解析 XML 数据

使用集合让一切井井有条 根据您的偏好保存内容并对其进行分类。

可扩展标记语言 (XML) 是一组以机器可读的形式对文档进行编码的规则。XML 是一种在互联网上共享数据的常用格式。经常更新内容的网站(例如新闻网站或博客)通常会提供 XML Feed,以便外部程序及时了解内容更改。上传和解析 XML 数据是联网应用的常见任务。本课将介绍如何解析 XML 文档并使用其数据。

如需详细了解如何在 Android 应用中创建基于网络的内容,请参阅 Web 应用

选择解析器

建议使用 XmlPullParser,这是一种在 Android 上解析 XML 的高效且可维护的方式。以前,Android 有此接口的两个实现:

任一选择都可以。此部分中的示例使用 ExpatPullParser(通过 Xml.newPullParser())。

分析 Feed

解析 Feed 的第一步是确定您感兴趣的字段。解析器会提取这些字段的数据,并忽略其余字段。

以下是示例应用中正在解析的 Feed 摘录。每个发布到 StackOverflow.com 的帖子都会以包含多个嵌套标签的 entry 标签形式显示在 Feed 中:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:creativeCommons="http://backend.userland.com/creativeCommonsRssModule" ...">
<title type="text">newest questions tagged android - Stack Overflow</title>
...
    <entry>
    ...
    </entry>
    <entry>
        <id>http://stackoverflow.com/q/9439999</id>
        <re:rank scheme="http://stackoverflow.com">0</re:rank>
        <title type="text">Where is my data file?</title>
        <category scheme="http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest/tags" term="android"/>
        <category scheme="http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest/tags" term="file"/>
        <author>
            <name>cliff2310</name>
            <uri>http://stackoverflow.com/users/1128925</uri>
        </author>
        <link rel="alternate" href="http://stackoverflow.com/questions/9439999/where-is-my-data-file" />
        <published>2012-02-25T00:30:54Z</published>
        <updated>2012-02-25T00:30:54Z</updated>
        <summary type="html">
            <p>I have an Application that requires a data file...</p>

        </summary>
    </entry>
    <entry>
    ...
    </entry>
...
</feed>

示例应用会提取 entry 标签及其嵌套标签 titlelinksummary 的数据。

实例化解析器

下一步是实例化解析器并启动解析过程。在此代码段中,解析器被初始化为不处理命名空间,并使用提供的 InputStream 作为输入。它通过调用 nextTag() 开始解析过程,并调用 readFeed() 方法,该方法将提取并处理应用感兴趣的数据:

Kotlin

// We don't use namespaces
private val ns: String? = null

class StackOverflowXmlParser {

    @Throws(XmlPullParserException::class, IOException::class)
    fun parse(inputStream: InputStream): List<*> {
        inputStream.use { inputStream ->
            val parser: XmlPullParser = Xml.newPullParser()
            parser.setFeature(XmlPullParser.FEATURE_PROCESS_NAMESPACES, false)
            parser.setInput(inputStream, null)
            parser.nextTag()
            return readFeed(parser)
        }
    }
 ...
}

Java

public class StackOverflowXmlParser {
    // We don't use namespaces
    private static final String ns = null;

    public List parse(InputStream in) throws XmlPullParserException, IOException {
        try {
            XmlPullParser parser = Xml.newPullParser();
            parser.setFeature(XmlPullParser.FEATURE_PROCESS_NAMESPACES, false);
            parser.setInput(in, null);
            parser.nextTag();
            return readFeed(parser);
        } finally {
            in.close();
        }
    }
 ...
}

读取 Feed

readFeed() 方法执行处理 Feed 的实际工作。它会查找标记为“entry”的元素作为以递归方式处理 Feed 的起点。如果某个标签不是 entry 标签,则会跳过它。以递归方式处理完整个 Feed 后,readFeed() 将返回 List,其中包含从 Feed 提取的条目(包括嵌套数据成员)。然后,解析器会返回此 List

Kotlin

@Throws(XmlPullParserException::class, IOException::class)
private fun readFeed(parser: XmlPullParser): List<Entry> {
    val entries = mutableListOf<Entry>()

    parser.require(XmlPullParser.START_TAG, ns, "feed")
    while (parser.next() != XmlPullParser.END_TAG) {
        if (parser.eventType != XmlPullParser.START_TAG) {
            continue
        }
        // Starts by looking for the entry tag
        if (parser.name == "entry") {
            entries.add(readEntry(parser))
        } else {
            skip(parser)
        }
    }
    return entries
}

