+---

+layout: post

+title: "Golang http handlers as middleware"

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

+comments: true

+categories: ["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.

+

+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.