自行用go的反射实现json序列化和反序列化功能
我在上一篇文章详细介绍了go反射的API用法，参见一篇带你全面掌握go反射的用法 - 张朝阳 - 博客园 (cnblogs.com)
go自带的json序列化就是通过反射来实现的，为了加深对反射API的掌握程度，我自动实现了类似json序列化和反序列化功能。
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package main
 
import (
    "bytes"
    "container/list"
    "encoding/json"
    "errors"
    "fmt"
    "reflect"
    "strconv"
    "strings"
)
 
type User struct {
    Name string
    Age  int
    Sex  byte `json:"gender"`
}
 
type Book struct {
    ISBN     string `json:"isbn"`
    Name     string
    Price    float32      `json:"price"`
    Author   *User        `json:"author"` //把指针去掉试试
    Keywords []string     `json:"kws"`
    Local    map[int]bool //TODO 暂不支持map
}
 
//由于json字符串里存在{}[]等嵌套情况，直接按,分隔是不合适的
func SplitJson(json string) []string {
    rect := make([]string, 0, 10)
    stack := list.New() //list是双端队列，用它来模拟栈
    beginIndex := 0
    for i, r := range json {
        if r == rune('{') || r == rune('[') {
            stack.PushBack(struct{}{}) //我们不关心栈里是什么，只关心栈里有没有元素
        } else if r == rune('}') || r == rune(']') {
            ele := stack.Back()
            if ele != nil {
                stack.Remove(ele) //删除栈顶元素
            }
        } else if r == rune(',') {
            if stack.Len() == 0 { //栈为空时才可以按,分隔
                rect = append(rect, json[beginIndex:i])
                beginIndex = i + 1
            }
        }
    }
    rect = append(rect, json[beginIndex:])
    return rect
}
 
func Marshal(v interface{}) ([]byte, error) {
    value := reflect.ValueOf(v)
    typ := value.Type() //跟typ := reflect.TypeOf(v)等价
    if typ.Kind() == reflect.Ptr {
        if value.IsNil() { //如果指向nil，直接输出null
            return []byte("null"), nil
        } else { //如果传的是指针类型，先解析指针
            typ = typ.Elem()
            value = value.Elem()
        }
    }
    bf := bytes.Buffer{} //存放序列化结果
    switch typ.Kind() {
    case reflect.String:
        return []byte(fmt.Sprintf("\"%s\"", value.String())), nil //取得reflect.Value对应的原始数据的值
    case reflect.Bool:
        return []byte(fmt.Sprintf("%t", value.Bool())), nil
    case reflect.Float32,
        reflect.Float64:
        return []byte(fmt.Sprintf("%f", value.Float())), nil
    case reflect.Uint,
        reflect.Uint8,
        reflect.Uint16,
        reflect.Uint32,
        reflect.Uint64,
        reflect.Int,
        reflect.Int8,
        reflect.Int16,
        reflect.Int32,
        reflect.Int64:
        return []byte(fmt.Sprintf("%v", value.Interface())), nil
    case reflect.Slice:
        if value.IsNil() {
            return []byte("null"), nil
        }
        bf.WriteByte('[')
        if value.Len() > 0 {
            for i := 0; i < value.Len(); i++ { //取得slice的长度
                if bs, err := Marshal(value.Index(i).Interface()); err != nil { //对slice的第i个元素进行序列化。递归
                    return nil, err
                } else {
                    bf.Write(bs)
                    bf.WriteByte(',')
                }
            }
            bf.Truncate(len(bf.Bytes()) - 1) //删除最后一个逗号
        }
        bf.WriteByte(']')
        return bf.Bytes(), nil
    case reflect.Map:
        if value.IsNil() {
            return []byte("null"), nil
        }
        bf.WriteByte('{')
        if value.Len() > 0 {
            for _, key := range value.MapKeys() {
                if keyBs, err := Marshal(key.Interface()); err != nil {
                    return nil, err
                } else {
                    bf.Write(keyBs)
                    bf.WriteByte(':')
                    v := value.MapIndex(key)
                    if vBs, err := Marshal(v.Interface()); err != nil {
                        return nil, err
                    } else {
                        bf.Write(vBs)
                        bf.WriteByte(',')
                    }
                }
            }
            bf.Truncate(len(bf.Bytes()) - 1) //删除最后一个逗号
        }
        bf.WriteByte('}')
        return bf.Bytes(), nil
    case reflect.Struct:
        bf.WriteByte('{')
        if value.NumField() > 0 {
            for i := 0; i < value.NumField(); i++ {
                fieldValue := value.Field(i)
                fieldType := typ.Field(i)
                name := fieldType.Name //如果没有json Tag，默认使用成员变量的名称
                if len(fieldType.Tag.Get("json")) > 0 {
                    name = fieldType.Tag.Get("json")
                }
                bf.WriteString("\"")
                bf.WriteString(name)
                bf.WriteString("\"")
                bf.WriteString(":")
                if bs, err := Marshal(fieldValue.Interface()); err != nil { //对value递归调用Marshal序列化
                    return nil, err
                } else {
                    bf.Write(bs)
                }
                bf.WriteString(",")
            }
            bf.Truncate(len(bf.Bytes()) - 1) //删除最后一个逗号
        }
        bf.WriteByte('}')
        return bf.Bytes(), nil
    default:
        return []byte(fmt.Sprintf("\"暂不支持该数据类型:%s\"", typ.Kind().String())), nil
    }
}
 