Java

private List readFeed(XmlPullParser parser) throws XmlPullParserException, IOException {
    List entries = new ArrayList();

    parser.require(XmlPullParser.START_TAG, ns, "feed");
    while (parser.next() != XmlPullParser.END_TAG) {
        if (parser.getEventType() != XmlPullParser.START_TAG) {
            continue;
        }
        String name = parser.getName();
        // Starts by looking for the entry tag
        if (name.equals("entry")) {
            entries.add(readEntry(parser));
        } else {
            skip(parser);
        }
    }
    return entries;
}

解析 XML

解析 XML Feed 的步骤如下:

  1. 按照分析 Feed 中所述,确定您希望包含在应用中的标签。此示例提取了 entry 标签及其嵌套标签 titlelinksummary 的数据。
  2. 创建以下方法:

    • 您感兴趣的每个标签的“read”方法。例如 readEntry()readTitle() 等等。解析器会从输入流中读取标签。当遇到名为 entrytitlelinksummary 的标签时,它会调用该标签的相应方法。否则,它会跳过该标签。
    • 为每个不同类型的标签提取数据并推动解析器解析下一个标签的方法。例如:
      • 对于 titlesummary 标签,解析器会调用 readText()。此方法通过调用 parser.getText() 提取这些标签的数据。
      • 对于 link 标签,解析器首先会确定链接是否为其感兴趣的类型,再提取该链接的数据。然后使用 parser.getAttributeValue() 提取该链接的值。
      • 对于 entry 标签,解析器会调用 readEntry()。此方法会解析条目的嵌套标签,并返回包含数据成员 titlelinksummaryEntry 对象。
    • 一种递归的辅助 skip() 方法。有关此主题的详细介绍,请参阅跳过您不感兴趣的标签

以下代码段展示了解析器如何解析条目、标题、链接和摘要。

Kotlin

data class Entry(val title: String?, val summary: String?, val link: String?)

// Parses the contents of an entry. If it encounters a title, summary, or link tag, hands them off
// to their respective "read" methods for processing. Otherwise, skips the tag.
@Throws(XmlPullParserException::class, IOException::class)
private fun readEntry(parser: XmlPullParser): Entry {
    parser.require(XmlPullParser.START_TAG, ns, "entry")
    var title: String? = null
    var summary: String? = null
    var link: String? = null
    while (parser.next() != XmlPullParser.END_TAG) {
        if (parser.eventType != XmlPullParser.START_TAG) {
            continue
        }
        when (parser.name) {
            "title" -> title = readTitle(parser)
            "summary" -> summary = readSummary(parser)
            "link" -> link = readLink(parser)
            else -> skip(parser)
        }
    }
    return Entry(title, summary, link)
}

// Processes title tags in the feed.
@Throws(IOException::class, XmlPullParserException::class)
private fun readTitle(parser: XmlPullParser): String {
    parser.require(XmlPullParser.START_TAG, ns, "title")
    val title = readText(parser)
    parser.require(XmlPullParser.END_TAG, ns, "title")
    return title
}

// Processes link tags in the feed.
@Throws(IOException::class, XmlPullParserException::class)
private fun readLink(parser: XmlPullParser): String {
    var link = ""
    parser.require(XmlPullParser.START_TAG, ns, "link")
    val tag = parser.name
    val relType = parser.getAttributeValue(null, "rel")
    if (tag == "link") {
        if (relType == "alternate") {
            link = parser.getAttributeValue(null, "href")
            parser.nextTag()
        }
    }
    parser.require(XmlPullParser.END_TAG, ns, "link")
    return link
}

// Processes summary tags in the feed.
@Throws(IOException::class, XmlPullParserException::class)
private fun readSummary(parser: XmlPullParser): String {
    parser.require(XmlPullParser.START_TAG, ns, "summary")
    val summary = readText(parser)
    parser.require(XmlPullParser.END_TAG, ns, "summary")
    return summary
}

// For the tags title and summary, extracts their text values.
@Throws(IOException::class, XmlPullParserException::class)
private fun readText(parser: XmlPullParser): String {
    var result = ""
    if (parser.next() == XmlPullParser.TEXT) {
        result = parser.text
        parser.nextTag()
    }
    return result
}
...

