dongxuxian1123
2016-11-29 11:30
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如何在golang中获取结构的json字段名称?

What is the way to get the json field names of this struct ?

type example struct {
    Id          int `json:"id"`
    CreatedAt   string `json:"created_at"`
    Tag         string `json:"tag"`
    Text        string `json:"text"`
    AuthorId    int `json:"author_id"`
}

I try to print the fields with this function :

func (b example) PrintFields() {
    val := reflect.ValueOf(b)
    for i := 0; i < val.Type().NumField(); i++ {
        fmt.Println(val.Type().Field(i).Name)
    }
}

Of course I get :

Id
CreatedAt
Tag
Text
AuthorId

But I would like something like :

id
created_at
tag
text
author_id
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5条回答 默认 最新

  • dongpangfu6322 2016-11-29 11:40
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    You use the StructTag type to get the tags. The documentation I linked has examples, look them up, but your code could be something like

    func (b example) PrintFields() {
        val := reflect.ValueOf(b)
        for i := 0; i < val.Type().NumField(); i++ {
            fmt.Println(val.Type().Field(i).Tag.Get("json"))
        }
    }
    

    NOTE The json tag format supports more than just field names, such as omitempty or string, so if you need an approach that takes care of that too, further improvements to the PrintFields function should be made:

    1. we need to check whether the json tag is - (i.e. json:"-")
    2. we need to check if name exists and isolate it

    Something like this:

    func (b example) PrintFields() {
        val := reflect.ValueOf(b)
        for i := 0; i < val.Type().NumField(); i++ {
            t := val.Type().Field(i)
            fieldName := t.Name
    
            if jsonTag := t.Tag.Get("json"); jsonTag != "" && jsonTag != "-" {
                if commaIdx := strings.Index(jsonTag, ","); commaIdx > 0 {
                    fieldName = jsonTag[:commaIdx]
                }
            }
    
    
            fmt.Println(fieldName)
        }
    }
    
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  • dongxieting9623 2016-11-29 11:41

    Use:

    func (b example) PrintFields() {
        val := reflect.ValueOf(b)
        t := val.Type()
        for i := 0; i < t.NumField(); i++ {
            fmt.Println(t.Field(i).Tag.Get("json"))
        }
    }
    

    See it in playground.

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  • dongpu1879 2016-11-29 11:43

    Instead of using StructField's Name, you can use Tag to get a StructTag object. See: https://golang.org/pkg/reflect/#StructTag

    Then you can use StructTag's Get or Lookup methods to get the json tag:

    Using Get:

    func (b example) PrintFields() {
        val := reflect.ValueOf(b)
        for i := 0; i < val.Type().NumField(); i++ {
            // prints empty line if there is no json tag for the field
            fmt.Println(val.Type().Field(i).Tag.Get("json"))
        }
    }
    

    Using Lookup:

    func (b example) PrintFields() {
        val := reflect.ValueOf(b)
        for i := 0; i < val.Type().NumField(); i++ {
            // skips fields without json tag
            if tag, ok := val.Type().Field(i).Tag.Lookup("json"); ok {
                fmt.Println(tag)
            }
        }
    }
    
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  • douzhao4071 2016-11-29 11:44

    Not the Name you are looking for. What you looking is the Tag

    func (b example) PrintFields() {
        val := reflect.ValueOf(b)
        for i := 0; i < val.Type().NumField(); i++ {
            fmt.Println(val.Type().Field(i).Tag.Get("json"))
        }
    }
    
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  • dongnanman9093 2019-02-05 21:48

    an updated version with a generic interface as parameter:

    func PrintFields(b interface{}) {
        val := reflect.ValueOf(b)
        for i := 0; i < val.Type().NumField(); i++ {
            t := val.Type().Field(i)
            fieldName := t.Name
    
            if jsonTag := t.Tag.Get("json"); jsonTag != "" && jsonTag != "-" {
                // check for possible comma as in "...,omitempty"
                var commaIdx int
                if commaIdx = strings.Index(jsonTag, ","); commaIdx < 0 {
                    commaIdx = len(jsonTag)
                }
                fieldName = jsonTag[:commaIdx]
            }
            fmt.Println(fieldName)
        }
    }
    
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