Golang JSON解码器无法识别类型

Question

Still a Golang beginner, I am trying to code a generic function to serve ReST requests. I pass a function to create a new resource (struct) with an interface implemented on it, because I would also invoke methods on the struct. When decoding the JSON, logging the type shows the correct (struct) type, but the JSON decoder seems to only recognize the interface, which it cannot decode to.

package main

import (
    "encoding/json"
    "github.com/julienschmidt/httprouter"
    "log"
    "net/http"
    "strings"
)

// general resource interface
type resource interface {
    // check semantics and return an array of errors or nil if no error found
    check() []string
    // update the resource in backend
    update() error
}

// specific resource named "anchor"
type anchor struct {
    ID   string `json:"id"`
    Name string `json:"name"`
}

func newAnchor() resource {
    return anchor{}
}

func (a anchor) check() []string {
    return nil
}

func (a anchor) update() error {
    return nil
}

// generic function to create (POST) a new resource
func restCreate(newResource func() resource) httprouter.Handle {
    return func(w http.ResponseWriter, r *http.Request, _ httprouter.Params) {
        const F = "restCreate"
        var checkErrs []string

        res := newResource()
        log.Printf("%s res type %T
", F, res)
        dcdr := json.NewDecoder(r.Body)
        err := dcdr.Decode(&res)
        log.Printf("%s Unmarshalled into %T: %+v
", F, res, res)
        if err == nil {
            checkErrs = res.check()
        }
        switch {
        case err != nil:
            w.WriteHeader(http.StatusInternalServerError)
            log.Printf("[ERR] %s: %v
", F, err)
        case checkErrs != nil:
            w.WriteHeader(http.StatusBadRequest)
            w.Write([]byte(strings.Join(checkErrs, "
")))
            log.Printf("%s: %v
", F, err)
        default:
            res.update()
            bs, _ := json.Marshal(res)
            w.Write(bs)
        }
    }
}

func main() {
    r := httprouter.New()
    r.POST("/anchors", restCreate(newAnchor))
    http.ListenAndServe(":8080", r)
}

The execution log shows:

restCreate res type main.anchor
restCreate Unmarshalled into main.anchor: {ID: Name:}
[ERR] restCreate: json: cannot unmarshal object into Go value of type main.resource

Why does Printf show the struct type and json.Decoder the interface?
I'd appreciate any indicator on what's going wrong and how to solve this in a generic way...

Answer 1

It is because you try to use a pointer to the interface to unmarshal into. You need return a pointer in a function

func newAnchor() resource {
    return &anchor{}
}

And you don't need to get address in this line: err := dcdr.Decode(&res)

Here is small working example: https://play.golang.org/p/3E0RmGTURO