Golang json Unmarshal "unexpected end of JSON input"

I am working on some code to parse the JSON data from an HTTP response. The code I have looks something like this:

type ResultStruct struct {
    result []map[string]string
}

var jsonData ResultStruct
err = json.Unmarshal(respBytes, &jsonData)

The json in the respBytes variable looks like this:

{
 "result": [
  {
   "id": "ID 1"
  },
  {
   "id": "ID 2"
  }
 ]
}

However, err is not nil. When I print it out it says unexpected end of JSON input. What is causing this? The JSON seems to valid. Does this error have something to do with my custom struct?

Thanks in advance!

The unexpected end of JSON input is the result of a syntax error in the JSON input (likely a missing ", }, or ]). The error does not depend on the type of the value that you are decoding to.

I ran the code with the example JSON input on the playground. It runs without error.

The code does not decode anything because the result field is not exported. If you export the result field:

type ResultStruct struct {
   Result []map[string]string
}

then the input is decoded as shown in this playground example.

I suspect that you are not reading the entire response body in your application. I suggest decoding the JSON input using:

err := json.NewDecoder(resp.Body).Decode(&jsonData)

The decoder reads directly from the response body.

You can also get this error if you're using json.RawMessage in an unexported field. For example, the following code produces the same error:

package main

import (
	"encoding/json"
	"fmt"
)

type MyJson struct {
	Foo bool `json:"foo"`
	bar json.RawMessage `json:"bar"`
}

type Bar struct {
	X int `json:"x"`
}

var respBytes = []byte(`
{
  "foo": true,
  "bar": { "x": 10 }
}`)

func main() {
	var myJson MyJson
	err := json.Unmarshal(respBytes, &myJson)
	if err != nil {
		fmt.Println(err)
		return
	}
	myBar := new(Bar)
	err = json.Unmarshal(myJson.bar, myBar)
	fmt.Println(err)
}

If you export "MyJson.bar" field (e.g. -> "MyJson.Bar", then the code works.

it is not the case here; but if you are getting this error loading json from a file it Will occur if the byte slice for the buffer is not initialized the the byte size of the file. [when you're new like me that happens! ] Since this is the first search result I got it still took some digging to figure out. In this use case the error is a bit misleading.

type GenesisResultStruct []GenesisField

fileinfo, _ := genesis.Stat()
bs := make([]byte, fileinfo.Size())
//bs := []byte {} // wrong!!
_, error := genesis.Read(bs)

if error != nil {
	fmt.Println("genesis read error: ", error)
	os.Exit(1)
}

var jsonData GenesisResultStruct
eGen = json.Unmarshal(bs, &jsonData)

if eGen != nil {
	fmt.Println("genesis unmarshal error: ", eGen)
	os.Exit(1)
}

Faced the same issue today.

You can also get this error if the respBytes is nil or there are no brackets [] if you are unmarshalling it to a slice.
In that case, you need to explicitly set respBytes.

As you are unmarshalling it to a slice, brackets [] are expected in the byte-slice

if src == nil {
	src = []byte("[]")
}