下面是一段Json格式的数据：

{"name":"lane","age":18,"email":["xxx@qq.com","yyy@qq.com"]}

接下来，我们写一段代码，把Json格式的数据做为字符串输入，我们的代码解析后输出到结构体；然后修改结构体，在以字符串格式输出。

1 将Json string解析为结构体(struct)

package main

import (
    "log"
    "encoding/json"
)

type Person struct {
    Name string
    Age int
    Email []string
}

var json_str = []byte(`{"name":"lane","age":18,"email":["xxx@qq.com","yyy@qq.com"]}`)

func main() {
    p := Person{}
    err := json.Unmarshal(json_str, &p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(p)
    }
}

2 将结构体(struct)转换为Json string

package main

import (
    "log"
    "encoding/json"
)

type Person struct {
    Name string
    Age int
    Email []string
}

var json_str = []byte(`{"name":"lane","age":18,"email":["xxx@qq.com","yyy@qq.com"]}`)

func main() {
    p := Person{}
    err := json.Unmarshal(json_str, &p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(p)
    }

    p.Age = 19
    json_str, err := json.Marshal(p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(string(json_str))
    }
}

3 使用Struct Tag

假设我们修改Json string，把age改为how-old，现在Json string是这样的：

{"name":"lane","how-old":18,"email":["xxx@qq.com","yyy@qq.com"]}

我们原来的代码还能正确运行吗？我们试一下：

package main

import (
    "log"
    "encoding/json"
)

type Person struct {
    Name string
    Age int
    Email []string
}

var json_str = []byte(`{"name":"lane","how-old":18,"email":["xxx@qq.com","yyy@qq.com"]}`)

func main() {
    p := Person{}
    err := json.Unmarshal(json_str, &p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(p)
    }

    p.Age = 19
    json_str, err := json.Marshal(p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(string(json_str))
    }
}

运行结果为：

2021/07/06 12:09:31 {lane 0 [xxx@qq.com yyy@qq.com]}
2021/07/06 12:09:31 {"Name":"lane","Age":19,"Email":["xxx@qq.com","yyy@qq.com"]}

使用Struct Tag会使情况变好：

Age int `json:"how-old"`

其中json:"how-old"为Struct Tag, 完整代码如下：

package main

import (
    "log"
    "encoding/json"
)

type Person struct {
    Name string
    Age int `json:"how-old"`
    Email []string
}

var json_str = []byte(`{"name":"lane","how-old":18,"email":["xxx@qq.com","yyy@qq.com"]}`)

func main() {
    p := Person{}
    err := json.Unmarshal(json_str, &p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(p)
    }

    p.Age = 19
    json_str, err := json.Marshal(p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(string(json_str))
    }
}

运行结果为：

2021/07/06 12:23:07 {lane 18 [xxx@qq.com yyy@qq.com]}
2021/07/06 12:23:07 {"Name":"lane","how-old":19,"Email":["xxx@qq.com","yyy@qq.com"]}

4 使用omitempty

如果把p.Email设为nil，

    p.Age = 19
    p.Email = nil    // <--- add this line 

则运行结果为：

2021/07/07 04:28:51 {lane 18 [xxx@qq.com yyy@qq.com]}
2021/07/07 04:28:51 {"Name":"lane","how-old":19,"Email":null}

其中，也可以省略掉"Email":null
方法是：

type Person struct {
    Name string
    Age int `json:"how-old"`
    Email []string `json:"email,omitempty"`    // use omitempty
}

下面是完整代码：

package main

import (
    "log"
    "encoding/json"
)

type Person struct {
    Name string
    Age int `json:"how-old"`
    Email []string `json:"email,omitempty"`
}

var json_str = []byte(`{"name":"lane","how-old":18,"email":["xxx@qq.com","yyy@qq.com"]}`)

func main() {
    p := Person{}
    err := json.Unmarshal(json_str, &p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(p)
    }

    p.Age = 19
    p.Email = nil
    json_str, err := json.Marshal(p)
    if err != nil {
        log.Print(err)
    } else {
        log.Print(string(json_str))
    }
}

运行结果：

2021/07/07 04:33:46 {lane 18 [xxx@qq.com yyy@qq.com]}
2021/07/07 04:33:46 {"Name":"lane","how-old":19}