jx

Package jx implements encoding and decoding of json [RFC 7159]. Lightweight fork of jsoniter.

go get github.com/go-faster/jx

• Usage and examples
• Roadmap
• Non-goals

Features

• Mostly zero-allocation and highly optimized
• Directly encode and decode json values
• No reflect or interface{}
• Pools and direct buffer access for less (or none) allocations
• Multi-pass decoding
• Validation

See usage for examples. Mostly suitable for fast low-level json manipulation with high control, for dynamic parsing and encoding of unstructured data. Used in ogen project for json (un)marshaling code generation based on json and OpenAPI schemas.

For example, we have following OpenTelemetry log entry:

{
  "Timestamp": "1586960586000000000",
  "Attributes": {
    "http.status_code": 500,
    "http.url": "http://example.com",
    "my.custom.application.tag": "hello"
  },
  "Resource": {
    "service.name": "donut_shop",
    "service.version": "2.0.0",
    "k8s.pod.uid": "1138528c-c36e-11e9-a1a7-42010a800198"
  },
  "TraceId": "13e2a0921288b3ff80df0a0482d4fc46",
  "SpanId": "43222c2d51a7abe3",
  "SeverityText": "INFO",
  "SeverityNumber": 9,
  "Body": "20200415T072306-0700 INFO I like donuts"
}

Flexibility of jx enables highly efficient semantic-aware encoding and decoding, e.g. using [16]byte for TraceId with zero-allocation hex encoding in json:

cpu: AMD Ryzen 9 7950X

See otel_test.go for example.

Why

Most of jsoniter issues are caused by necessity to be drop-in replacement for standard encoding/json. Removing such constrains greatly simplified implementation and reduced scope, allowing to focus on json stream processing.

• Commas are handled automatically while encoding
• Raw json, Number and Base64 support
• Reduced scope
  • No reflection
  • No encoding/json adapter
  • 3.5x less code (8.5K to 2.4K SLOC)
• Fuzzing, improved test coverage
• Drastically refactored and simplified
  • Explicit error returns
  • No Config or API

Usage

• Decoding
• Encoding
• Writer
• Raw message
• Number
• Base64
• Validation
• Multi pass decoding

Decode

Use jx.Decoder. Zero value is valid, but constructors are available for convenience:

• jx.Decode(reader io.Reader, bufSize int) for io.Reader
• jx.DecodeBytes([]byte) for byte slices
• jx.DecodeStr(string) for strings

To reuse decoders and their buffers, use jx.GetDecoder and jx.PutDecoder alongside with reset functions:

• jx.Decoder.Reset(io.Reader) to reset to new io.Reader
• jx.Decoder.ResetBytes([]byte) to decode another byte slice

Decoder is reset on PutDecoder.

d := jx.DecodeStr(`{"values":[4,8,15,16,23,42]}`)

// Save all integers from "values" array to slice.
var values []int

// Iterate over each object field.
if err := d.Obj(func(d *jx.Decoder, key string) error {
    switch key {
    case "values":
        // Iterate over each array element.
        return d.Arr(func(d *jx.Decoder) error {
            v, err := d.Int()
            if err != nil {
                return err
            }
            values = append(values, v)
            return nil
        })
    default:
        // Skip unknown fields if any.
        return d.Skip()
    }
}); err != nil {
    panic(err)
}

fmt.Println(values)
// Output: [4 8 15 16 23 42]

Encode

Use jx.Encoder. Zero value is valid, reuse with jx.GetEncoder, jx.PutEncoder and jx.Encoder.Reset(). Encoder is reset on PutEncoder.

var e jx.Encoder
e.ObjStart()           // {
e.FieldStart("values") // "values":
e.ArrStart()           // [
for _, v := range []int{4, 8, 15, 16, 23, 42} {
    e.Int(v)
}
e.ArrEnd() // ]
e.ObjEnd() // }
fmt.Println(e)
fmt.Println("Buffer len:", len(e.Bytes()))
// Output: {"values":[4,8,15,16,23,42]}
// Buffer len: 28

Writer

Use jx.Writer for low level json writing.

