Golang net / http和Gorilla：在处理程序之前运行代码

Is it possible using the net/http package and/or any of the gorilla libraries to make some code execute on EVERY URL before going to the handler? For example, to check if a connection is coming from a black listed IP address?

Create a handler that invokes another handler after checking the IP address:

type checker struct {
   h http.Handler
}

func (c checker) ServeHTTP(w http.ResponseWriter, r *http.Request) {
   if blackListed(r.RemoteAddr) {
      http.Error(w, "not authorized", http.StatusForbidden)
      return
   }
   c.h.ServeHTTP(w, r)
}

Pass this handler to ListenAndServe instead of your original handler. For example, if you had:

err := http.ListenAndServe(addr, mux)

change the code to

err := http.ListenAndServe(addr, checker{mux})

This also applies to all the variations of ListenAndServe. It works with http.ServeMux, Gorilla mux and other routers.

If you want to use the default muxer, which i find is common, you can create middleware like so:

func mustBeLoggedIn(handler func(http.ResponseWriter, *http.Request)) func(http.ResponseWriter, *http.Request) {
    return func(w http.ResponseWriter, r *http.Request) {
        // Am i logged in?
        if ...not logged in... {
            http.Error(w, err.Error(), http.StatusUnauthorized)
            return
        }
        // Pass through to the original handler.
        handler(w, r)
    }
}

Use it like so:

http.HandleFunc("/some/priveliged/action", mustBeLoggedIn(myVanillaHandler))
http.ListenAndServe(":80", nil)