Go Web 编程入门：中间件

前言

中间件通常位于前端的客户端站点请求和请求的后端资源之间。中间件非常有用，尤其是当我们希望在进行 API 调用之前进行一些验证，例如请求方法、标头和 JWT 等。

日志中间件

http.Handler 包装器是一个具有一个输入参数和一个输出参数的函数，两者都是 http.Handler 类型。

func Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        //code before make the backend api call

	next.ServeHTTP(w, r) //serve and handle the http request

        //code after make the backend api call
	})
}

现在，而不是直接服务和监听我们的 http 处理程序，例如：

http.ListenAndServe(":3000", handler)

我们现在可以将它包装在我们的中间件中并变成：

http.ListenAndServe(":3000", Middleware(handler))

接下来将介绍如何创建一个 Go 语言的日志中间件。

中间件只需将 http.HandlerFunc 作为其参数之一，将其包装并返回一个新的 http.HandlerFunc 供服务器调用。

新建一个 main.go 文件：

package main

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

func logging(f http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        log.Println(r.URL.Path)
        f(w, r)
    }
}

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

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

func main() {
    http.HandleFunc("/foo", logging(foo))
    http.HandleFunc("/bar", logging(bar))

    http.ListenAndServe(":8080", nil)
}

创建 mod 文件：

$ go mod init main.go 
go: creating new go.mod: module main.go
go: to add module requirements and sums:
        go mod tidy

打开另一个终端：

$ curl -s http://localhost:8080/foo
foo
$ curl -s http://localhost:8080/bar
bar
$ curl -s http://localhost:8080/foo\?bar
foo

运行上述代码：

$ go run main.go         
2022/05/25 23:12:00 /foo
2022/05/25 23:12:09 /bar
2022/05/25 23:12:16 /foo

中间件框架

中间件本身只是将 a 作为其参数之一，将其包装并返回一个新的供服务器调用。在这里，我们定义了一种新类型，它最终使将多个中间件链接在一起变得更加容易。这个想法的灵感来自于 Mat Ryers 关于构建 API 的演讲。

这个片段详细解释了如何创建一个新的中间件。在下面的完整示例中，代码结构示例如下：

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
}

完整测试

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)
}

运行代码：

$ go run main.go 

打开另外的终端：

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

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

$ go run main.go         
2022/05/25 23:20:01 / 21.595µs
2022/05/25 23:20:13 / 5.129µs

参考链接：

• Middleware with GO (jeffdevslife.com)