No automatic commas or indentation for lowest possible overhead, useful for code generated json encoding.

Raw

Use jx.Decoder.Raw to read raw json values, similar to json.RawMessage.

d := jx.DecodeStr(`{"foo": [1, 2, 3]}`)

var raw jx.Raw
if err := d.Obj(func(d *jx.Decoder, key string) error {
    v, err := d.Raw()
    if err != nil {
        return err
    }
    raw = v
    return nil
}); err != nil {
    panic(err)
}

fmt.Println(raw.Type(), raw)
// Output:
// array [1, 2, 3]

Number

Use jx.Decoder.Num to read numbers, similar to json.Number. Also supports number strings, like "12345", which is common compatible way to represent uint64.

d := jx.DecodeStr(`{"foo": "10531.0"}`)

var n jx.Num
if err := d.Obj(func(d *jx.Decoder, key string) error {
    v, err := d.Num()
    if err != nil {
        return err
    }
    n = v
    return nil
}); err != nil {
    panic(err)
}

fmt.Println(n)
fmt.Println("positive:", n.Positive())

// Can decode floats with zero fractional part as integers:
v, err := n.Int64()
if err != nil {
    panic(err)
}
fmt.Println("int64:", v)
// Output:
// "10531.0"
// positive: true
// int64: 10531

Base64

Use jx.Encoder.Base64 and jx.Decoder.Base64 or jx.Decoder.Base64Append.

Same as encoding/json, base64.StdEncoding or [RFC 4648].

var e jx.Encoder
e.Base64([]byte("Hello"))
fmt.Println(e)

data, _ := jx.DecodeBytes(e.Bytes()).Base64()
fmt.Printf("%s", data)
// Output:
// "SGVsbG8="
// Hello

Validate

Check that byte slice is valid json with jx.Valid:

fmt.Println(jx.Valid([]byte(`{"field": "value"}`))) // true
fmt.Println(jx.Valid([]byte(`"Hello, world!"`)))    // true
fmt.Println(jx.Valid([]byte(`["foo"}`)))            // false

Capture

The jx.Decoder.Capture method allows to unread everything is read in callback. Useful for multi-pass parsing:

d := jx.DecodeStr(`["foo", "bar", "baz"]`)
var elems int
// NB: Currently Capture does not support io.Reader, only buffers.
if err := d.Capture(func(d *jx.Decoder) error {
	// Everything decoded in this callback will be rolled back.
	return d.Arr(func(d *jx.Decoder) error {
		elems++
		return d.Skip()
	})
}); err != nil {
	panic(err)
}
// Decoder is rolled back to state before "Capture" call.
fmt.Println("Read", elems, "elements on first pass")
fmt.Println("Next element is", d.Next(), "again")

// Output:
// Read 3 elements on first pass
// Next element is array again

ObjBytes

The Decoder.ObjBytes method tries not to allocate memory for keys, reusing existing buffer.

d := DecodeStr(`{"id":1,"randomNumber":10}`)
d.ObjBytes(func(d *Decoder, key []byte) error {
    switch string(key) {
    case "id":
    case "randomNumber":
    }
    return d.Skip()
})

Roadmap

• Rework and export Any
• Support Raw for io.Reader
• Support Capture for io.Reader
• Improve Num
  • Better validation on decoding
  • Support BigFloat and BigInt
  • Support equivalence check, like eq(1.0, 1) == true
• Add non-callback decoding of objects

Non-goals

• Code generation for decoding or encoding
• Replacement for encoding/json
• Reflection or interface{} based encoding or decoding
• Support for json path or similar

This package should be kept as simple as possible and be used as low-level foundation for high-level projects like code generator.

License

MIT, same as jsoniter