Go XML编码问题

Question

I need to produce the following xml :

<AccessControlList>
    <Grant>
        <Grantee
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
            <ID>xxxx-xx</ID>
            <DisplayName>rahul.khan</DisplayName>
        </Grantee>
        <Permission>FULL_CONTROL</Permission>
    </Grant>
</AccessControlList>

My struct is defined like this :

type Grantee struct {
Xmlns_xsi   string `xml:"xmlns xsi,attr,omitempty"`
Xsi_type    string `xml:"http://www.w3.org/2001/XMLSchema-instance type,attr,omitempty"`
ID          string `xml:",omitempty"`
DisplayName string `xml:",omitempty"`

}

However when i marshall this structure the resulting xml document that is produced looks like this :

<AccessControlList>
    <Grant>
        <Grantee
            xmlns:XMLSchema-instance="http://www.w3.org/2001/XMLSchema-instance" XMLSchema-instance:type="CanonicalUser">
            <ID>xxxx-xx</ID>
            <DisplayName>rahul.khan</DisplayName>
        </Grantee>
        <Permission>FULL_CONTROL</Permission>
    </Grant>
</AccessControlList>

As a result, when the doc is unmarshalled, the type field doesn't seem to be parsed by the aws-go-sdk.

For e.g this is the unmarhsalled output that i need to get

Grantee: {
    DisplayName: "rahul.khan",
    ID: "xxxx-xx",
    Type: "CanonicalUser"
  },
  Permission: "FULL_CONTROL"
}

Instead what I get is this :

  Grantee: {
    DisplayName: "rahul.khan",
    ID: "xxxx-xx"
  },
  Permission: "FULL_CONTROL"
}

The Type attribute seems to be missing from the unmarshalled output. The only difference that I see in the xml document produced by my code and from aws is this line

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser" v/s

xmlns:XMLSchema-instance="http://www.w3.org/2001/XMLSchema-instance" XMLSchema-instance:type="CanonicalUser"

Appreciate if someone can help me understand how to fix this issue ?

Answer 1

This struct Marshals to the XML you need, using just endcoding/xml.Marshal():

type Grantee struct {
    Xmlns_xsi   string `xml:"xmlns:xsi,attr,omitempty"`  //set namespace prefix explicitly
    Xsi_type    string `xml:"xsi:type,attr,omitempty"`   //set namespace:attr  explicitly
    ID          string `xml:",omitempty"`
    DisplayName string `xml:",omitempty"`
}

g := Grantee{
    DisplayName: "rahul.khan",
    ID:          "xxxx-xx",
    Xsi_type:    "CanonicalUser",
    Xmlns_xsi:   "http://www.w3.org/2001/XMLSchema-instance", // set namespace URI as the value of the field
}

gm, err := xml.MarshalIndent(g, "", "  ")
if err != nil {
    fmt.Printf("error: %v", err)
    return
}

fmt.Printf("%+s
", gm)

//<Grantee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
//  <ID>xxxx-xx</ID>
//  <DisplayName>rahul.khan</DisplayName>
//</Grantee>

https://play.golang.org/p/zrVlmktLyZu

Notice that the xmlns:xsi and xsi:type are just set as explicit attribute names, including the colons. As you found, if you leave a space between the namespace and attribute name, encoding/xml makes changes in an attempt to clean up the namespaces in the output (like using the final part of a namespace-URL as the namespace prefix: e.g. XMLSchema-instance).

I recognize that this XML may not Unmarshal into your Go struct as you would like (as discussed in many golang issues, such as the one referenced by @Adrian in the comment to your question). But, if you're submitting to the S3 Rest API to change the ACL, it looks like this xml will be correct). The aws-go-sdk appears to use their own function to deserialise XML, so give it a try.