Custom MarshalJSON() never gets called in Go

Question

I've written custom versions of MarshalJSON and UnmarshalJSON. My UnmarshalJSON gets called the way I want it to, but I can't get it to work with MarshalJSON. Here's code that summarizes my problem:

package main

import (
    "bytes"
    "encoding/json"
    "fmt"
    "log"
    "os"
)

type myStruct struct {
    Data string `json:"data"`
}

func (s *myStruct) MarshalJSON() ([]byte, error) {
    return []byte(`{"data":"charlie"}`), nil
}

func (s *myStruct) UnmarshalJSON(b []byte) error {
    // Insert the string directly into the Data member
    return json.Unmarshal(b, &s.Data)
}

func main() {
    // Create a struct with initial content "alpha"
    ms := myStruct{"alpha"}

    // Replace content with "bravo" using custom UnmarshalJSON() (SUCCESSFUL)
    if err := json.NewDecoder(bytes.NewBufferString(`"bravo"`)).Decode(&ms); err != nil {
        log.Fatal(err)
    }

    // Use custom MarshalJSON() to get "charlie" back (UNSUCCESSFUL)
    if err := json.NewEncoder(os.Stdout).Encode(ms); err != nil {
        log.Fatal(err)
    }

    // Trying another method (UNSUCCESSFUL)
    if ret, err := json.Marshal(ms); err != nil {
        log.Fatal(err)
    } else {
        fmt.Println(string(ret))
    }

    // Verify that the Marshaler interface is correctly implemented
    var marsh json.Marshaler
    marsh = &ms
    ret, _ := marsh.MarshalJSON()
    fmt.Println(string(ret)) // Prints "charlie"
}

In short, the program encodes the struct "automatically" in two ways, and then finally calls MarshalJSON manually. The response I want is "charlie". Running the code generates the following output:

{"data":"bravo"}
{"data":"bravo"}
{"data":"charlie"}

Try it at Go Playground: http://play.golang.org/p/SJ05S8rAYN

Answer 1

In this part of the code, ms gets copied into an interface{} variable:

// Trying another method (UNSUCCESSFUL)
if ret, err := json.Marshal(ms); err != nil {

The problem is that this variable does not implement the json.Marshaler interface, since MarshalJSON is not in the method set for myStruct (only for *myStruct).

The fix is to either (a) make your MarshalJSON method take a non-pointer receiver (which will mean it gets a copy of the struct: possibly costly if it is large), or (b) marshal a pointer to the struct (as Kavu mentioned in a comment).

The reason for this behaviour is that Go doesn't let you take a pointer to the value stored inside an interface variable, instead requiring you to make a copy of the value whenever you want to access it. While the language has syntactic sugar to convert ms.MarshalJSON() into (&ms).MarshalJSON() as a way to access the method with a pointer receiver, this can not be done for a value stored in an interface variable. For this reason, the method is not considered to be in its method set.