Go语言标准库之JSON编解码

基本的类型

Go语言中的数据类型和JSON的数据类型的关系

• bool -> JSON boolean
• float64 -> JSON numbers
• string -> JSON strings

• nil -> JSON null

  Struct to JSON

  Go包encoding/json中的json.Marshal方法，可以将struct编码为JSON数据

package main

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

type FruitBasket struct {
    Name    string
    Fruit   []string
    Id      int64  `json:"ref"`
    private string // 这个字段不会被编码
    Created time.Time
}

func main() {
    basket := FruitBasket{
        Name:    "Standard",
        Fruit:   []string{"Apple", "Banana", "Orange"},
        Id:      999,
        private: "Second-rate",
        Created: time.Now(),
    }

    jsonData, err := json.Marshal(basket)
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println(string(jsonData))
}

输出结果

{"Name":"Standard","Fruit":["Apple","Banana","Orange"],"ref":999,"Created":"2019-01-01T11:40:05.9885387+08:00"}

• 只有public字段才能被导出
• json:tag可以定义导出字段的名称

• nil导出为null

  格式化输出

  可以使用json.MarshalIndent方法来格式化输出的JSON数据

formatData, err := json.MarshalIndent(basket, "", "    ")
fmt.Println(string(formatData))

格式化输出结果是

{
    "Name": "Standard",
    "Fruit": [
        "Apple",
        "Banana",
        "Orange"
    ],
    "ref": 999,
    "Created": "2019-01-01T11:45:49.0969176+08:00"
}

JSON to Struct

Go包中json.Unmarshal方法，解析 JSON数据

jsonStr := []byte(`
    {
        "Name": "Standard",
        "Fruit": [
            "Apple",
            "Banana",
            "Orange"
        ],
        "ref": 999,
        "Created": "2019-01-01T11:45:49.0969176+08:00"
    }
    `)

    var basketStruct FruitBasket
    err = json.Unmarshal(jsonStr, &basketStruct)
    if err != nil {
        log.Println(err)
    }
    fmt.Println(basketStruct.Name, basketStruct.Fruit)
    fmt.Println(basketStruct.Created)

输出结果是

Standard [Apple Banana Orange]
2019-01-01 11:45:49.0969176 +0800 CST

编码格式化任意类型的对象和数组

map[string]interface{}可以编码任意类型的 JSON对象

[]interface{}可以编码任意类型的JSON数组

arbitData := []byte(`{"Name":"Eve","Age":6,"Parents":["Alice","Bob"]}`)
    var v interface{}
    err = json.Unmarshal(arbitData, &v)
    if err != nil {
        log.Println(err)
    }
    data := v.(map[string]interface{})
    for k, v := range data {
        switch v := v.(type) {
        case string:
            fmt.Println(k, v, "(string)")
        case float64:
            fmt.Println(k, v, "(float64)")
        case []interface{}:
            fmt.Println(k, v, "(array)")
            for i, u := range v {
                fmt.Println("    ", i, u)
            }
        default:
            fmt.Println(k, v, "(unknown)")
        }
    }

输出结果是

Name Eve (string)
Age 6 (float64)
Parents [Alice Bob] (array)
     0 Alice
     1 Bob

编解码JSON文件

Go中encoding/json中的json.Decoder和json.Encoder方法可以编解码JSON格式文件

例子：

jsonstr := `
    {"Name": "Alice", "Age": 25}
    {"Name": "Bob", "Age": 22}`
    reader := strings.NewReader(jsonstr)
    writer := os.Stdout

    dec := json.NewDecoder(reader)
    enc := json.NewEncoder(writer)

    for {
        var m map[string]interface{}
        if err := dec.Decode(&m); err == io.EOF {
            break
        } else if err != nil {
            log.Fatal(err)
        }

        for k := range m {
            if k == "Age" {
                delete(m, k)
            }
        }

        if err = enc.Encode(&m); err != nil {
            log.Println(err)
        }
    }

输出结果是

{"Name":"Alice"}
{"Name":"Bob"}

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