func Unmarshal(data []byte, v interface{}) error {
    s := string(data)
    //去除前后的连续空格
    s = strings.TrimLeft(s, " ")
    s = strings.TrimRight(s, " ")
    if len(s) == 0 {
        return nil
    }
    typ := reflect.TypeOf(v)
    value := reflect.ValueOf(v)
    if typ.Kind() != reflect.Ptr { //因为要修改v，必须传指针
        return errors.New("must pass pointer parameter")
    }
 
    typ = typ.Elem() //解析指针
    value = value.Elem()
 
    switch typ.Kind() {
    case reflect.String:
        if s[0] == '"' && s[len(s)-1] == '"' {
            value.SetString(s[1 : len(s)-1]) //去除前后的""
        } else {
            return fmt.Errorf("invalid json part: %s", s)
        }
    case reflect.Bool:
        if b, err := strconv.ParseBool(s); err == nil {
            value.SetBool(b)
        } else {
            return err
        }
    case reflect.Float32,
        reflect.Float64:
        if f, err := strconv.ParseFloat(s, 64); err != nil {
            return err
        } else {
            value.SetFloat(f) //通过reflect.Value修改原始数据的值
        }
    case reflect.Int,
        reflect.Int8,
        reflect.Int16,
        reflect.Int32,
        reflect.Int64:
        if i, err := strconv.ParseInt(s, 10, 64); err != nil {
            return err
        } else {
            value.SetInt(i) //有符号整型通过SetInt
        }
    case reflect.Uint,
        reflect.Uint8,
        reflect.Uint16,
        reflect.Uint32,
        reflect.Uint64:
        if i, err := strconv.ParseUint(s, 10, 64); err != nil {
            return err
        } else {
            value.SetUint(i) //无符号整型需要通过SetUint
        }
    case reflect.Slice:
        if s[0] == '[' && s[len(s)-1] == ']' {
            arr := SplitJson(s[1 : len(s)-1]) //去除前后的[]
            if len(arr) > 0 {
                slice := reflect.ValueOf(v).Elem()                    //别忘了，v是指针
                slice.Set(reflect.MakeSlice(typ, len(arr), len(arr))) //通过反射创建slice
                for i := 0; i < len(arr); i++ {
                    eleValue := slice.Index(i)
                    eleType := eleValue.Type()
                    if eleType.Kind() != reflect.Ptr {
                        eleValue = eleValue.Addr()
                    }
                    if err := Unmarshal([]byte(arr[i]), eleValue.Interface()); err != nil {
                        return err
                    }
                }
            }
        } else if s != "null" {
            return fmt.Errorf("invalid json part: %s", s)
        }
    case reflect.Map:
        if s[0] == '{' && s[len(s)-1] == '}' {
            arr := SplitJson(s[1 : len(s)-1]) //去除前后的{}
            if len(arr) > 0 {
                mapValue := reflect.ValueOf(v).Elem()                //别忘了，v是指针
                mapValue.Set(reflect.MakeMapWithSize(typ, len(arr))) //通过反射创建map
                kType := typ.Key()                                   //获取map的key的Type
                vType := typ.Elem()                                  //获取map的value的Type
                for i := 0; i < len(arr); i++ {
                    brr := strings.Split(arr[i], ":")
                    if len(brr) != 2 {
                        return fmt.Errorf("invalid json part: %s", arr[i])
                    }
 
                    kValue := reflect.New(kType) //根据Type创建指针型的Value
                    if err := Unmarshal([]byte(brr[0]), kValue.Interface()); err != nil {
                        return err
                    }
                    vValue := reflect.New(vType) //根据Type创建指针型的Value
                    if err := Unmarshal([]byte(brr[1]), vValue.Interface()); err != nil {
                        return err
                    }
                    mapValue.SetMapIndex(kValue.Elem(), vValue.Elem()) //往map里面赋值
                }
            }
        } else if s != "null" {
            return fmt.Errorf("invalid json part: %s", s)
        }
    case reflect.Struct:
        if s[0] == '{' && s[len(s)-1] == '}' {
            arr := SplitJson(s[1 : len(s)-1])
            if len(arr) > 0 {
                fieldCount := typ.NumField()
                //建立json tag到FieldName的映射关系
                tag2Field := make(map[string]string, fieldCount)
                for i := 0; i < fieldCount; i++ {
                    fieldType := typ.Field(i)
                    name := fieldType.Name
                    if len(fieldType.Tag.Get("json")) > 0 {
                        name = fieldType.Tag.Get("json")
                    }
                    tag2Field[name] = fieldType.Name
                }
 
