CODEWITHSUNDEEP
jsongo
mhcbinder
mhcbinder

Reputation: 49

Golang JSON decoder does not recognize type

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

Upvotes: 0

Views: 2051

Answers (2)

Pavlo Strokov
Pavlo Strokov

Reputation: 2087

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

Upvotes: 2

yazgazan
yazgazan

Reputation: 378

You cannot Unmarshal into an interface unless variable is holding a pointer to the concrete type you desire, as json.Decode won't know which concrete type to use. You have two workarounds at your disposal:

Having newResource return a concrete type under the hood:

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

This way json.Decode knows to unmarshal your JSON into an anchor.

Use newAnchor instead of newResource: this will be more readable in your restCreate function, and is more idiomatic[1].

[1] http://idiomaticgo.com/post/best-practice/accept-interfaces-return-structs/

Upvotes: 1

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