545 lines
15 KiB
Markdown
545 lines
15 KiB
Markdown
---
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title: Go's http package by example
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---
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Go's [http](http://golang.org/pkg/net/http/) package has turned into one of my
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favorite things about the Go programming language. Initially it appears to be
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somewhat complex, but in reality it can be broken down into a couple of simple
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components that are extremely flexible in how they can be used. This guide will
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cover the basic ideas behind the http package, as well as examples in using,
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testing, and composing apps built with it.
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This guide assumes you have some basic knowledge of what an interface in Go is,
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and some idea of how HTTP works and what it can do.
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## Handler
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The building block of the entire http package is the `http.Handler` interface,
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which is defined as follows:
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```go
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type Handler interface {
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ServeHTTP(ResponseWriter, *Request)
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}
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```
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Once implemented the `http.Handler` can be passed to `http.ListenAndServe`,
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which will call the `ServeHTTP` method on every incoming request.
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`http.Request` contains all relevant information about an incoming http request
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which is being served by your `http.Handler`.
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The `http.ResponseWriter` is the interface through which you can respond to the
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request. It implements the `io.Writer` interface, so you can use methods like
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`fmt.Fprintf` to write a formatted string as the response body, or ones like
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`io.Copy` to write out the contents of a file (or any other `io.Reader`). The
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response code can be set before you begin writing data using the `WriteHeader`
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method.
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Here's an example of an extremely simple http server:
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```go
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package main
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import (
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"fmt"
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"log"
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"net/http"
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)
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type helloHandler struct{}
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func (h helloHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "hello, you've hit %s\n", r.URL.Path)
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}
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func main() {
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err := http.ListenAndServe(":9999", helloHandler{})
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log.Fatal(err)
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}
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```
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`http.ListenAndServe` serves requests using the handler, listening on the given
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address:port. It will block unless it encounters an error listening, in which
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case we `log.Fatal`.
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Here's an example of using this handler with curl:
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```
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~ $ curl localhost:9999/foo/bar
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hello, you've hit /foo/bar
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```
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## HandlerFunc
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Often defining a full type to implement the `http.Handler` interface is a bit
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overkill, especially for extremely simple `ServeHTTP` functions like the one
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above. The `http` package provides a helper function, `http.HandlerFunc`, which
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wraps a function which has the signature
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`func(w http.ResponseWriter, r *http.Request)`, returning an `http.Handler`
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which will call it in all cases.
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The following behaves exactly like the previous example, but uses
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`http.HandlerFunc` instead of defining a new type.
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```go
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package main
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import (
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"fmt"
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"log"
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"net/http"
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)
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func main() {
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h := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "hello, you've hit %s\n", r.URL.Path)
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})
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err := http.ListenAndServe(":9999", h)
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log.Fatal(err)
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}
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```
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## ServeMux
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On their own, the previous examples don't seem all that useful. If we wanted to
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have different behavior for different endpoints we would end up with having to
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parse path strings as well as numerous `if` or `switch` statements. Luckily
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we're provided with `http.ServeMux`, which does all of that for us. Here's an
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example of it being used:
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```go
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package main
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import (
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"fmt"
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"log"
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"net/http"
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)
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func main() {
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h := http.NewServeMux()
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h.HandleFunc("/foo", func(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintln(w, "Hello, you hit foo!")
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})
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h.HandleFunc("/bar", func(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintln(w, "Hello, you hit bar!")
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})
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h.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
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w.WriteHeader(404)
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fmt.Fprintln(w, "You're lost, go home")
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})
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err := http.ListenAndServe(":9999", h)
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log.Fatal(err)
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}
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```
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The `http.ServeMux` is itself an `http.Handler`, so it can be passed into
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`http.ListenAndServe`. When it receives a request it will check if the request's
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path is prefixed by any of its known paths, choosing the longest prefix match it
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can find. We use the `/` endpoint as a catch-all to catch any requests to
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unknown endpoints. Here's some examples of it being used:
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```
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~ $ curl localhost:9999/foo
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Hello, you hit foo!
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~ $ curl localhost:9999/bar
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Hello, you hit bar!
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~ $ curl localhost:9999/baz
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You're lost, go home
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```
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`http.ServeMux` has both `Handle` and `HandleFunc` methods. These do the same
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thing, except that `Handle` takes in an `http.Handler` while `HandleFunc` merely
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takes in a function, implicitly wrapping it just as `http.HandlerFunc` does.
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### Other muxes
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There are numerous replacements for `http.ServeMux` like
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[gorilla/mux](http://www.gorillatoolkit.org/pkg/mux) which give you things like
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automatically pulling variables out of paths, easily asserting what http methods
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are allowed on an endpoint, and more. Most of these replacements will implement
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`http.Handler` like `http.ServeMux` does, and accept `http.Handler`s as
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arguments, and so are easy to use in conjunction with the rest of the things
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I'm going to talk about in this post.
