Marshal of json.RawMessage

Question

Please find the code here http://play.golang.org/p/zdQ14ItNBZ

I am keeping JSON data as RawMessage, but cannot decode it out. I need the containing struct to be Marshalled and Unmarshalled, but I would expect still be able to get the JSON field.

code:

package main

import (
    "encoding/json"
    "fmt"
)

type Data struct {
    Name string
    Id   int
    Json json.RawMessage
}
type Data2 struct {
    Name string
    Id   int
}


func main() {

    tmp := Data2{"World", 2}

    b, err := json.Marshal(tmp)
    if err != nil {
        fmt.Println("Error %s", err.Error())
    }
    fmt.Println("b %s", string(b))

    test := Data{"Hello", 1, b}
    b2, err := json.Marshal(test)
    if err != nil {
        fmt.Println("Error %s", err.Error())
    }

    fmt.Println("b2 %s", string(b2))

    var d Data
    err = json.Unmarshal(b2, &d)
    if err != nil {
        fmt.Println("Error %s", err.Error())
    }
    fmt.Println("d.Json %s", string(d.Json))

    var tmp2 Data2
    err = json.Unmarshal(d.Json, &tmp2)
    if err != nil {
        fmt.Println("Error %s", err.Error())
    }
    fmt.Println("Data2 %+v", tmp2)

}

out:

b %s {"Name":"World","Id":2}
b2 %s {"Name":"Hello","Id":1,"Json":"eyJOYW1lIjoiV29ybGQiLCJJZCI6Mn0="}
d.Json %s "eyJOYW1lIjoiV29ybGQiLCJJZCI6Mn0="
Error %s json: cannot unmarshal string into Go value of type main.Data2
Data2 %+v { 0}

Solution 1

the methods on json.RawMessage all take a pointer receiver, which is why you're not able to utilize any of them; you don't have a pointer.

This "works" in the sense that it executes, but this is likely not the strategy that you want: http://play.golang.org/p/jYvh8nHata

basically you need this:

type Data struct {
    Name string
    Id   int
    Json *json.RawMessage
}

and then propagate that change through the rest of your program. What... what are you actually trying to do?

Solution 2

jorellis answer is correct for versions of Go before 1.8.

Go 1.8 and newer will correctly handle marshalling of both a pointer and non-pointer json.RawMessage.

Fixing commit: https://github.com/golang/go/commit/1625da24106b610f89ff7a67a11581df95f8e234

Comments

[bsr]

Thanks a lot. I knew about pointer receiver, and tried locally. might have made a mistake. I use this store some cached values, which just needed as an http json response. thanks again for the help.

[codefreak]

is there any equivalent of json.RawMessage in encoding/xml package?

[Sam Whited]

@codefreak a bit late, but the encoding/xml package lets you EncodeToken directly, or use an ",innerxml" tag to unmarshal "raw" xml bytes.