how to find, "invalid character ',' looking for beginning of value" error message

I have a short Go program that runs the go list -json command for several packages, stores the output of each run of the command in a json.RawMessage, appends each json.RawMessage into a slice of json.RawMessages, and then returns the result to the server after concatenating each of the json.RawMessages together and compacting the json. However, there is an error message that gets produced when I run json.Compact that I can't locate the source of. Googling this error message reveals that most people who seem to encounter it--whether it's for an invalid , or some other character--have a hard time finding the source of it.

invalid character ',' looking for beginning of value

The code with comments is available to view here on play.golang.org (although it won't run there) and also below.

Question: can you explain the source of this error and how to prevent it?

(Note, some of the packages were included just for testing purposes)

package main
import (
"expvar"
"encoding/json"
"bytes"
"fmt"
"github.com/go-martini/martini"
"github.com/zenazn/goji"
"github.com/zenazn/goji/web"
"go/build"
"log"
"math/rand"
"net/http"
_ "net/http/pprof"
"os/exec"
)
type myType struct {
J []json.RawMessage
}
var pack map[string]string
type GoList struct {
Imports []string
}
type Import struct {
Dir        string
ImportPath string
Name       string
Target     string
Standard   bool
Root       string
GoFiles    []string
Imports    []string
Deps       []string
}
const contentTypeJSON = "application/json"
func main() {
http.HandleFunc("/importgraph", func(w http.ResponseWriter, r *http.Request) { importGraph(w, r) })
http.HandleFunc("/", handler)
http.ListenAndServe(":8080", nil)
}
func handler(w http.ResponseWriter, r *http.Request) {
fmt.Println("Inside handler")
fmt.Fprintf(w, "Hello world from my Go program!")
}
func importGraph(w http.ResponseWriter, r *http.Request) {
pack = make(map[string]string)
var t myType
cmd := exec.Command("go", "list", "-json")
stdout, err := cmd.Output()
if err != nil {
println(err.Error())
return
}
var list GoList
err = json.Unmarshal(stdout, &list)
for _, d := range list.Imports {
//get the imports for each of the packages listed by go list -json
t.imports(d)
}
var buff bytes.Buffer
//concatenate the separate json.RawMessages together into json
buff.WriteByte('[')
for i, j := range t.J {
if i != 0 {
buff.WriteByte(',')
}
buff.Write([]byte(j))
}
buff.WriteByte(']')
var buffer bytes.Buffer
if err := json.Compact(&buffer, buff.Bytes()); err != nil {
println(err.Error()) //error message: invalid character ',' looking for beginning of value
return
}
w.Header().Set("Content-Type", contentTypeJSON)
w.Write(buffer.Bytes())
}
func (myObj *myType) imports(pk string) error {
cmd := exec.Command("go", "list", "-json", pk)
stdout, _ := cmd.Output()
pack[pk] = pk
var deplist Import
json.Unmarshal(stdout, &deplist)
var newj json.RawMessage
json.Unmarshal(stdout, &newj)
myObj.J = append(myObj.J, newj)
for _, imp := range deplist.Imports {
if _, ok := pack[imp]; !ok {
myObj.imports(imp) //recursive call to get the imports of the imports etc
}
}
return nil
}

First, as has been commented, are you sure you can't use
the go/build package directly rather than running go list?

I Wouldn't use println (or fmt.Println) inside HTTP handlers. It's much better to use log.Println and/or get the error into the ResponseWriter. Also, it's a good idea to wrap your ListenAndServe call with log.Fatal.

When printing/logging error values you can just use err, no need to have err.Error().

