Golang json Unmarshal "unexpected end of JSON input"

Question

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!

Answer 1

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.

Answer 2

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 (eg -> "MyJson.Bar", then the code works.

Answer 3

it is not the case here; but if you are getting this error loading json from a file the 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)
}

Answer 4

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("[]")
}

Answer 5

I encountered the same error while trying to using a select query from the DB (using sqlx).

The code was something like this:

result := []*userRow{}
err := q.Select(&result, getUserByIDQuery, userID)
if err != nil {
    return false, e.Error(codes.Internal, e.DBError, err)
}

Where userRow is defined like

type userRow struct {
    UserId        string
    FirstName     string
    LastName      string
}

To solve the issue I had, I added a db tag to the struct:

type userRow struct {
    UserId        string `db:"user_id"`
    FirstName     string `db:"first_name"`
    LastName      string `db:"last_name"`
}

Hope this can help someone in the furute.