json - 如何将 JSON 解码为通过反射创建的值？

Question

package controllers

import (
    "encoding/json"
    "errors"
    "io"
    "io/ioutil"
    "reflect"
)

func GetTypeFromReq(c *App, ty interface{}) (interface{}, error) {
    //get the type we are going to marshall into
    item := reflect.ValueOf(ty)

    //define and set the error that we will be returning to null
    var retErr error
    retErr = nil

    //extract the body from the request and defer closing of the body
    body, err := ioutil.ReadAll(io.LimitReader(c.Request.Body, 1048576))
    defer c.Request.Body.Close()

    //handle errors and unmarshal our data
    if err != nil {
        retErr = errors.New("Failed to Read body: " + err.Error())
    } else if err = json.Unmarshal(body, &item); err != nil {
        retErr = errors.New("Unmarshal Failed: " + err.Error())
    }

    return item, retErr
}

我试图将一个类型和一个请求传递到一个函数中，然后在该函数中将请求解编码到一个变量中并将其返回。

我认为我的方法是错误的，因为当我尝试这样做时:

inter, err := GetTypeFromReq(&c, models.User{})
if err != nil {
    revel.ERROR.Println(err.Error())
}
user := inter.(models.User)

我收到错误“接口(interface)转换:接口(interface) {} 是 reflect.Value，不是 models.User”

关于如何解决这个问题的任何提示？

Answer 1

以下是修改函数以使其按预期工作的方法:

func GetTypeFromReq(c *App, ty interface{}) (interface{}, error) {
  // Allocate new value with same type as ty
  v := reflect.New(reflect.TypeOf(ty))

  //define and set the error that we will be returning to null
  var retErr error
  retErr = nil

  //extract the body from the request and defer closing of the body
  body, err := ioutil.ReadAll(io.LimitReader(c.Request.Body, 1048576))
  defer c.Request.Body.Close()

  //handle errors and unmarshal our data
  if err != nil {
    retErr = errors.New("Failed to Read body: " + err.Error())
  } else if err = json.Unmarshal(body, v.Interface()); err != nil {
    retErr = errors.New("Unmarshal Failed: " + err.Error())
  }

  // v holds a pointer, call Elem() to get the value.
  return v.Elem().Interface(), retErr
}

注意对 Interface() 的调用获取 reflect.Value 的当前值。

这是一种避免反射和类型断言的方法:

func GetFromReq(c *App, item interface{}) error {
  //extract the body from the request and defer closing of the body
  body, err := ioutil.ReadAll(io.LimitReader(c.Request.Body, 1048576))
  defer c.Request.Body.Close()

  //handle errors and unmarshal our data
  if err != nil {
    retErr = errors.New("Failed to Read body: " + err.Error())
  } else if err = json.Unmarshal(body, item); err != nil {
    retErr = errors.New("Unmarshal Failed: " + err.Error())
  }
  return retErr
}

像这样使用它:

var user models.User
err := GetFromReq(&c, &user)
if err != nil {
   revel.ERROR.Println(err.Error())
}

使用 JSON decoder简化代码:

func GetFromReq(c *App, item interface{}) error {
  defer c.Request.Body.Close()
  return json.NewDecoder(io.LimitReader(c.Request.Body, 1048576)).Deocode(item)
}

如果 c.Request 是一个 *http.Request 并且 c.Response 是一个 http.ResponseWriter ，然后将函数写成:

func GetFromReq(c *App, item interface{}) error {
  return json.NewDecoder(http.MaxBytesReaer(c.Response, c.Request.Body, 1048576)).Deocode(item)
}

net/http 服务器中的请求体无需关闭。使用 MaxBytesReader代替 io.LimitReader 以防止客户端意外或恶意发送大请求并浪费服务器资源。