Java

public static class Entry {
    public final String title;
    public final String link;
    public final String summary;

    private Entry(String title, String summary, String link) {
        this.title = title;
        this.summary = summary;
        this.link = link;
    }
}

// Parses the contents of an entry. If it encounters a title, summary, or link tag, hands them off
// to their respective "read" methods for processing. Otherwise, skips the tag.
private Entry readEntry(XmlPullParser parser) throws XmlPullParserException, IOException {
    parser.require(XmlPullParser.START_TAG, ns, "entry");
    String title = null;
    String summary = null;
    String link = null;
    while (parser.next() != XmlPullParser.END_TAG) {
        if (parser.getEventType() != XmlPullParser.START_TAG) {
            continue;
        }
        String name = parser.getName();
        if (name.equals("title")) {
            title = readTitle(parser);
        } else if (name.equals("summary")) {
            summary = readSummary(parser);
        } else if (name.equals("link")) {
            link = readLink(parser);
        } else {
            skip(parser);
        }
    }
    return new Entry(title, summary, link);
}

// Processes title tags in the feed.
private String readTitle(XmlPullParser parser) throws IOException, XmlPullParserException {
    parser.require(XmlPullParser.START_TAG, ns, "title");
    String title = readText(parser);
    parser.require(XmlPullParser.END_TAG, ns, "title");
    return title;
}

// Processes link tags in the feed.
private String readLink(XmlPullParser parser) throws IOException, XmlPullParserException {
    String link = "";
    parser.require(XmlPullParser.START_TAG, ns, "link");
    String tag = parser.getName();
    String relType = parser.getAttributeValue(null, "rel");
    if (tag.equals("link")) {
        if (relType.equals("alternate")){
            link = parser.getAttributeValue(null, "href");
            parser.nextTag();
        }
    }
    parser.require(XmlPullParser.END_TAG, ns, "link");
    return link;
}

// Processes summary tags in the feed.
private String readSummary(XmlPullParser parser) throws IOException, XmlPullParserException {
    parser.require(XmlPullParser.START_TAG, ns, "summary");
    String summary = readText(parser);
    parser.require(XmlPullParser.END_TAG, ns, "summary");
    return summary;
}

// For the tags title and summary, extracts their text values.
private String readText(XmlPullParser parser) throws IOException, XmlPullParserException {
    String result = "";
    if (parser.next() == XmlPullParser.TEXT) {
        result = parser.getText();
        parser.nextTag();
    }
    return result;
}
  ...
}

跳过您不感兴趣的标签

上述 XML 解析步骤之一是解析器跳过它不感兴趣的标签。下面是解析器的 skip() 方法:

Kotlin

@Throws(XmlPullParserException::class, IOException::class)
private fun skip(parser: XmlPullParser) {
    if (parser.eventType != XmlPullParser.START_TAG) {
        throw IllegalStateException()
    }
    var depth = 1
    while (depth != 0) {
        when (parser.next()) {
            XmlPullParser.END_TAG -> depth--
            XmlPullParser.START_TAG -> depth++
        }
    }
}

Java

private void skip(XmlPullParser parser) throws XmlPullParserException, IOException {
    if (parser.getEventType() != XmlPullParser.START_TAG) {
        throw new IllegalStateException();
    }
    int depth = 1;
    while (depth != 0) {
        switch (parser.next()) {
        case XmlPullParser.END_TAG:
            depth--;
            break;
        case XmlPullParser.START_TAG:
            depth++;
            break;
        }
    }
 }

其工作原理如下:

  • 如果当前事件不是 START_TAG,则会抛出异常。
  • 它会使用 START_TAG 以及直到匹配的 END_TAG(含)的所有事件。
  • 为确保其在遇到正确的 END_TAG 时停止,而非在遇到原始 START_TAG 之后的首个标签时停止,它会不断追踪嵌套深度。

因此,如果当前元素具有嵌套元素,在解析器使用了原始 START_TAG 及其匹配的 END_TAG 之间的所有事件之前,depth 的值不会为 0。例如,看看解析器如何跳过拥有 <name><uri> 这 2 个嵌套元素的 <author> 元素:

  • 第一次经历 while 循环时,解析器在 <author> 之后遇到的下一个标签是 <name>START_TAGdepth 的值递增到 2。
  • 第二次经历 while 循环时,解析器遇到的下一个标签是 END_TAG </name>depth 的值递减为 1。
  • 第三次经历 while 循环时,解析器遇到的下一个标签是 START_TAG <uri>depth 的值递增到 2。
  • 第四次经历 while 循环时,解析器遇到的下一个标签是 END_TAG </uri>depth 的值递减为 1。
  • 第五次,也就是最后一次经历 while 循环时,解析器遇到的下一个标签是 END_TAG </author>depth 的值递减为 0,这表明该方法已成功跳过 <author> 元素。

