go interface 转 string,解析json

package main

import (
	"encoding/json"
	"fmt"
)

var user map[string]interface{}

func main() {
	userJson := "{\"username\":\"system\",\"password\":\"123456\"}"
	json.Unmarshal([]byte(userJson), &user)
	fmt.Println(user) //打印结果:map[password:123456 username:system]
	username := user["username"]
	fmt.Println("username  ==", username) //username  == system
	//判断username的长度不为0,
	//if len(username) != 0 {//直接使用,报错invalid argument username (type interface {}) for len
	if len(username.(string)) != 0 {//通过.(string)转换成string类型
		fmt.Println("输入合法")
	} else {
		fmt.Println("输入不合法")
	}
}