golang-地图中的功能为<nil>

Question

I am trying to build a simple router in GO, I have a get method on a struct that should be passing a callback to the Get route map with the url as the key, it seems that fmt.Println(urlCallback) is returning a nil value and causes a runtime panic if i tried to call it, coming from a javascript background I am only just coming to grips with pointers and the like and feel it may have something to do with this,if someone could tell me why the passed func is nil that would be great.

Here is my "Router" package.

package Router

import (
    "fmt"
    "net/http"
    "net/url"
    "log"
)

type Res http.ResponseWriter
type Req *http.Request

type RouteMap map[*url.URL]func(Res, Req) 
type MethodMap map[string]RouteMap

type Router struct {
    Methods MethodMap
}

func (router *Router) Get(urlString string, callback func(Res, Req)) {
    parsedUrl, err := url.Parse(urlString)

    if(err != nil) {
        panic(err)
    }

    fmt.Println(parsedUrl)

    router.Methods["GET"][parsedUrl] = callback
}

func (router *Router) initMaps() {
    router.Methods = MethodMap{}
    router.Methods["GET"] = RouteMap{}
}

func (router Router) determineHandler(res http.ResponseWriter, req *http.Request) {
    fmt.Println(req.URL)
    fmt.Println(req.Method)

    methodMap := router.Methods[req.Method]
    urlCallback := methodMap[req.URL]

    fmt.Println(methodMap)
    fmt.Println(urlCallback)
}

func (router Router) Serve(host string, port string) {
    fullHost := host + ":" + port

    fmt.Println("Router is now serving to:" + fullHost)
    http.HandleFunc("/", router.determineHandler)

    err := http.ListenAndServe(fullHost, nil)

    if err == nil {
        fmt.Println("Router is now serving to:" + fullHost)
    } else {
        fmt.Println("An error occurred")
        log.Fatal(err)
    }
}


func NewRouter() Router {
    newRouter := Router{}
    newRouter.initMaps()

    return newRouter
}

and my main.

package main

import (
    "./router"
    "fmt"
)

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

    router.Get("/test", func(Router.Res, Router.Req) {
        fmt.Println("In test woohooo!")
    })

    router.Serve("localhost", "8888")
}

Answer 1

You are using *URL.url objects for map keys. Since two different objects won't be the same, you can't access the key for that path again. It's panicking because

urlCallback := methodMap[req.URL]

isn't an existing key, and so you're accessing a nil value. What you probably want to do in this case is use the Path property of the URL.url object.

So you'd have:

type RouteMap map[string]func(Res, Req)

In Get():

router.Methods["GET"][parsedUrl.Path] = callback

For determineRouter(), you can do this:

urlCallback, exists := methodMap[req.URL.Path]
if exists != false {
    urlCallback(res, req)
}

This adds a check to see if the key exists before it tries to call it.