使用 XML 数据

示例应用在 AsyncTask 内获取并解析 XML Feed。这样会使处理过程脱离主界面线程。处理完成后,应用会更新主 activity (NetworkActivity) 中的界面。

在下面的代码段中,loadPage() 方法会执行以下操作:

  • 使用 XML Feed 的网址初始化字符串变量。
  • 如果用户的设置和网络连接允许,则调用 new DownloadXmlTask().execute(url)。此操作将实例化新的 DownloadXmlTask 对象(AsyncTask 子类)并运行其 execute() 方法,该方法会下载和解析 Feed 并返回要在界面中显示的字符串结果。

Kotlin

class NetworkActivity : Activity() {

    companion object {

        const val WIFI = "Wi-Fi"
        const val ANY = "Any"
        const val SO_URL = "http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest"
        // Whether there is a Wi-Fi connection.
        private var wifiConnected = false
        // Whether there is a mobile connection.
        private var mobileConnected = false

        // Whether the display should be refreshed.
        var refreshDisplay = true
        // The user's current network preference setting.
        var sPref: String? = null
    }

    ...

    // Uses AsyncTask subclass to download the XML feed from stackoverflow.com.
    // Uses AsyncTask to download the XML feed from stackoverflow.com.
    fun loadPage() {

        if (sPref.equals(ANY) && (wifiConnected || mobileConnected)) {
            DownloadXmlTask().execute(SO_URL)
        } else if (sPref.equals(WIFI) && wifiConnected) {
            DownloadXmlTask().execute(SO_URL)
        } else {
            // show error
        }
    }

    ...
}

Java

public class NetworkActivity extends Activity {
    public static final String WIFI = "Wi-Fi";
    public static final String ANY = "Any";
    private static final String URL = "http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest";

    // Whether there is a Wi-Fi connection.
    private static boolean wifiConnected = false;
    // Whether there is a mobile connection.
    private static boolean mobileConnected = false;
    // Whether the display should be refreshed.
    public static boolean refreshDisplay = true;
    public static String sPref = null;

    ...

    // Uses AsyncTask to download the XML feed from stackoverflow.com.
    public void loadPage() {

        if((sPref.equals(ANY)) && (wifiConnected || mobileConnected)) {
            new DownloadXmlTask().execute(URL);
        }
        else if ((sPref.equals(WIFI)) && (wifiConnected)) {
            new DownloadXmlTask().execute(URL);
        } else {
            // show error
        }
    }

下面显示的 AsyncTask 子类 DownloadXmlTask 实现了以下 AsyncTask 方法:

  • doInBackground() 会执行方法 loadXmlFromNetwork()。它以参数形式传入 Feed 网址。方法 loadXmlFromNetwork() 获取并处理 Feed。完成后,它会传回一个结果字符串。
  • onPostExecute() 接受返回的字符串,并在界面中显示该字符串。

Kotlin

// Implementation of AsyncTask used to download XML feed from stackoverflow.com.
private inner class DownloadXmlTask : AsyncTask<String, Void, String>() {
    override fun doInBackground(vararg urls: String): String {
        return try {
            loadXmlFromNetwork(urls[0])
        } catch (e: IOException) {
            resources.getString(R.string.connection_error)
        } catch (e: XmlPullParserException) {
            resources.getString(R.string.xml_error)
        }
    }

    override fun onPostExecute(result: String) {
        setContentView(R.layout.main)
        // Displays the HTML string in the UI via a WebView
        findViewById<WebView>(R.id.webview)?.apply {
            loadData(result, "text/html", null)
        }
    }
}

Java

// Implementation of AsyncTask used to download XML feed from stackoverflow.com.
private class DownloadXmlTask extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... urls) {
        try {
            return loadXmlFromNetwork(urls[0]);
        } catch (IOException e) {
            return getResources().getString(R.string.connection_error);
        } catch (XmlPullParserException e) {
            return getResources().getString(R.string.xml_error);
        }
    }

    @Override
    protected void onPostExecute(String result) {
        setContentView(R.layout.main);
        // Displays the HTML string in the UI via a WebView
        WebView myWebView = (WebView) findViewById(R.id.webview);
        myWebView.loadData(result, "text/html", null);
    }
}

以下是从 DownloadXmlTask 调用的方法 loadXmlFromNetwork()。它会执行以下操作:

  1. 实例化 StackOverflowXmlParser。此外,它还为 List 形式的 Entry 对象(entries,包括 titleurlsummary)创建变量,以保存从 XML Feed 中提取的这些字段的值。
  2. 调用 downloadUrl(),它会获取 Feed 并将其作为 InputStream 返回。
  3. 使用 StackOverflowXmlParser 解析 InputStreamStackOverflowXmlParser 用 Feed 中的数据填充 entries List
  4. 处理 entries List,并将 Feed 数据与 HTML 标记相结合。
  5. 返回一个 HTML 字符串,AsyncTaskonPostExecute() 方法会将其显示在主 activity 界面中。

Kotlin

// Uploads XML from stackoverflow.com, parses it, and combines it with
// HTML markup. Returns HTML string.
@Throws(XmlPullParserException::class, IOException::class)
private fun loadXmlFromNetwork(urlString: String): String {
    // Checks whether the user set the preference to include summary text
    val pref: Boolean = PreferenceManager.getDefaultSharedPreferences(this)?.run {
        getBoolean("summaryPref", false)
    } ?: false

    val entries: List<Entry> = downloadUrl(urlString)?.use { stream ->
        // Instantiate the parser
        StackOverflowXmlParser().parse(stream)
    } ?: emptyList()

    return StringBuilder().apply {
        append("<h3>${resources.getString(R.string.page_title)}</h3>")
        append("<em>${resources.getString(R.string.updated)} ")
        append("${formatter.format(rightNow.time)}</em>")
        // StackOverflowXmlParser returns a List (called "entries") of Entry objects.
        // Each Entry object represents a single post in the XML feed.
        // This section processes the entries list to combine each entry with HTML markup.
        // Each entry is displayed in the UI as a link that optionally includes
        // a text summary.
        entries.forEach { entry ->
            append("<p><a href='")
            append(entry.link)
            append("'>" + entry.title + "</a></p>")
            // If the user set the preference to include summary text,
            // adds it to the display.
            if (pref) {
                append(entry.summary)
            }
        }
    }.toString()
}

// Given a string representation of a URL, sets up a connection and gets
// an input stream.
@Throws(IOException::class)
private fun downloadUrl(urlString: String): InputStream? {
    val url = URL(urlString)
    return (url.openConnection() as? HttpURLConnection)?.run {
        readTimeout = 10000
        connectTimeout = 15000
        requestMethod = "GET"
        doInput = true
        // Starts the query
        connect()
        inputStream
    }
}

Java

// Uploads XML from stackoverflow.com, parses it, and combines it with
// HTML markup. Returns HTML string.
private String loadXmlFromNetwork(String urlString) throws XmlPullParserException, IOException {
    InputStream stream = null;
    // Instantiate the parser
    StackOverflowXmlParser stackOverflowXmlParser = new StackOverflowXmlParser();
    List<Entry> entries = null;
    String title = null;
    String url = null;
    String summary = null;
    Calendar rightNow = Calendar.getInstance();
    DateFormat formatter = new SimpleDateFormat("MMM dd h:mmaa");

    // Checks whether the user set the preference to include summary text
    SharedPreferences sharedPrefs = PreferenceManager.getDefaultSharedPreferences(this);
    boolean pref = sharedPrefs.getBoolean("summaryPref", false);

    StringBuilder htmlString = new StringBuilder();
    htmlString.append("<h3>" + getResources().getString(R.string.page_title) + "</h3>");
    htmlString.append("<em>" + getResources().getString(R.string.updated) + " " +
            formatter.format(rightNow.getTime()) + "</em>");

    try {
        stream = downloadUrl(urlString);
        entries = stackOverflowXmlParser.parse(stream);
    // Makes sure that the InputStream is closed after the app is
    // finished using it.
    } finally {
        if (stream != null) {
            stream.close();
        }
     }

    // StackOverflowXmlParser returns a List (called "entries") of Entry objects.
    // Each Entry object represents a single post in the XML feed.
    // This section processes the entries list to combine each entry with HTML markup.
    // Each entry is displayed in the UI as a link that optionally includes
    // a text summary.
    for (Entry entry : entries) {
        htmlString.append("<p><a href='");
        htmlString.append(entry.link);
        htmlString.append("'>" + entry.title + "</a></p>");
        // If the user set the preference to include summary text,
        // adds it to the display.
        if (pref) {
            htmlString.append(entry.summary);
        }
    }
    return htmlString.toString();
}

// Given a string representation of a URL, sets up a connection and gets
// an input stream.
private InputStream downloadUrl(String urlString) throws IOException {
    URL url = new URL(urlString);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setReadTimeout(10000 /* milliseconds */);
    conn.setConnectTimeout(15000 /* milliseconds */);
    conn.setRequestMethod("GET");
    conn.setDoInput(true);
    // Starts the query
    conn.connect();
    return conn.getInputStream();
}