CODEWITHSUNDEEP
jsongostreamdecodeunmarshalling
Brad Peabody
Brad Peabody

Reputation: 11377

How to read a JSON object in Go without decoding it (for use in reading a large stream)

I am reading JSON in response to an HTTP endpoint and would like to extract the contents of an array of objects which is nested inside. The response can be large so I am trying to use a streaming approach instead of just json.Unmarshal'ing the whole thing. The JSON looks like so:

{
"useless_thing_1": { /* etc */ },
"useless_thing_2": { /* etc */ },
"the_things_i_want": [
  { /* complex object I want to json.Unmarshal #1 */ },
  { /* complex object I want to json.Unmarshal #2 */ },
  { /* complex object I want to json.Unmarshal #3 */ },
  /* could be many thousands of these */
],
"useless_thing_3": { /* etc */ },
}

The json library provided with Go has json.Unmarshal which works well for complete JSON objects. It also has json.Decoder which can unmarshal full objects or provide individual tokens. I can use this tokenizer to carefully go through and extract things but the logic to do so is somewhat complex and I cannot then easily still use json.Unmarshal on the object after I've read it as tokens.

The json.Decoder is buffered which makes it difficult to read one object (i.e. { /* complex object I want to json.Unmarshal #1 */ }) and then consume the , myself and make a new json.Decoder - because it will try to consume the comma itself. This is the approach I tried and haven't been able to make work.

I'm looking for a better solution to this problem. Here is the broken code when I tried to manually consume the commas:

// code here that naively looks for `"the_things_i_want": [` and
// puts the next bytes after that in `buffer`

// this is the rest of the stream starting from `{ /* complex object I want to json.Unmarshal #1 */ },`
in := io.MultiReader(buffer, res.Body) 

dec := json.NewDecoder(in)

for {

    var p MyComplexThing
    err := dec.Decode(&p)
    if err != nil {
        panic(err)
    }

    // steal the comma from in directly - this does not work because the decoder buffer's its input
    var b1 [1]byte
    _, err = io.ReadAtLeast(in, b1[:], 1) // returns random data from later in the stream
    if err != nil {
        panic(err)
    }
    switch b1[0] {
    case ',':
        // skip over it
    case ']':
        break // we're done
    default:
        panic(fmt.Errorf("Unexpected result from read %#v", b1))
    }
}

Upvotes: 3

Views: 2487

Answers (1)

Thundercat
Thundercat

Reputation: 120941

Use Decoder.Token and Decoder.More to decode a JSON document as a stream.

Walk through the document with Decoder.Token to the JSON value of interest. Call Decoder.Decode unmarshal the JSON value to a Go value. Repeat as needed to slurp up all values of interest.

Here's some code with commentary explaining how it works:

func decode(r io.Reader) error {
    d := json.NewDecoder(r)

    // We expect that the JSON document is an object.
    if err := expect(d, json.Delim('{')); err != nil {
        return err
    }

    // While there are fields in the object...
    for d.More() {

        // Get field name
        t, err := d.Token()
        if err != nil {
            return err
        }

        // Skip value if not the field that we are looking for.
        if t != "the_things_i_want" {
            if err := skip(d); err != nil {
                return err
            }
            continue
        }

        // We expect JSON array value for the field.
        if err := expect(d, json.Delim('[')); err != nil {
            return err
        }

        // While there are more JSON array elements...
        for d.More() {

            // Unmarshal and process the array element.

            var m map[string]interface{}
            if err := d.Decode(&m); err != nil {
                return err
            }
            fmt.Printf("found %v\n", m)
        }

        // We are done decoding the array.
        return nil

    }
    return errors.New("things I want not found")
}

// skip skips the next value in the JSON document.
func skip(d *json.Decoder) error {
    n := 0
    for {
        t, err := d.Token()
        if err != nil {
            return err
        }
        switch t {
        case json.Delim('['), json.Delim('{'):
            n++
        case json.Delim(']'), json.Delim('}'):
            n--
        }
        if n == 0 {
            return nil
        }
    }
}

// expect returns an error if the next token in the document is not expectedT.
func expect(d *json.Decoder, expectedT interface{}) error {
    t, err := d.Token()
    if err != nil {
        return err
    }
    if t != expectedT {
        return fmt.Errorf("got token %v, want token %v", t, expectedT)
    }
    return nil
}

Run it on the playground.

Upvotes: 6

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