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fastjson: fast JSON encoder for Go

Package fastjson provides a library and code generator for fast JSON encoding.

The supplied code generator (cmd/generate-fastjson) generates JSON marshalling methods for all exported types within a specified package.

Requirements

Go 1.8+

License

Apache 2.0.

Installation

go get -u go.elastic.co/fastjson/...

Code generation

Package fastjson is intended to be used with the accompanying code generator, cmd/generate-fastjson. This code generator will parse the Go code of a specified package, and write out fastjson marshalling method (MarshalFastJSON) definitions for the exported types in the package.

You can provide your own custom marshalling logic for a type by defining a MarshalFastJSON method for it. The generator will not generate methods for those types with existing marshalling methods.

Usage

generate-fastjson <package>
  -f    remove the output file if it exists
  -o string
        file to which output will be written (default "-")

Custom omitempty extension

The standard json struct tags defined by encoding/json are honoured, enabling you to generate fastjson-marshalling code for your existing code.

We extend the omitempty option by enabling you to define an unexported method on your type T, func (T) isZero() bool, which will be called to determine whether or not the value is considered empty. This enables omitempty on non-pointer struct types.

Example

Given the following package:

package example

type Foo struct {
	Bar Bar `json:",omitempty"`
}

type Bar struct {
	Baz Baz
	Qux *Qux `json:"quux,omitempty"`
}

func (b Bar) isZero() bool {
	return b == (Bar{})
}

type Baz struct {
}

func (Baz) MarshalFastJSON(w *fastjson.Writer) error {
}

type Qux struct{}

Assuming we're in the package directory, we would generate the methods like so, which will write a Go file to stdout:

generate-fastjson .

Output:

// Code generated by "generate-fastjson". DO NOT EDIT.

package example

import (
	"go.elastic.co/fastjson"
)

func (v *Foo) MarshalFastJSON(w *fastjson.Writer) error {
	w.RawByte('{')
	if !v.Bar.isZero() {
		w.RawString("\"Bar\":")
		if err := v.Bar.MarshalFastJSON(w); err != nil && firstErr == nil {
			firstErr == err
		}
	}
	w.RawByte('}')
	return nil
}

func (v *Bar) MarshalFastJSON(w *fastjson.Writer) error {
	var firstErr error
	w.RawByte('{')
	w.RawString("\"Baz\":")
	if err := v.Baz.MarshalFastJSON(w); err != nil && firstErr == nil {
		firstErr = err
	}
	if v.Qux != nil {
		w.RawString(",\"quux\":")
		if err := v.Qux.MarshalFastJSON(w); err != nil && firstErr == nil {
			firstErr == err
		}
	}
	w.RawByte('}')
	return firstErr
}

func (v *Qux) MarshalFastJSON(w *fastjson.Writer) error {
	w.RawByte('{')
	w.RawByte('}')
	return nil
}

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Package fastjson provides a Go library and code generator for fast JSON encoding.

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