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## Composability
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When I say that the `http` package is composable I mean that it is very easy to
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create re-usable pieces of code and glue them together into a new working
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application. The `http.Handler` interface is the way all pieces communicate with
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each other. Here's an example of where we use the same `http.Handler` to handle
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multiple endpoints, each slightly differently:
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```go
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package main
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import (
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"fmt"
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"log"
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"net/http"
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)
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type numberDumper int
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func (n numberDumper) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "Here's your number: %d\n", n)
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}
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func main() {
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h := http.NewServeMux()
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h.Handle("/one", numberDumper(1))
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h.Handle("/two", numberDumper(2))
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h.Handle("/three", numberDumper(3))
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h.Handle("/four", numberDumper(4))
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h.Handle("/five", numberDumper(5))
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h.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
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w.WriteHeader(404)
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fmt.Fprintln(w, "That's not a supported number!")
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})
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err := http.ListenAndServe(":9999", h)
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log.Fatal(err)
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}
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```
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`numberDumper` implements `http.Handler`, and can be passed into the
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`http.ServeMux` multiple times to serve multiple endpoints. Here's it in action:
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```
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~ $ curl localhost:9999/one
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Here's your number: 1
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~ $ curl localhost:9999/five
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Here's your number: 5
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~ $ curl localhost:9999/bazillion
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That's not a supported number!
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```
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## Testing
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Testing http endpoints is extremely easy in Go, and doesn't even require you to
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actually listen on any ports! The `httptest` package provides a few handy
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utilities, including `NewRecorder` which implements `http.ResponseWriter` and
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allows you to effectively make an http request by calling `ServeHTTP` directly.
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Here's an example of a test for our previously implemented `numberDumper`,
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commented with what exactly is happening:
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```go
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package main
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import (
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"fmt"
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"net/http"
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"net/http/httptest"
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. "testing"
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)
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func TestNumberDumper(t *T) {
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// We first create the http.Handler we wish to test
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n := numberDumper(1)
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// We create an http.Request object to test with. The http.Request is
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// totally customizable in every way that a real-life http request is, so
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// even the most intricate behavior can be tested
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r, _ := http.NewRequest("GET", "/one", nil)
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// httptest.Recorder implements the http.ResponseWriter interface, and as
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// such can be passed into ServeHTTP to receive the response. It will act as
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// if all data being given to it is being sent to a real client, when in
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// reality it's being buffered for later observation
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w := httptest.NewRecorder()
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// Pass in our httptest.Recorder and http.Request to our numberDumper. At
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// this point the numberDumper will act just as if it was responding to a
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// real request
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n.ServeHTTP(w, r)
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// httptest.Recorder gives a number of fields and methods which can be used
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// to observe the response made to our request. Here we check the response
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// code
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if w.Code != 200 {
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t.Fatalf("wrong code returned: %d", w.Code)
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}
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// We can also get the full body out of the httptest.Recorder, and check
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// that its contents are what we expect
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body := w.Body.String()
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if body != fmt.Sprintf("Here's your number: 1\n") {
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t.Fatalf("wrong body returned: %s", body)
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}
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}
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```
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In this way it's easy to create tests for your individual components that you
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are using to build your application, keeping the tests near to the functionality
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they're testing.
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Note: if you ever do need to spin up a test server in your tests, `httptest`
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also provides a way to create a server listening on a random open port for use
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in tests as well.
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## Middleware
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Serving endpoints is nice, but often there's functionality you need to run for
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*every* request before the actual endpoint's handler is run. For example, access
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logging. A middleware component is one which implements `http.Handler`, but will
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actually pass the request off to another `http.Handler` after doing some set of
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actions. The `http.ServeMux` we looked at earlier is actually an example of
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middleware, since it passes the request off to another `http.Handler` for actual
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processing. Here's an example of our previous example with some logging
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middleware:
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```go
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package main
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import (
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"fmt"
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"log"
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"net/http"
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)
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type numberDumper int
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func (n numberDumper) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "Here's your number: %d\n", n)
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}
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func logger(h http.Handler) http.Handler {
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return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
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log.Printf("%s requested %s", r.RemoteAddr, r.URL)
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h.ServeHTTP(w, r)
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})
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}
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func main() {
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h := http.NewServeMux()
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h.Handle("/one", numberDumper(1))
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h.Handle("/two", numberDumper(2))
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h.Handle("/three", numberDumper(3))
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h.Handle("/four", numberDumper(4))
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h.Handle("/five", numberDumper(5))
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h.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
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w.WriteHeader(404)
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fmt.Fprintln(w, "That's not a supported number!")
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})
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hl := logger(h)
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err := http.ListenAndServe(":9999", hl)
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log.Fatal(err)
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}
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```
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`logger` is a function which takes in an `http.Handler` called `h`, and returns
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a new `http.Handler` which, when called, will log the request it was called with
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and then pass off its arguments to `h`. To use it we pass in our
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`http.ServeMux`, so all incoming requests will first be handled by the logging
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middleware before being passed to the `http.ServeMux`.
