Middleware (Advanced)

这个例子将展示如何在GO中创建更高级的中间件。一个中间件只需要一个http.HandlerFunc 作为它的一个参数，装饰它并把它作为一个新的 http.HandlerFunc 给服务器调用.

在这里，我们定义一个新的类型 Middleware, 它可以更容易的链接多个中间件。这个想法灵感来自于Mat Ryer 谈论如果建立APIs. 你可以从 这里查看更详细的内容.

这个片段详细展示了如何创建一个新的中间件。此处下面是完整的例子，这里我们减少了一些样板代码.

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
}

下面是完整的例子:

// 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)
}

$ 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