剖析 XML 資料

可擴充標記語言 (XML) 是一套編碼規則,用於建立機器可讀取的文件格式,它也是在網際網路上分享資料時常用的一種格式。

如果網站經常更新內容 (例如新聞網站或網誌),通常就會提供 XML 動態饋給,如此一來,外部程式就能掌握內容變更。上傳及剖析 XML 資料是網路連線應用程式的常見工作。本主題將說明如何剖析 XML 文件並使用其資料。

如要進一步瞭解如何在 Android 應用程式中建立網頁內容,請參閱「網頁內容」。

選擇剖析器

建議您使用 XmlPullParser,這是一種在 Android 上有效率且可維護的 XML 剖析方法。Android 有兩種實作此介面的方法:

兩者皆可使用。本節中的範例使用 Xml.newPullParser()ExpatPullParser

分析動態饋給

剖析動態饋給的第一步,要決定對哪些欄位感興趣。剖析器會擷取這些欄位的資料,並忽略其餘的部分。

請查看以下範例應用程式中的剖析動態饋給摘錄。每篇張貼至 StackOverflow.com 的貼文都會以 entry 標記的形式顯示在動態饋給中,其中包含多個巢狀標記:

<?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>

範例應用程式會擷取 entry 標記及其巢狀標記 titlelinksummary 的資料。

將剖析器例項化

剖析動態饋給的下一步,是將剖析器執行個體化並啟動剖析程序。這個程式碼片段會將剖析器初始化,使其不處理命名空間,並使用已提供的 InputStream 做為輸入內容。系統會透過 nextTag() 呼叫啟動剖析程序,並叫用 readFeed() 方法,以擷取並處理應用程式感興趣的資料:

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();
        }
    }
 ...
}

讀取動態饋給

實際負責處理動態饋給資料的是 readFeed() 方法。這個方法會找出標記為「entry」的元素,做為遞迴處理動態饋給的起點。如果不是 entry 標記,則可略過。整個動態饋給都以週期性方式處理後,readFeed() 會傳回 List,其中包含從動態饋給中擷取的項目 (包括巢狀資料成員)。接著,剖析器會傳回這個 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;
}

剖析 XML

剖析 XML 動態饋給的步驟如下:

  1. 按照「分析動態饋給」一文的說明,找出您要納入應用程式的標記。這個範例會擷取 entry 標記及其巢狀標記 titlelinksummary 的資料。
  2. 建立以下方法:

    • 您要納入的每個標記都有一種「讀取」方法,例如 readEntry()readTitle()。剖析器會讀取輸入資料流中的標記。當遇到本範例中名為 entrytitlelinksummary 的標記時,它會為該標記呼叫適當方法,其他的標記則會略過。
    • 為其他類型的各標記擷取資料的方法,以及讓剖析器對下一個標記進行剖析的方法。在這個例子中,相關方法如下:
      • 剖析器會針對 titlesummary 標記呼叫 readText()。這個方法會呼叫 parser.getText() 來擷取這些標記的資料。
      • 如果是 link 標記,剖析器會先確定連結是否為其有興趣的類型,再擷取該連結的資料。接著,剖析器會使用 parser.getAttributeValue() 擷取連結的值。
      • 針對 entry 標記,剖析器會呼叫 readEntry()。這個方法會剖析該項目的巢狀標記,並傳回包含資料成員 titlelinksummaryEntry 物件。
    • 遞迴的 skip() 輔助方法。如要進一步瞭解這個主題,請參閱「跳過您不需要的標記」。

這段程式碼顯示剖析器如何剖析項目、標題、連結和摘要。

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;
}
  ...
}

跳過您不需要的標記

剖析器必須略過不感興趣的標記。以下是剖析器的 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;
        }
    }
 }

運作方式如下:

  • 如果目前事件不是 START_TAG,就會擲回例外狀況。
  • 這個事件會使用 START_TAG,以及直到比對至 END_TAG 的所有事件。
  • 它會追蹤巢狀深度,確保其在正確的 END_TAG 時停止,而不是在遇到原始 START_TAG 之後偵測到的第一個標記時停止。

因此,如果目前元素具有巢狀元素,則直到剖析器使用在原始 START_TAG 和其相符 END_TAG 之間的所有事件前,depth 的值不為 0,。例如,請思考剖析器如何略過具有 2 個巢狀元素 <name><uri><author> 元素:

  • 第一次通過 while 迴圈時,剖析器在 <author> 後遇到的下一個標記是 <name>START_TAGdepth 的值會遞增至 2。
  • 第二次通過 while 迴圈時,剖析器遇到的下一個標記是 END_TAG </name>depth 的值會減少至 1。
  • 第三次通過 while 迴圈時,剖析器遇到的下一個標記是 START_TAG <uri>depth 的值會遞增至 2。
  • 第四次通過 while 迴圈時,剖析器遇到的下一個標記是 END_TAG </uri>depth 的值會減少至 1。
  • 第五次 (最後一次) 通過 while 迴圈,剖析器遇到的下一個標記是 END_TAG </author>depth 的值會減少至 0,表示已成功略過 <author> 元素。

使用 XML 資料

範例應用程式會以非同步方式擷取和剖析 XML 資訊動態饋給。這麼做就不會在主 UI 執行緒上進行處理程序。處理程序完成後,應用程式會在自己的主要活動中更新 UI (NetworkActivity)。

在以下摘錄的程式碼中,loadPage() 方法會執行以下操作:

  • 使用 XML 動態饋給的網址來初始化字串變數。
  • 如果使用者的設定和網路連線允許,請叫用 downloadXml(url) 方法。這個方法會下載並剖析動態饋給,並傳回要在使用者介面中顯示的結果。

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 方法會在 Kotlin 中呼叫下列方法:

  • lifecycleScope.launch(Dispatchers.IO) 會使用 Kotlin 協同程式在 IO 執行緒上啟動 loadXmlFromNetwork() 方法,並將動態饋給網址視為參數傳遞。loadXmlFromNetwork() 方法會擷取及處理動態饋給。作業完畢後,它會傳回結果字串。
  • withContext(Dispatchers.Main) 會使用 Kotlin 協同程式返回主執行緒,接收傳回的字串並將其顯示在使用者介面中。

在 Java 程式設計語言中,處理程序如下:

  • Executor 會在背景執行緒上執行 loadXmlFromNetwork() 方法,並將動態饋給網址視為參數傳遞。loadXmlFromNetwork() 方法會擷取及處理動態饋給。作業完畢後,它會傳回結果字串。
  • Handler 會呼叫 post 以返回主執行緒,接收傳回的字串並將其顯示在使用者介面中。

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);
        });
    });
}

下一個程式碼片段顯示了從 downloadXml 叫用的 loadXmlFromNetwork() 方法。這個檔案可執行以下作業:

  1. StackOverflowXmlParser 例項化。它也會為 Entry 物件 (entries) 的 Listtitleurlsummary 建立變數,並為這些欄位保留從 XML 動態饋給擷取的值。
  2. 呼叫 downloadUrl() 以擷取動態饋給並將其做為 InputStream 傳回。
  3. 使用 StackOverflowXmlParser 剖析 InputStreamStackOverflowXmlParser 會將來自動態饋給的資料填入 entriesList
  4. 處理 entries List,並將動態饋給資料和 HTML 標記合併。
  5. 傳回在主要活動 UI 中顯示的 HTML 字串。

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();
}