Middleware (高级)

本示例将展示如何在Go中创建更高级版本的中间件。

中间件本身只接受 http.HandlerFunc 作为其参数之一，包装它并返回新的 http.HandlerFunc的服务调用。

在这里，我们定义了一种新型 Middleware，它最终使将多个中间件链接在一起变得更容易。这个想法受到 Mat Ryers’ 关于构建API的讨论的启发。你可以在 here找到更详细的解释，包括演讲。

这个片段详细解释了如何创建新的中间件。在下面的完整示例中，我们通过一些样板代码减少了这个版本。

func createNewMiddleware() Middleware {

    // Create a new Middleware
    middleware := func(next http.HandlerFunc) http.HandlerFunc {

        // Define the http.HandlerFunc which is called by the server eventually
        handler := func(w http.ResponseWriter, r *http.Request) {

            // ... do middleware things

            // Call the next middleware/handler in chain
            next(w, r)
        }

        // Return newly created handler
        return handler
    }

    // Return newly created middleware
    return middleware
}

完整例子

// advanced-middleware.go
package main

import (
    "fmt"
    "log"
    "net/http"
    "time"
)

type Middleware func(http.HandlerFunc) http.HandlerFunc

// Logging logs all requests with its path and the time it took to process
func Logging() Middleware {

    // Create a new Middleware
    return func(f http.HandlerFunc) http.HandlerFunc {

        // Define the http.HandlerFunc
        return func(w http.ResponseWriter, r *http.Request) {

            // Do middleware things
            start := time.Now()
            defer func() { log.Println(r.URL.Path, time.Since(start)) }()

            // Call the next middleware/handler in chain
            f(w, r)
        }
    }
}

// Method ensures that url can only be requested with a specific method, else returns a 400 Bad Request
func Method(m string) Middleware {

    // Create a new Middleware
    return func(f http.HandlerFunc) http.HandlerFunc {

        // Define the http.HandlerFunc
        return func(w http.ResponseWriter, r *http.Request) {

            // Do middleware things
            if r.Method != m {
                http.Error(w, http.StatusText(http.StatusBadRequest), http.StatusBadRequest)
                return
            }

            // Call the next middleware/handler in chain
            f(w, r)
        }
    }
}

// Chain applies middlewares to a http.HandlerFunc
func Chain(f http.HandlerFunc, middlewares ...Middleware) http.HandlerFunc {
    for _, m := range middlewares {
        f = m(f)
    }
    return f
}

func Hello(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintln(w, "hello world")
}

func main() {
    http.HandleFunc("/", Chain(Hello, Method("GET"), Logging()))
    http.ListenAndServe(":8080", nil)
}

run

$ go run advanced-middleware.go
2017/02/11 00:34:53 / 0s

$ curl -s http://localhost:8080/
hello world

$ curl -s -XPOST http://localhost:8080/
Bad Request