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Here's an example log entry which is output when the `/five` endpoint is hit:
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```
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2015/06/30 20:15:41 [::1]:34688 requested /five
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```
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## Middleware chaining
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Being able to chain middleware together is an incredibly useful ability which we
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get almost for free, as long as we use the signature
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`func(http.Handler) http.Handler`. A middleware component returns the same type
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which is passed into it, so simply passing the output of one middleware
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component into the other is sufficient.
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However, more complex behavior with middleware can be tricky. For instance, what
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if you want a piece of middleware which takes in a parameter upon creation?
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Here's an example of just that, with a piece of middleware which will set a
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header and its value for all requests:
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```go
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package main
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import (
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"fmt"
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"log"
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"net/http"
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)
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type numberDumper int
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func (n numberDumper) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "Here's your number: %d\n", n)
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}
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func logger(h http.Handler) http.Handler {
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return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
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log.Printf("%s requested %s", r.RemoteAddr, r.URL)
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h.ServeHTTP(w, r)
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})
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}
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type headerSetter struct {
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key, val string
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handler http.Handler
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}
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func (hs headerSetter) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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w.Header().Set(hs.key, hs.val)
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hs.handler.ServeHTTP(w, r)
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}
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func newHeaderSetter(key, val string) func(http.Handler) http.Handler {
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return func(h http.Handler) http.Handler {
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return headerSetter{key, val, h}
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}
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}
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func main() {
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h := http.NewServeMux()
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h.Handle("/one", numberDumper(1))
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h.Handle("/two", numberDumper(2))
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h.Handle("/three", numberDumper(3))
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h.Handle("/four", numberDumper(4))
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h.Handle("/five", numberDumper(5))
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h.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
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w.WriteHeader(404)
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fmt.Fprintln(w, "That's not a supported number!")
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})
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hl := logger(h)
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hhs := newHeaderSetter("X-FOO", "BAR")(hl)
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err := http.ListenAndServe(":9999", hhs)
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log.Fatal(err)
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}
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```
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And here's the curl output:
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```
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~ $ curl -i localhost:9999/three
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HTTP/1.1 200 OK
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X-Foo: BAR
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Date: Wed, 01 Jul 2015 00:39:48 GMT
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Content-Length: 22
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Content-Type: text/plain; charset=utf-8
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Here's your number: 3
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```
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`newHeaderSetter` returns a function which accepts and returns an
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`http.Handler`. Calling that returned function with an `http.Handler` then gets
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you an `http.Handler` which will set the header given to `newHeaderSetter`
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before continuing on to the given `http.Handler`.
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This may seem like a strange way of organizing this; for this example the
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signature for `newHeaderSetter` could very well have looked like this:
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```
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func newHeaderSetter(key, val string, h http.Handler) http.Handler
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```
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And that implementation would have worked fine. But it would have been more
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difficult to compose going forward. In the next section I'll show what I mean.
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## Composing middleware with alice
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[Alice](https://github.com/justinas/alice) is a very simple and convenient
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helper for working with middleware using the function signature we've been using
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thusfar. Alice is used to create and use chains of middleware. Chains can even
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be appended to each other, giving even further flexibility. Here's our previous
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example with a couple more headers being set, but also using alice to manage the
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added complexity.
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```go
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package main
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import (
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"fmt"
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"log"
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"net/http"
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"github.com/justinas/alice"
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)
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type numberDumper int
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func (n numberDumper) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "Here's your number: %d\n", n)
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}
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func logger(h http.Handler) http.Handler {
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return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
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log.Printf("%s requested %s", r.RemoteAddr, r.URL)
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h.ServeHTTP(w, r)
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})
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}
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type headerSetter struct {
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key, val string
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handler http.Handler
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}
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func (hs headerSetter) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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w.Header().Set(hs.key, hs.val)
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hs.handler.ServeHTTP(w, r)
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}
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func newHeaderSetter(key, val string) func(http.Handler) http.Handler {
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return func(h http.Handler) http.Handler {
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return headerSetter{key, val, h}
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}
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}
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func main() {
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h := http.NewServeMux()
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h.Handle("/one", numberDumper(1))
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h.Handle("/two", numberDumper(2))
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h.Handle("/three", numberDumper(3))
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h.Handle("/four", numberDumper(4))
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fiveHS := newHeaderSetter("X-FIVE", "the best number")
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h.Handle("/five", fiveHS(numberDumper(5)))
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h.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
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w.WriteHeader(404)
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fmt.Fprintln(w, "That's not a supported number!")
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})
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chain := alice.New(
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newHeaderSetter("X-FOO", "BAR"),
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newHeaderSetter("X-BAZ", "BUZ"),
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logger,
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).Then(h)
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err := http.ListenAndServe(":9999", chain)
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log.Fatal(err)
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}
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```
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In this example all requests will have the headers `X-FOO` and `X-BAZ` set, but
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the `/five` endpoint will *also* have the `X-FIVE` header set.
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## Fin
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Starting with a simple idea of an interface, the `http` package allows us to
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create for ourselves an incredibly useful and flexible (yet still rather simple)
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ecosystem for building web apps with re-usable components, all without breaking
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our static checks.
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