go-json处理的问题

1、通过Decoder来解析json串

package main

import (
	"encoding/json"
	"fmt"
	"io"
	"log"
	"strings"
)

func main() {
	const jsonStream = `
	{"Name":"Ed","Text":"Knock knock."}
	{"Name":"Sam","Text":"Who's there?"}
	`
	type Message struct {
		Name, Text string
	}

	dec := json.NewDecoder(strings.NewReader(jsonStream))
	for {
		var m Message
		if err := dec.Decode(&m); err == io.EOF {
			break
		} else if err != nil {
			log.Fatal(err)
		}
		fmt.Printf("%s:%s\n", m.Name, m.Text)
	}
}
//output
//Ed:Knock knock.
//Sam:Who's there?

2、marshal对struct进行json字符串化

package main

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

func main() {
	type ColorGroup struct {
		ID     int
		Name   string
		Colors []string
	}
	group := ColorGroup{
		ID:     1,
		Name:   "Reds",
		Colors: []string{"Crimson", "Red", "Ruby", "Marroon"},
	}
	b, err := json.Marshal(group)
	if err != nil {
		fmt.Println("err:", err)
	}
	os.Stdout.Write(b)
}
//output
//{"ID":1,"Name":"Reds","Colors":["Crimson","Red","Ruby","Marroon"]}

package main

import (
	"encoding/json"
	"fmt"
)

type Color struct {
	Red string `json:"r"` //json中key是r
	Yel string `json:"yel,omitempty"`//json中key是yel，omitempty表示当Yel为空时，json中不输出
	Blue string `json:"-"`//json序列化是忽略
	Age string `json:",-"`//json序列化时使用默认的Age key，
	name string `json:"name"`//小写的名字json序列化忽略,私有成员变量，json包无法访问
}

func main()  {
	color := Color{
		Red:"256",
		Yel:"20",
		Blue:"10",
		Age:"10",
		name:"chen",
	}
	bs,_ := json.Marshal(color)
	fmt.Printf("%s\n",bs)
}

//{"r":"256","yel":"20","Age":"10"}

3、Unmarshal 解析json

json规范中，没有区分整数和浮点数

package main

import (
	"encoding/json"
	"fmt"
)

//指定反解析的json struct类型。 fansCount会转换为int64类型。不会有精度的丢失。
type User struct {
	Name string
	FansCount int64
}

func main()  {
	const jsonStream = `
        {"name":"ethancai", "fansCount": 9223372036854775807}`
        var user User
        err := json.Unmarshal([]byte(jsonStream),&user)
        if err != nil {
        	fmt.Println("error:",err)
		}
		fmt.Printf("%+v \n",user)
}

//{Name:ethancai FansCount:9223372036854775807}

package main

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

func main()  {
	const jsonStream = `{"name":"ethancai", "fansCount": 9223372036854775807}`
	var user interface{} //不指定反序列化的类型,
	err := json.Unmarshal([]byte(jsonStream),&user)
	if err != nil {
		fmt.Println("err",err)
	}
	m := user.(map[string]interface{})

	fansCount := m["fansCount"] //没有类型，反序列化时会默认转换为float64,如果整数过大，会丢失精度
	fmt.Printf("%+v\n",reflect.TypeOf(fansCount).Name())
	fmt.Printf("%+v\n",fansCount.(float64))
}
//float64
//9.223372036854776e+18

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name string
	FansCount interface{} //不指定变量类型
}
func main()  {
	const jsonStream = `{"name":"ethancai", "fansCount": 9223372036854775807}`
	var user User
	err := json.Unmarshal([]byte(jsonStream),&user)
	if err != nil {
		fmt.Println("err",err)
	}
	fmt.Printf("%+v \n",user)
}
//{Name:ethancai FansCount:9.223372036854776e+18}

为了解决精度丢失的问题，需要引入json.Number类型

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name string
	FansCount json.Number //使用json.Number类型，本质是string类型。不会有精度丢失
}
func main()  {
	const jsonStream = `{"name":"ethancai", "fansCount": 9223372036854775807}`
	var user User
	err := json.Unmarshal([]byte(jsonStream),&user)
	if err != nil {
		fmt.Println("err",err)
	}
	fmt.Printf("%+v \n",user)
	num ,_ := user.FansCount.Int64() //然后转换成想要的类型
	fmt.Println(num)
}
//{Name:ethancai FansCount:9223372036854775807}
//9223372036854775807