介绍
在Go中，中间件可用于在处理程序函数之前和之后执行代码。 它使用单功能接口的能力。可以随时引入而不会影响其他中间件。对于如身份验证日志功能可以在开发的后期阶段添加，而不会影响现有代码。

中间件的签名应为(http.ResponseWriter，* http.Request)，即为http.handlerFunc类型。

普通的请求处理:

func loginHandler(w http.ResponseWriter, r *http.Request) {
      // Steps to login
}

func main() {
    http.HandleFunc("/login", loginHandler)
    http.ListenAndServe(":8080", nil)
}

添加计算时间中间件; 需要handlerFunc来执行

// logger中间件记录处理每个请求所花费的时间
func Logger(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        startTime := time.Now()
        h.ServeHttp(w,r)
        endTime := time.Since(startTime)
        log.Printf("%s %d %v", r.URL, r.Method, endTime)
    })
}

func loginHandler(w http.ResponseWriter, r *http.Request) {
            // Steps to login
}

func main() {
    http.HandleFunc("/login", Logger(loginHandler))
    http.ListenAndServe(":8080", nil)
}

CORS Middleware

func CORS(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        origin := r.Header.Get("Origin")
        w.Header().Set("Access-Control-Allow-Origin", origin)
        if r.Method == "OPTIONS" {
            w.Header().Set("Access-Control-Allow-Credentials", "true")
            w.Header().Set("Access-Control-Allow-Methods", "GET,POST")

            w.RespWriter.Header().Set("Access-Control-Allow-Headers", "Content-Type, X-CSRF-Token, Authorization")
            return
        } else {
            h.ServeHTTP(w, r)
        }
    })
}

func main() {
    http.HandleFunc("/login", Logger(CORS(loginHandler)))
    http.ListenAndServe(":8080", nil)
}

Auth Middleware

func Authenticate(h http.Handler) http.Handler {
    return CustomHandlerFunc(func(w *http.ResponseWriter, r *http.Request) {
        // extract params from req
        // post params | headers etc
        if CheckAuth(params) {
            log.Println("Auth Pass")
            // pass control to next middleware in chain or handler func
            h.ServeHTTP(w, r)
        } else {
            log.Println("Auth Fail")
            // Responsd Auth Fail
        }
    })
}

Recovery防止服务器崩溃

func Recovery(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request){
        defer func() {
            if err := recover(); err != nil {
                // respondInternalServerError
            }
        }()
        h.ServeHTTP(w , r)
    })
}