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+---
+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.