gorilla/mux

Package gorilla/mux implements a request router and dispatcher for matching incoming requests to their respective handler.

The name mux stands for "HTTP request multiplexer". Like the standard http.ServeMux, mux.Router matches incoming requests against a list of registered routes and calls a handler for the route that matches the URL or other conditions. The main features are:

• It implements the http.Handler interface so it is compatible with the standard http.ServeMux.
• Requests can be matched based on URL host, path, path prefix, schemes, header and query values, HTTP methods or using custom matchers.
• URL hosts, paths and query values can have variables with an optional regular expression.
• Registered URLs can be built, or "reversed", which helps maintaining references to resources.
• Routes can be used as subrouters: nested routes are only tested if the parent route matches. This is useful to define groups of routes that share common conditions like a host, a path prefix or other repeated attributes. As a bonus, this optimizes request matching.

---

• Install
• Examples
• Matching Routes
• Static Files
• Serving Single Page Applications (e.g. React, Vue, Ember.js, etc.)
• Registered URLs
• Walking Routes
• Graceful Shutdown
• Middleware
• Handling CORS Requests
• Testing Handlers
• Full Example

---

Install

With a correctly configured Go toolchain:

go get -u github.com/gorilla/mux

Examples

Let's start registering a couple of URL paths and handlers:

func main() {
    r := mux.NewRouter()
    r.HandleFunc("/", HomeHandler)
    r.HandleFunc("/products", ProductsHandler)
    r.HandleFunc("/articles", ArticlesHandler)
    http.Handle("/", r)
}

Here we register three routes mapping URL paths to handlers. This is equivalent to how http.HandleFunc() works: if an incoming request URL matches one of the paths, the corresponding handler is called passing (http.ResponseWriter, *http.Request) as parameters.

Paths can have variables. They are defined using the format {name} or {name:pattern}. If a regular expression pattern is not defined, the matched variable will be anything until the next slash. For example:

r := mux.NewRouter()
r.HandleFunc("/products/{key}", ProductHandler)
r.HandleFunc("/articles/{category}/", ArticlesCategoryHandler)
r.HandleFunc("/articles/{category}/{id:[0-9]+}", ArticleHandler)

The names are used to create a map of route variables which can be retrieved calling mux.Vars():

func ArticlesCategoryHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    w.WriteHeader(http.StatusOK)
    fmt.Fprintf(w, "Category: %v\n", vars["category"])
}

And this is all you need to know about the basic usage. More advanced options are explained below.

Matching Routes

Routes can also be restricted to a domain or subdomain. Just define a host pattern to be matched. They can also have variables:

r := mux.NewRouter()
// Only matches if domain is "www.example.com".
r.Host("www.example.com")
// Matches a dynamic subdomain.
r.Host("{subdomain:[a-z]+}.example.com")

There are several other matchers that can be added. To match path prefixes:

r.PathPrefix("/products/")

...or HTTP methods:

r.Methods("GET", "POST")

...or URL schemes:

r.Schemes("https")

...or header values:

r.Headers("X-Requested-With", "XMLHttpRequest")

...or query values:

r.Queries("key", "value")

...or to use a custom matcher function:

r.MatcherFunc(func(r *http.Request, rm *RouteMatch) bool {
    return r.ProtoMajor == 0
})

...and finally, it is possible to combine several matchers in a single route:

r.HandleFunc("/products", ProductsHandler).
  Host("www.example.com").
  Methods("GET").
  Schemes("http")

Routes are tested in the order they were added to the router. If two routes match, the first one wins:

r := mux.NewRouter()
r.HandleFunc("/specific", specificHandler)
r.PathPrefix("/").Handler(catchAllHandler)

Setting the same matching conditions again and again can be boring, so we have a way to group several routes that share the same requirements. We call it "subrouting".

For example, let's say we have several URLs that should only match when the host is www.example.com. Create a route for that host and get a "subrouter" from it:

r := mux.NewRouter()
s := r.Host("www.example.com").Subrouter()

Then register routes in the subrouter:

s.HandleFunc("/products/", ProductsHandler)
s.HandleFunc("/products/{key}", ProductHandler)
s.HandleFunc("/articles/{category}/{id:[0-9]+}", ArticleHandler)

The three URL paths we registered above will only be tested if the domain is www.example.com, because the subrouter is tested first. This is not only convenient, but also optimizes request matching. You can create subrouters combining any attribute matchers accepted by a route.