Further, when you actually want to do something more detailed than just reporting/logging the error message you can look at it's type and other info. For example, log.Printf("verbose error info: %#v", err) gives:

&json.SyntaxError{msg:"invalid character ',' looking for beginning of value", Offset:0}

I tried this because I know the json package returns various error types with additional info and I was hoping the offset value would be of help. If it had been then something like this might have been helpful:

if err := json.Compact(…) {
if err != nil {
log.Println("json.Compact:", err)
if serr, ok := err.(*json.SyntaxError); ok {
log.Println("Occurred at offset:", serr.Offset)
// … something to show the data in buff around that offset …
}
}
}

But offset zero isn't helpful

So although this doesn't identify you problem hopefully
it can be of some help to your further investigation.

Edit:

So after adding:

log.Println("Write file:", ioutil.WriteFile("data.json", buff.Bytes(), 0600))

to the above error handling block I then ran a JSON validator on the resultant file and it identified this piece:

        "XTestImports": [
"io",
"log",
"net"
]
},,{
"Dir": "/usr/local/go/src/mime",
"ImportPath": "mime",
"Name": "mime",

Note the double ,,.

That should tell you whete the error in your code is.
But if not, you need to skip empty entries, either when processing t.J or when you build it. The later is better and just involves:

	if len(newj) > 0 {
myObj.J = append(myObj.J, newj)
}

(where btw you don't check for errors from json.Unmarshal so it's not clear if that is supposed to ever be empty or if it's empty due to a preceeding error. Never ignore error returns!)

I also encountered the same error message in a Go program, but the error message was within the HTTP response error, in HTML format when my HTTP response parser expected JSON.

For me, the solution was to change my request to include setting the Content-Type header to application/json. How you do this depends on which http client library you happen to be using; if you have access to the http.Header core type, you can set the header with .Set(...).

I realize the scope of this fix for me may not apply to the original question, but I came here first after googling and thought this would help others, since the message was not particularly obvious at first glance. The hint is that the invalid < character is the first HTML character in the error/response, which is likely the result of the request type not being set to application/json, thus the server responds with a non JSON response.

For me the issue was I was trying to parse the already parsed JSON.

I was also facing this error "invalid character 'N' looking for beginning of value".
This error was coming while "unmarshalling the non-json response into a json". I was expecting a json response, so wrote go code to unmarshal it into a json. But, due to URL change, the response that I was getting was a text ie. "404 Not found" error, which cannot be unmarshalled into a json.
"invalid character 'N' looking for beginning of value"

So, to summarise, this error appears when we are trying to unmarshal a non-json response (text/html/xml) into json.

Reason for this eerie error message is :

// When unmarshaling quoted strings, invalid UTF-8 or
// invalid UTF-16 surrogate pairs are not treated as an error.
// Instead, they are replaced by the Unicode replacement
// character U+FFFD.

https://golang.org/src/encoding/json/decode.go

In my case I saved my json as string then parsed it by :
stringData = JSON.parse(myJsonString)

I also had the same error another time using gin-context-ShouldBind() (https://godoc.org/github.com/gin-gonic/gin#Context.ShouldBind) and mapping my json to go object:
error was because it needs a json as string, so I used : JSON.stringify(jsonObject) when sending my request from front-end part.

And in case someone has the same problem as me, I needed to call JSON.stringify on my post data.

I encountered a similar problem with my error message being:

> invalid character 'I' looking for beginning of value

In my case, i was trying to decode BSON using json.Unmarshal. Json doesn't recognize the ISODate type, which caused this error.

I had a similar issue. For me I omitted the first letter of my authorization token. So instead of

"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJlbWFpbCI6InJhcGhhZWxuZ0BlbWFpbC5jb20iLCJleHAiOjE2MTM5NTQzMjB9.yPGC937VNAF8Qg05Z1x3fZ3zu_MUs-cA_Iag5-4RcJE"

I used this

"yJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJlbWFpbCI6InJhcGhhZWxuZ0BlbWFpbC5jb20iLCJleHAiOjE2MTM5NTQzMjB9.yPGC937VNAF8Qg05Z1x3fZ3zu_MUs-cA_Iag5-4RcJE"