                for _, ele := range arr {
                    brr := strings.SplitN(ele, ":", 2) //json的value里可能存在嵌套，所以用:分隔时限定个数为2
                    if len(brr) == 2 {
                        tag := strings.Trim(brr[0], " ")
                        if tag[0] == '"' && tag[len(tag)-1] == '"' { //json的key肯定是带""的
                            tag = tag[1 : len(tag)-1]                        //去除json key前后的""
                            if fieldName, exists := tag2Field[tag]; exists { //根据json key(即json tag)找到对应的FieldName
                                fieldValue := value.FieldByName(fieldName)
                                fieldType := fieldValue.Type()
                                if fieldType.Kind() != reflect.Ptr {
                                    //如果内嵌不是指针，则声明时已经用0值初始化了，此处只需要根据json改写它的值
                                    fieldValue = fieldValue.Addr()                                            //确保fieldValue指向指针类型，因为接下来要把fieldValue传给Unmarshal
                                    if err := Unmarshal([]byte(brr[1]), fieldValue.Interface()); err != nil { //递归调用Unmarshal，给fieldValue的底层数据赋值
                                        return err
                                    }
                                } else {
                                    //如果内嵌的是指针，则需要通过New()创建一个实例(申请内存空间)。不能给New()传指针型的Type，所以调一下Elem()
                                    newValue := reflect.New(fieldType.Elem())                               //newValue代表的是指针
                                    if err := Unmarshal([]byte(brr[1]), newValue.Interface()); err != nil { //递归调用Unmarshal，给fieldValue的底层数据赋值
                                        return err
                                    }
                                    value.FieldByName(fieldName).Set(newValue) //把newValue赋给value的Field
                                }
 
                            } else {
                                fmt.Printf("字段%s找不到\n", tag)
                            }
                        } else {
                            return fmt.Errorf("invalid json part: %s", tag)
                        }
                    } else {
                        return fmt.Errorf("invalid json part: %s", ele)
                    }
                }
            }
        } else if s != "null" {
            return fmt.Errorf("invalid json part: %s", s)
        }
    default:
        fmt.Printf("暂不支持类型:%s\n", typ.Kind().String())
    }
    return nil
}
 
func main() {
    user := User{
        Name: "钱钟书",
        Age:  57,
        Sex:  1,
    }
    book := Book{
        ISBN:     "4243547567",
        Name:     "围城",
        Price:    34.8,
        Author:   &user,                      //改成nil试试
        Keywords: []string{"爱情", "民国", "留学"}, //把这一行注释掉试一下，测测null
        Local:    map[int]bool{2: true, 3: false},
    }
 
    if bytes, err := Marshal(user); err != nil { //也可以给Marshal传指针类型
        fmt.Printf("序列化失败: %v\n", err)
    } else {
        fmt.Println(string(bytes))
        var u User
        if err = Unmarshal(bytes, &u); err != nil {
            fmt.Printf("反序列化失败: %v\n", err)
        } else {
            fmt.Printf("user name %s\n", u.Name)
        }
    }
 
    if bytes, err := json.Marshal(user); err != nil {
        fmt.Printf("序列化失败: %v\n", err)
    } else {
        fmt.Println(string(bytes))
        var u User
        if err = json.Unmarshal(bytes, &u); err != nil {
            fmt.Printf("反序列化失败: %v\n", err)
        } else {
            fmt.Printf("user name %s\n", u.Name)
        }
    }
 
    if bytes, err := Marshal(book); err != nil { //也可以给Marshal传指针类型
        fmt.Printf("序列化失败: %v\n", err)
    } else {
        fmt.Println(string(bytes))
        var b Book //必须先声明值类型，再通过&给Unmarshal传一个指针参数。因为声明值类型会初始化为0值，而声明指针都没有创建底层的内存空间
        if err = Unmarshal(bytes, &b); err != nil {
            fmt.Printf("反序列化失败: %v\n", err)
        } else {
            fmt.Printf("book name %s author name %s local %v\n", b.Name, b.Author.Name, b.Local)
        }
    }
 
    if bytes, err := json.Marshal(book); err != nil {
        fmt.Printf("序列化失败: %v\n", err)
    } else {
        fmt.Println(string(bytes))
        var b Book
        if err = json.Unmarshal(bytes, &b); err != nil {
            fmt.Printf("反序列化失败: %v\n", err)
        } else {
            fmt.Printf("book name %s author name %s local %v\n", b.Name, b.Author.Name, b.Local)
        }
    }
}