解析 XML 数据

可扩展标记语言 (XML) 是一组以机器可读形式对文档进行编码时需遵从的规则。它是一组在互联网上共享数据的普遍格式。

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。本主题将介绍如何解析 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,请参阅基于网络的内容

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。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 ExpatPullParserXml.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。

下面是从示例应用中已解析的 Feed 摘引的代码段。每个发布到 StackOverflow.com 的帖子都会以包含多个嵌套标记的 entry 标记的形式显示在 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

解析 Feed 的下一步是实例化解析器并启动解析过程。此代码段会初始化一个解析器,使其不处理命名空间并将提供的 InputStream 用作输入。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:

    • 您要包含的每个标签的“read”方法,例如 readEntry()readTitle()。解析器会从输入流中读取标签。当遇到此示例中名为 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。在此示例中,相关方法如下所示:
      • 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

解析器需要跳过不感兴趣的标签。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

示例应用会异步提取并解析 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 downloadXml(url) 方法。此方法会下载并解析 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
    }
    ...
    // Asynchronously downloads the XML feed from stackoverflow.com.
    fun loadPage() {

        if (sPref.equals(ANY) && (wifiConnected || mobileConnected)) {
            downloadXml(SO_URL)
        } else if (sPref.equals(WIFI) && wifiConnected) {
            downloadXml(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;
    ...
    // Asynchronously downloads the XML feed from stackoverflow.com.
    public void loadPage() {

        if((sPref.equals(ANY)) && (wifiConnected || mobileConnected)) {
            downloadXml(URL);
        }
        else if ((sPref.equals(WIFI)) && (wifiConnected)) {
            downloadXml(URL);
        } else {
            // Show error.
        }
    }

downloadXml 方法在 Kotlin 中调用以下方法:

  • lifecycleScope.launch(Dispatchers.IO),它使用 Kotlin 协程在 IO 线程上启动 loadXmlFromNetwork() 方法。它以参数形式传入 Feed 网址。loadXmlFromNetwork() 方法获取并处理 Feed。完成后,它会传回一个结果字符串。
  • withContext(Dispatchers.Main),它使用 Kotlin 协程返回到主线程,接受返回的字符串,并在界面中显示该字符串。

在 Java 编程语言中,该过程如下:

  • Executor 在后台线程上执行 loadXmlFromNetwork() 方法。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。
  • Handler 会调用 post 以返回到主线程,接受返回的字符串,并在界面中显示该字符串。

Kotlin

// Implementation of Kotlin coroutines used to download XML feed from stackoverflow.com.
private fun downloadXml(vararg urls: String) {
    var result: String? = null
    lifecycleScope.launch(Dispatchers.IO) {
        result = try {
            loadXmlFromNetwork(urls[0])
        } catch (e: IOException) {
            resources.getString(R.string.connection_error)
        } catch (e: XmlPullParserException) {
            resources.getString(R.string.xml_error)
        }
        withContext(Dispatchers.Main) {
            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 Executor and Handler used to download XML feed asynchronously from stackoverflow.com.
private void downloadXml(String... urls) {
    ExecutorService executor = Executors.newSingleThreadExecutor();
    Handler handler = new Handler(Looper.getMainLooper());
    executor.execute(() -> {
        String result;
            try {
                result = loadXmlFromNetwork(urls[0]);
            } catch (IOException e) {
                result = getResources().getString(R.string.connection_error);
            } catch (XmlPullParserException e) {
                result = getResources().getString(R.string.xml_error);
            }
        String finalResult = result;
        handler.post(() -> {
            setContentView(R.layout.main);
            // Displays the HTML string in the UI via a WebView.
            WebView myWebView = (WebView) findViewById(R.id.webview);
            myWebView.loadData(finalResult, "text/html", null);
        });
    });
}

下一个代码段中显示了从 downloadXml 调用的 loadXmlFromNetwork() 方法。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. 返回在主 activity 界面中显示的 HTML 字符串。

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 ->
        // Instantiates 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;
    // Instantiates 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();
}