JSON ENCODING

JSON is a structured data exchange format. We can encode structs, maps and slices to JSON. In Go, this is called Marshalling. Decoding is called Unmarshalling.

We use JSON because:

It is Human readable.
Computers can easily understand it.
Widely used and supported.

Encoding values to JSON

Example

Encode list of users to JSON.

type user struct {
	Name        string      `json:"username"`
	Password    string      `json:"-"`
	Permissions permissions `json:"perms,omitempty"`
}

*Note: JSON package only encodes exported fields

json:"username" encodes the field as username.

json:"-" encodes the field but excludes it.

json:"perms,omitempty" encodes the field as perms and prevents encoding if it has a zero-value.

json:",omitempty" only prevents encoding if it has a zero-value.

func main() {
	users := []user{
		{"inanc", "1234", nil},
		{"fab", "42", permissions{"admin": true}},
		{"dave", "644", permissions{"write": true}},
	}

	// json.Marshal(users)
	out, err := json.MarshalIndent(users, "", "\t")
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Println(string(out))
}

json.Marshal encodes a value to JSON. json.MarshalIndent is an alternative to json.Marshal which formats the output to indent it.

go run main.go > users.json

This command saves the output to a file named users.json.

Field Tags

Field Tags associates a static string metadata to a field. They are part of a struct's type and are compile-time constants.

They are mostly used for controlling the encoding/decoding behavior.

We enclose Field Tags in a raw string literal because they usually include double-quotes.

We denote a Field tag as a key-value. The key denotes a package name which means only the package(in this case JSON package) will read the tag. The value may contain multiple options and are conventionally enclosed in double-quotes seperated by a comma.

We use the options to tell the json package how to encode and decode a field.

Decoding values from JSON

Example

In the program below, we read the json data into a []byte and then decode it into a []user

type user struct {
	Name        string          `json:"username"`
	Permissions map[string]bool `json:"permissions"`
}

func main() {
	var input []byte
	for in := bufio.NewScanner(os.Stdin); in.Scan(); {
		input = append(input, in.Bytes()...)
	}

	var users []user

	err := json.Unmarshal(input, &users)
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Println(users)
}

json:"username" tells the json package to get the data from the username field into the Name field.

json.Unmarshal decodes from JSON to a value.

& finds the memory address of the users variable which it passess to the Unmarshal function.

# README

JSON ENCODING

Encoding values to JSON

Example

Field Tags

Decoding values from JSON

Example