在Golang中解组简单的xml时出错

Question

I'm trying to write a very simple parser in Go for a large xml file (the dblp.xml), an excerpt of which is below:

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE dblp SYSTEM "dblp.dtd">
<dblp>
    <article key="journals/cacm/Gentry10" mdate="2010-04-26">
        <author>Craig Gentry</author>
        <title>Computing arbitrary functions of encrypted data.</title>
        <pages>97-105</pages>
        <year>2010</year>
        <volume>53</volume>
        <journal>Commun. ACM</journal>
        <number>3</number>
        <ee>http://doi.acm.org/10.1145/1666420.1666444</ee>
        <url>db/journals/cacm/cacm53.html#Gentry10</url>
    </article>

    <article key="journals/cacm/Gentry10" mdate="2010-04-26">
        <author>Craig Gentry Number2</author>
        <title>Computing arbitrary functions of encrypted data.</title>
        <pages>97-105</pages>
        <year>2010</year>
        <volume>53</volume>
        <journal>Commun. ACM</journal>
        <number>3</number>
        <ee>http://doi.acm.org/10.1145/1666420.1666444</ee>
        <url>db/journals/cacm/cacm53.html#Gentry10</url>
    </article>
</dblp>

My code is as follows and it looks like there's something going on at xml.Unmarshal(byteValue, &articles), as I cannot get any of the xml's values in the output. Can you help me with what is wrong with my code?

package main

import (
    "encoding/xml"
    "fmt"
    "io/ioutil"
    "os"
)

// Contains the array of articles in the dblp xml
type Dblp struct {
    XMLName xml.Name  `xml:"dblp"`
    Dblp    []Article `xml:"article"`
}

// Contains the article element tags and attributes
type Article struct {
    XMLName xml.Name `xml:"article"`
    Key     string   `xml:"key,attr"`
    Year    string   `xml:"year"`
}

func main() {
    xmlFile, err := os.Open("TestDblp.xml")
    if err != nil {
        fmt.Println(err)
    }

    fmt.Println("Successfully Opened TestDblp.xml")
    // defer the closing of our xmlFile so that we can parse it later on
    defer xmlFile.Close()

    // read our opened xmlFile as a byte array.
    byteValue, _ := ioutil.ReadAll(xmlFile)

    var articles Dblp
    fmt.Println("Entered var")
    // we unmarshal our byteArray which contains our
    // xmlFiles content into 'users' which we defined above
    xml.Unmarshal(byteValue, &articles)

    for i := 0; i < len(articles.Dblp); i++ {
        fmt.Println("Entered loop")
        fmt.Println("get title: " + articles.Dblp[i].Key)
        fmt.Println("get year: " + articles.Dblp[i].Year)
    }

}

Answer 1

You have a specific line in your code that returns an error

xml.Unmarshal(byteValue, &articles)

If you change that to

err = xml.Unmarshal(byteValue, &articles)
if err != nil {
    fmt.Println(err.Error())
}

You'll see an error being reported: xml: encoding "ISO-8859-1" declared but Decoder.CharsetReader is nil. As a best practice, you should always check for errors being returned.

To fix that, you can either remove the encoding attribute (encoding="ISO-8859-1") from the XML or change the code of your unmarshalling a bit:

package main

import (
    "encoding/xml"
    "fmt"
    "io"
    "os"

    "golang.org/x/text/encoding/charmap"
)

// Contains the array of articles in the dblp xml
type Dblp struct {
    XMLName xml.Name  `xml:"dblp"`
    Dblp    []Article `xml:"article"`
}

// Contains the article element tags and attributes
type Article struct {
    XMLName xml.Name `xml:"article"`
    Key     string   `xml:"key,attr"`
    Year    string   `xml:"year"`
}

func main() {
    xmlFile, err := os.Open("dblp.xml")
    if err != nil {
        fmt.Println(err)
    }

    fmt.Println("Successfully Opened TestDblp.xml")
    // defer the closing of our xmlFile so that we can parse it later on
    defer xmlFile.Close()

    var articles Dblp
    decoder := xml.NewDecoder(xmlFile)
    decoder.CharsetReader = makeCharsetReader
    err = decoder.Decode(&articles)
    if err != nil {
        fmt.Println(err)
    }

    for i := 0; i < len(articles.Dblp); i++ {
        fmt.Println("Entered loop")
        fmt.Println("get title: " + articles.Dblp[i].Key)
        fmt.Println("get year: " + articles.Dblp[i].Year)
    }
}

func makeCharsetReader(charset string, input io.Reader) (io.Reader, error) {
    if charset == "ISO-8859-1" {
        // Windows-1252 is a superset of ISO-8859-1, so should do here
        return charmap.Windows1252.NewDecoder().Reader(input), nil
    }
    return nil, fmt.Errorf("Unknown charset: %s", charset)
}

running the above program results in:

Successfully Opened TestDblp.xml
Entered var
Entered loop
get title: journals/cacm/Gentry10
get year: 2010
Entered loop
get title: journals/cacm/Gentry10
get year: 2010