how to build own simple router in golang?

Question

I am building own router in golang to understand logic , however I am getting 404 error not found normally, i able to running to server but I wrote function in hello name, it is not running. what could reason for this ?

package main
import (
    "fmt"
    "log"
    "net/http"
    "strings"
    "time"
)

var Session *http.Server
var r Router

func Run(port string) {
    Session = &http.Server{
        Addr:           port,
        Handler:        &r,
        ReadTimeout:    10 * time.Second,
        WriteTimeout:   10 * time.Second,
        MaxHeaderBytes: 1 << 20,
    }
    log.Fatal(Session.ListenAndServe())
}

type Handle func(http.ResponseWriter, *http.Request)

type Router struct {
    mux map[string]Handle
}

func newRouter() *Router {
    return &Router{
        mux: make(map[string]Handle),
    }
}

func (r *Router) Add(path string, handle Handle) {
    r.mux[path] = handle
}

func GetHeader(url string) string {
    sl := strings.Split(url, "/")
    return fmt.Sprintf("/%s", sl[1])
}

func (rt *Router) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    head := GetHeader(r.URL.Path)
    h, ok := rt.mux[head]
    if ok {
        h(w, r)
        return
    }
    http.NotFound(w, r)
}

func hello(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "%s", "hello world")
}

func main() {
    r := newRouter()
    r.Add("/hello", hello)
    Run(":8080")
}

Answer 1

Your main function, by using the := notation, is declaring and initialising a new local variable called r, whose scope is different from the global variable you have on the top of your listing.

This means that the r variable that you're adding the handler to, is not the global one that you're trying to assign to the Http.Server struct, therefore your ServeHTTP will not find any mapping at all into its own receiver r.

A solution is to declare the r Router as a pointer to a Router, like this:

var r *Router

and in your handler, you can pass directly the pointer without any dereference:

Session = &http.Server{
        Addr:           port,
        Handler:        r,
        ReadTimeout:    10 * time.Second,
        WriteTimeout:   10 * time.Second,
        MaxHeaderBytes: 1 << 20,
    }

Of course, remember to initialise the variable by assigning the pointer value and not declaring a new local one:

func main() {
    r = newRouter()
    r.Add("/hello", hello)
    Run(":8080")
}