JSON

Go offers built-in support for JSON encoding and decoding, including to and from built-in and custom data types.

package main

import (
    "encoding/json"
    "fmt"
    "os"
)

// We’ll use these two structs to demonstrate encoding and decoding of custom types below.
type response1 struct {
    Page   int
    Fruits []string
}

// Only exported fields will be encoded/decoded in JSON.
// Fields must start with capital letters to be exported.
type response2 struct {
    Page   int      `json:"page"`
    Fruits []string `json:"fruits"`
}

func main() {

    // First we’ll look at encoding basic data types to JSON strings.
    // Here are some examples for atomic values.
    bolB, _ := json.Marshal(true)
    fmt.Println(string(bolB))

    intB, _ := json.Marshal(1)
    fmt.Println(string(intB))

    fltB, _ := json.Marshal(2.34)
    fmt.Println(string(fltB))

    strB, _ := json.Marshal("gopher")
    fmt.Println(string(strB))

    // And here are some for slices and maps
    // which encode to JSON arrays and objects as you’d expect.
    slcD := []string{"apple", "peach", "pear"}
    slcB, _ := json.Marshal(slcD)
    fmt.Println(string(slcB))

    mapD := map[string]int{"apple": 5, "lettuce": 7}
    mapB, _ := json.Marshal(mapD)
    fmt.Println(string(mapB))

    // The JSON package can automatically encode your custom data types.
    // It will only include exported fields in the 
    // encoded output and will by default use those names as the JSON keys.
    res1D := &response1{
        Page:   1,
        Fruits: []string{"apple", "peach", "pear"}}
    res1B, _ := json.Marshal(res1D)
    fmt.Println(string(res1B))

    // You can use tags on struct field declarations 
    // to customize the encoded JSON key names.
    // Check the definition of response2 above to see an example of such tags.
    res2D := &response2{
        Page:   1,
        Fruits: []string{"apple", "peach", "pear"}}
    res2B, _ := json.Marshal(res2D)
    fmt.Println(string(res2B))

    // Now let’s look at decoding JSON data into Go values.
    // Here’s an example for a generic data structure.
    byt := []byte(`{"num":6.13,"strs":["a","b"]}`)

    // We need to provide a variable where 
    // the JSON package can put the decoded data.
    // This map[string]interface{} will hold a map of strings to arbitrary data types.
    var dat map[string]interface{}

    // Here’s the actual decoding, and a check for associated errors.
    if err := json.Unmarshal(byt, &dat); err != nil {
        panic(err)
    }
    fmt.Println(dat)

    // In order to use the values in the decoded map
    // we’ll need to convert them to their appropriate type.
    // For example here we convert the value in num to the expected float64 type.
    num := dat["num"].(float64)
    fmt.Println(num)

    // Accessing nested data requires a series of conversions.
    strs := dat["strs"].([]interface{})
    str1 := strs[0].(string)
    fmt.Println(str1)

    // We can also decode JSON into custom data types.
    // This has the advantages of adding additional 
    // type-safety to our programs and eliminating the need 
    // for type assertions when accessing the decoded data.
    str := `{"page": 1, "fruits": ["apple", "peach"]}`
    res := response2{}
    json.Unmarshal([]byte(str), &res)
    fmt.Println(res)
    fmt.Println(res.Fruits[0])

    // In the examples above we always used bytes and strings
    // as intermediates between the data and JSON representation on standard out.
    // We can also stream JSON encodings directly to os.Writers
    // like os.Stdout or even HTTP response bodies.
    enc := json.NewEncoder(os.Stdout)
    d := map[string]int{"apple": 5, "lettuce": 7}
    enc.Encode(d)
}

$ go run json.go
true
1
2.34
"gopher"
["apple","peach","pear"]
{"apple":5,"lettuce":7}
{"Page":1,"Fruits":["apple","peach","pear"]}
{"page":1,"fruits":["apple","peach","pear"]}
map[num:6.13 strs:[a b]]
6.13
a
{1 [apple peach]}
apple
{"apple":5,"lettuce":7}

We’ve covered the basic of JSON in Go here, but check out the JSON and Go blog post and JSON package docs for more.