our `run_server` function, which returns a future.
    // As with every `async fn`, for `run_server` to do anything,
    // the returned future needs to be run. Additionally,
    // we need to convert the returned future from a futures 0.3 future into a
    // futures 0.1 future.
    let futures_03_future = run_server(addr);
    let futures_01_future = futures_03_future.unit_error().boxed().compat();

    // Finally, we can run the future to completion using the `run` function
    // provided by Hyper.
    run(futures_01_future);
}

如果你现在cargo run，你应该看到信息“Listening on http://127.0.0.1:3000“打印在你的终端上。如果你在你选择的浏览器中，打开这个网址，你会看到“hello, world!”出现在浏览器中。祝贺你！您刚刚在 Rust 中编写了，第一个异步 web 服务器。

您还可以检查 request(请求) 本身，它包含诸如，request 的 URI、HTTP 版本、header 和其他元数据等信息。例如，我们可以打印出请求的 URI，如下所示：

println!("Got request at {:?}", req.uri());

您可能已经注意到，在处理请求时，我们还没有做任何异步操作，我们只是立即响应，所以我们没有利用上async fn给我们的灵活性。与其只返回静态消息，不如尝试使用 Hyper 的 HTTP 客户端，将用户的请求代理到另一个网站。

我们首先解析出要请求的 URL：

let url_str = "http://www.rust-lang.org/en-US/";
let url = url_str.parse::<Uri>().expect("failed to parse URL");

然后，我们可以新建一个新的hyper::Client，并使用它，制造一个GET请求，将响应返回给用户：

let res = Client::new().get(url).compat().await;
// Return the result of the request directly to the user
println!("request finished-- returning response");
res

Client::get会返回一个hyper::client::FutureResponse，它实现了Future<Output = Result<Response, Error>>（或Future<Item = Response, Error = Error>在 futures 0.1 版）。当我们.await以后，会发送一个 HTTP 请求，挂起当前任务，并在响应可用时，任务会排队等待继续。

现在，如果你现在cargo run，在浏览器中打开http://127.0.0.1:3000/foo，您将看到 Rust 主页和以下终端输出：

Listening on http://127.0.0.1:3000
Got request at /foo
making request to http://www.rust-lang.org/en-US/
request finished-- returning response

祝贺你！你只是代理了一个 HTTP 请求。