Subrouters can be used to create domain or path "namespaces": you define subrouters in a central place and then parts of the app can register its paths relatively to a given subrouter.

There's one more thing about subroutes. When a subrouter has a path prefix, the inner routes use it as base for their paths:

r := mux.NewRouter()
s := r.PathPrefix("/products").Subrouter()
// "/products/"
s.HandleFunc("/", ProductsHandler)
// "/products/{key}/"
s.HandleFunc("/{key}/", ProductHandler)
// "/products/{key}/details"
s.HandleFunc("/{key}/details", ProductDetailsHandler)

Static Files

Note that the path provided to PathPrefix() represents a "wildcard": calling PathPrefix("/static/").Handler(...) means that the handler will be passed any request that matches "/static/*". This makes it easy to serve static files with mux:

func main() {
    var dir string

    flag.StringVar(&dir, "dir", ".", "the directory to serve files from. Defaults to the current dir")
    flag.Parse()
    r := mux.NewRouter()

    // This will serve files under http://localhost:8000/static/<filename>
    r.PathPrefix("/static/").Handler(http.StripPrefix("/static/", http.FileServer(http.Dir(dir))))

    srv := &http.Server{
        Handler:      r,
        Addr:         "127.0.0.1:8000",
        // Good practice: enforce timeouts for servers you create!
        WriteTimeout: 15 * time.Second,
        ReadTimeout:  15 * time.Second,
    }

    log.Fatal(srv.ListenAndServe())
}

Serving Single Page Applications

Most of the time it makes sense to serve your SPA on a separate web server from your API, but sometimes it's desirable to serve them both from one place. It's possible to write a simple handler for serving your SPA (for use with React Router's BrowserRouter for example), and leverage mux's powerful routing for your API endpoints.

package main

import (
	"encoding/json"
	"log"
	"net/http"
	"os"
	"path/filepath"
	"time"

	"github.com/gorilla/mux"
)

// spaHandler implements the http.Handler interface, so we can use it
// to respond to HTTP requests. The path to the static directory and
// path to the index file within that static directory are used to
// serve the SPA in the given static directory.
type spaHandler struct {
	staticPath string
	indexPath  string
}

// ServeHTTP inspects the URL path to locate a file within the static dir
// on the SPA handler. If a file is found, it will be served. If not, the
// file located at the index path on the SPA handler will be served. This
// is suitable behavior for serving an SPA (single page application).
func (h spaHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// Join internally call path.Clean to prevent directory traversal
	path := filepath.Join(h.staticPath, r.URL.Path)

	// check whether a file exists or is a directory at the given path
	fi, err := os.Stat(path)
	if os.IsNotExist(err) || fi.IsDir() {
		// file does not exist or path is a directory, serve index.html
		http.ServeFile(w, r, filepath.Join(h.staticPath, h.indexPath))
		return
	}

	if err != nil {
		// if we got an error (that wasn't that the file doesn't exist) stating the
		// file, return a 500 internal server error and stop
		http.Error(w, err.Error(), http.StatusInternalServerError)
        return
	}

	// otherwise, use http.FileServer to serve the static file
	http.FileServer(http.Dir(h.staticPath)).ServeHTTP(w, r)
}

func main() {
	router := mux.NewRouter()

	router.HandleFunc("/api/health", func(w http.ResponseWriter, r *http.Request) {
		// an example API handler
		json.NewEncoder(w).Encode(map[string]bool{"ok": true})
	})

	spa := spaHandler{staticPath: "build", indexPath: "index.html"}
	router.PathPrefix("/").Handler(spa)

	srv := &http.Server{
		Handler: router,
		Addr:    "127.0.0.1:8000",
		// Good practice: enforce timeouts for servers you create!
		WriteTimeout: 15 * time.Second,
		ReadTimeout:  15 * time.Second,
	}

	log.Fatal(srv.ListenAndServe())
}

Registered URLs

Now let's see how to build registered URLs.

Routes can be named. All routes that define a name can have their URLs built, or "reversed". W