----

-title: "Golang HTTP Handlers as Middleware"

-date: 2013-10-07T08:52:00Z

-comments: true

-tags: ["go", "http"]

----

-

-Most modern web stacks allow the "filtering" of requests via stackable/composable middleware, allowing you to cleanly separate cross-cutting concerns from your web application. This weekend I needed to hook into go's ```http.FileServer``` and was pleasantly surprised how easy it was to do.

-

-<!--more-->

-

-Let's start with a basic file server for ```/tmp```:

-

-```go main.go

-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```:

-

-```go

-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:

-

-```go

-type Handler interface {

- ServeHTTP(ResponseWriter, *Request)

-}

-```

-

-Because of go's granular interfaces, any object can be a ```Handler``` so long as it implements ```ServeHTTP```. It seems all we need to do is construct our own ```Handler``` that wraps ```http.FileServer```'s handler. There's a built in helper for turning ordinary functions into handlers called ```http.HandlerFunc```:

-

-```go

-type HandlerFunc func(ResponseWriter, *Request)

-```

-

-Then we just wrap ```http.FileServer``` like so:

-

-```go main.go

-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](http://golang.org/pkg/net/http/#Handler) like [TimeoutHandler](http://golang.org/pkg/net/http/#TimeoutHandler) and [RedirectHandler](http://golang.org/pkg/net/http/#RedirectHandler) that can be mixed and matched the same way.