golang invalid memory address or nil pointer dereference

Question

情况是这样的，我是golang初学者，在学习《go语言编程》网络编程这章的时候根据书上的代码尝试敲了下，发现报错.

Warn: panic in  0x12cfe40 runtime error: invalid memory address or nil pointer dereference

以下是服务端代码以及报错信息

package main

import (
    "net/http"
    "os"
    "io"
    "io/ioutil"
    "log"
    "html/template"
    "path"
    "runtime/debug"
)
const (
    UPLOAD_DIR = "./uploads"
)

const (
    TEMPLATE_DIR = "./views"
)

const (
    ListDir = 0x0001
)
var templates = make(map[string]*template.Template)

func staticDirHandle(mux *http.ServeMux, prefix string, staticDir string, flags int) {
    mux.HandleFunc(prefix, func(w http.ResponseWriter, r *http.Request) {
        file := staticDir + r.URL.Path[len(prefix)-1 :]
        if (flags & ListDir) == 0 {
            if exits := isExists(file); !exits {
                http.NotFound(w, r)
                return
            }
        }
        http.ServeFile(w, r, file)
    })
}

func init() {

    fileInfoArr, err := ioutil.ReadDir(TEMPLATE_DIR)
    if err != nil {
        panic(err)
        return
    }

    var templateName, templatePath string

    for _, fileInfo := range fileInfoArr  {
        templateName = fileInfo.Name()
        if ext := path.Ext(templateName); ext != ".html" {
            continue
        }
        templatePath = TEMPLATE_DIR + "/" + templateName
        log.Println("Loading template:", templatePath)
        t := template.Must(template.ParseFiles(templatePath))
        templates[templateName] = t
    }
}

func check(err error) {
    if err != nil {
        panic(err)
    }

}

func renderHtml(w http.ResponseWriter, tmpl string, locals map[string]interface{}) error {
    err := templates[tmpl].Execute(w, locals)
    return err
}

func uploadHandler(w http.ResponseWriter, r * http.Request)  {
    if r.Method == "GET" {

        //读取指定模版的内容
        if err := renderHtml(w, "upload", nil); err != nil {
            http.Error(w, err.Error(), http.StatusInternalServerError)
            return
        }
        //根据模版语法渲染
        return
    }

    if r.Method == "POST" {
        //return the first from the image
        f, h, err := r.FormFile("image")
        check(err)
        //h:Fileheader describes the file info
        filename := h.Filename
        //f:a interface to access the file
        defer f.Close()

        t, err := os.Create(UPLOAD_DIR + "/" + filename)
        check(err)
        defer t.Close()

        //t.writer, f.reader
        _, err = io.Copy(t, f)
        check(err)

        //redirect a new url
        //StatusFound RFC
        http.Redirect(w, r, "/view?id=" + filename, http.StatusFound)
    }
}

func viewHandler(w http.ResponseWriter, r *http.Request) {
    imageId := r.FormValue("id")
    imagePath := UPLOAD_DIR + "/" + imageId
    if exits := isExists(imagePath); !exits {
        http.NotFound(w, r)
        return
    }
    w.Header().Set("Content-Type", "image")
    http.ServeFile(w, r, imagePath)
}

func isExists(path string) bool {
    _, err := os.Stat(path)
    if err == nil {
        return true
    }
    return os.IsExist(err)
}

func listHandler(w http.ResponseWriter, r *http.Request)  {
    fileInfoArr, err := ioutil.ReadDir("./uploads")
    check(err)
    locals := make(map[string] interface{})
    var images []string


    for _, fileInfo := range fileInfoArr {
        images = append(images, fileInfo.Name())
    }

    locals["images"] = images
    err = renderHtml(w, "list", locals)
    check(err)
}

func safeHandler(fn http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        defer func() {
            if e, ok := recover().(error); ok {
                http.Error(w, e.Error(),http.StatusInternalServerError)
                log.Println("Warn: panic in ", fn, e)
                log.Println(string(debug.Stack()))
            }
        }()
        fn(w, r)
    }
}



func main() {

    mux := http.NewServeMux()
    staticDirHandle(mux, "/assets/", "./public", 0)
    http.HandleFunc("/", safeHandler(listHandler))
    http.HandleFunc("/view", safeHandler(viewHandler))
    http.HandleFunc("/upload", safeHandler(uploadHandler))
    err := http.ListenAndServe(":8080", nil)
    if err != nil {
        log.Fatal("listenAndServe: ", err.Error())
    }
}

请求各位能帮我解答疑惑，十分感谢。

Answer 1

这是一个很明显的错误，你稍微 debug 一下就很明了了。

提示一点：从你的 templates map 中没有找到你传递的模板名，而 go 中的 map 索引如果没有找到并不会报错，而是返回一个该类型的默认值

Answer 2

这一行

 err := templates[tmpl].Execute(w, locals)

要先检查map里面有没有

t, ok := templates[tmpl]
if !ok {
    return errors.New("没有找到模板")
}
err := t.Execute(w, locals)
return err