Golang struct having io.Reader unable to serialize

I am trying to serialize below struct into byte[] to store it into DB and then while reading it from DB I am deserializing it.

type Response struct {
   	Headers map[string][]string
   	Body    io.Reader
   	Status  int
}

Below is the code how I am creating response object and setting up the value for it.

resp := new(Response)
resp.Body = bytes.NewReader(outBytes) //outBytes is byte[]
resp.Headers.SetKeyValue("Content-Type", "text/json") //SetKeyValue is the method created for adding headers
resp.Headers.SetKeyValue("Url-Type", "broker")
resp.Status = 200

I am using json.Marshal() to serialize the resp object as below.

b, _ := json.Marshal(resp)

Below is the code, I am using to deserialize.

var r Response
r.Body = &bytes.Buffer{}
json.Unmarshal(b,&r)

Problem is with deserialization, I am not able to get the resp.Body object. It is always nil or blank in spite of setting body object (see above). I am able to get Headers and Status field of the struct back from deserialize but not Body.

I know there is something to be handle with Body field which is an io.Reader.

Any help would be really great.

Short Answer : JSON marshaller will not use Read() function to read the string from io.Reader . Instead of using io.Reader you may use a type that implements Marshaler interface.

How Marshaller works :
Marshal traverses the value v recursively. If an encountered value implements the Marshaler interface and is not a nil pointer, Marshal calls its MarshalJSON method to produce JSON. If no MarshalJSON method is present but the value implements encoding.TextMarshaler instead, Marshal calls its MarshalText method. The nil pointer exception is not strictly necessary but mimics a similar, necessary exception in the behavior of UnmarshalJSON.

Otherwise, Marshal uses the following type-dependent default encodings:

• Boolean values encode as JSON booleans.
• Floating point, integer, and Number values encode as JSON numbers.

Implementaton
This is what you may do

type Response struct {
	Headers map[string][]string
	Body    *JSONReader
	Status  int
}

type JSONReader struct {
	*bytes.Reader
}

func NewJSONReader(outBytes []byte) *JSONReader {
	jr := new(JSONReader)
	jr.Reader = bytes.NewReader(outBytes)
	return jr
}

func (js JSONReader) MarshalJSON() ([]byte, error) {
	data, err := ioutil.ReadAll(js.Reader)
	if err != nil {
		return nil, err
	}
	data = []byte(`"` + string(data) + `"`)
	return data, nil
}

// UnmarshalJSON sets *jr to a copy of data.
func (jr *JSONReader) UnmarshalJSON(data []byte) error {
	if jr == nil {
		return errors.New("json.JSONReader: UnmarshalJSON on nil pointer")
	}
	if data == nil {
		return nil
	}
	data = []byte(strings.Trim(string(data), "\""))
	jr.Reader = bytes.NewReader(data)
	return nil
}

Here is a go playground link with the implementation and sample use : link

Overview

io.Reader is an interface so it can't be marshaled. Each marshaling struct attribute must implement Marshaler interface to be marshaled. You could declare your own marshaler wrapper struct to marshal data from bytes.Reader.

Why interface can't be marshaled?

Interfaces in Go provide a way to specify the behavior of an object: if something can do this, then it can be used here. In opposite Go’s structs are typed collections of fields. They’re useful for grouping data together to form records. Go supports methods defined on struct types not interface types.

Implementation

type Response struct {
    Body *MarshalableReader
}

type MarshalableReader struct {
    *bytes.Reader
}

func (r MarshalableReader) MarshalJSON() ([]byte, error) {
    data, err := ioutil.ReadAll(r.Reader)
    if err != nil {
        return nil, err
    }
    return []byte(fmt.Sprintf("\"%s\"", data)), nil
}

func main() {
    resp := Response{&MarshalableReader{bytes.NewReader([]byte("Blah Blah"))}}

    marshaled, err := json.Marshal(resp)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("JSON: %s\n", marshaled)
}                                                                                               

I am using "encoding/json" package : https://golang.org/pkg/encoding/json/

As I can use the http.ResponseWriter to send JSON Response. Here is two functions you could use to send JSON and read JSON from the body :

// GetJSONContent returns the JSON content of a request
func GetJSONContent(v interface{}, r *http.Request) error {
	defer r.Body.Close()
	return json.NewDecoder(r.Body).Decode(v)
}

// JSONWithHTTPCode Json Output with an HTTP code
func JSONWithHTTPCode(w http.ResponseWriter, d interface{}, code int) {
	w.Header().Set("Content-Type", "application/json; charset=UTF-8")
	w.WriteHeader(code)
	if d != nil {
		err := json.NewEncoder(w).Encode(d)
		if err != nil {
			panic(err)
		}
	}
}

And then in your handlers just use those func as follow :

// Handler handler
func Handler(w http.ResponseWriter, r *http.Request) {
	    s := YourStruct{}
        err = GetJSONContent(s, r) 
        if err != nil {
            panic(err)
        }
		return
	}

	JSONWithHTTPCode(w, s, http.StatusOK)
}

Hope it helped