使用Go解析XML，并以小写形式包含多个元素

Question

I don't know how to make this code work. I'm just trying to parse a simple XML file like this:

package main

import (
    "encoding/xml"
    "fmt"
)

type Data struct {
    XMLName xml.Name `xml:"data"`
    Nam     string   `xml:"nam,attr"`
}

type Struct struct {
    XMLName xml.Name `xml:"struct"`
    Data    []Data
}

func main() {

    x := `  
        <struct>
            <data nam="MESSAGE_TYPE">     
            </data>
            <data nam="MESSAGE_TYPE2">
            </data>
        </struct>
    `
    s := Struct{}
    err := xml.Unmarshal([]byte(x), &s)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%v
", s)
    fmt.Println(s.Data)
}

what I got is:

{{ struct} []}
[]

But when I change the "data" elements to uppercase, like this:

package main

import (
    "encoding/xml"
    "fmt"
)

type Data struct {
    XMLName xml.Name `xml:"Data"`
    Nam     string   `xml:"nam,attr"`
}

type Struct struct {
    XMLName xml.Name `xml:"struct"`
    Data    []Data
}

func main() {

    x := `  
        <struct>
            <Data nam="MESSAGE_TYPE">     
            </Data>
            <Data nam="MESSAGE_TYPE2">
            </Data>
        </struct>
    `
    s := Struct{}
    err := xml.Unmarshal([]byte(x), &s)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%v
", s)
    fmt.Println(s.Data)
}

I got this:

{{ struct} [{{ Data} MESSAGE_TYPE} {{ Data} MESSAGE_TYPE2}]}
[{{ Data} MESSAGE_TYPE} {{ Data} MESSAGE_TYPE2}]

Can anybody tell me why?

Answer 1

If you do not put XML annotation on the structure field the name of the field is taken as name of XML element.

In the documentation on Unmarshal in endoding/xml package we can find the following:

Unmarshal maps an XML element to a struct using the following rules. In the rules, the tag of a field refers to the value associated with the key 'xml' in the struct field's tag (see the example above).

• If the XML element contains a sub-element whose name matches a field without any mode flags (",attr", ",chardata", etc), Unmarshal maps the sub-element to that struct field.

The matching is case sensitive so it makes a difference in your case.

I recommend annotating the structure like this to fit the actual data:

type Struct struct {
    XMLName xml.Name `xml:"struct"`
    Data    []Data   `xml:"data"`
}