轉者按：本文介紹瞭如何hook一個http的處理函式，從而加入自定義的內容。

Let’s start with a basic file server for /tmp

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func main() {
    http.ListenAndServe(":8080", http.FileServer(http.Dir("/tmp")))
}

This starts up a local file server at :8080. How can we hook into this so we can run some code before file requests are served? Let’s look at the method signature for http.ListenAndServe:

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func
 
 ListenAndServe(addr string, handler Handler) error

So it looks like http.FileServer returns a Handler that knows how to serve files given a root directory. Now let’s look at the Handler interface:

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type Handler interface {
    ServeHTTP(ResponseWriter, *Request)
}

Because of go’s granular interfaces, any object can be a Handler

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type HandlerFunc func(ResponseWriter, *Request)

Then we just wrap http.FileServer like so:

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func OurLoggingHandler(h http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    fmt.Println(*r.URL)
    h.ServeHTTP(w, r)
  })
}
func main() {
    fileHandler := http.FileServer(http.Dir("/tmp"))
    wrappedHandler := OurLoggingHandler(fileHandler)
    http.ListenAndServe(":8080", wrappedHandler)
}

Go has a bunch of other builtin handlers like TimeoutHandler and RedirectHandler that can be mixed and matched the same way.

type TraceHandler struct {
        h http.Handler
        n int
}

func (r *TraceHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
        r.n++
        fmt.Printf("counter = %d\n", r.n) //why counter always zero
        fmt.Println("get", req.URL.Path, " from ", req.RemoteAddr)
        r.h.ServeHTTP(w, req)
}

func main() {
        port := "9090" //Default port
        if len(os.Args) > 1 {
                port = strings.Join(os.Args[1:2], "")
        }
        h := http.StripPrefix("/icclogs/", http.FileServer(http.Dir("./logs/")))
        http.Handle("/icclogs/", &TraceHandler{h: h, n: 0})

        println("Listening on port ", port, "...")
        err := http.ListenAndServe(":"+port, nil) //設定監聽的埠

        if err != nil {
                log.Fatal("ListenAndServe: ", err)
        }
}