Android Training - 网络操作(3) - 解析XML数据
可扩展标记语言(XML)是使用机器可读形式按照一套规则编码的的文档。XML是互联网分享数据的通用格式。网站提供一个XML种子来让外部程序同步更新网站内容。上传和解析XML是网络应用程序常做的工作。下面就学习怎么解析并使用XML文件。
选择一个解析器我们推荐XmlPullParser,它是Android上高效,并且可维护的解析XML的方法。以前Android有两个实现XmlPullParser接口的方法:通过XmlPullParserFactory.newPullParser()取得KXmlParser。通过Xml.newPullParser()取得ExpatPullParcer。两个选择都很好,下面的例子使用第二种。
分析Feed解析feed的第一步就是决定你感兴趣的字段,解析器可以挑选你需要的数据,忽略不需要的。
下面是从某个网站载录下来的部分feed文件:
<?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>我们提取entry标记中的内嵌标记title,link和summary的数据。
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FeedreadFeed()方法执行实际的处理feed的工作。它查找entry元素作为起点,然后递归地处理feed。如果不的entry标记,就跳过。整个feed被递归处理完后,readFeed()返回一个list,包含了整个提取后的数据:
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下面是解析的步骤:
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;} ...}跳过你不关心的Tag下面是skip()方法的实例:
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; } } }它是怎么工作的呢:如果当前事件不是一个START_TAG就抛出一个异常。有START_TAG就一定有对应的END_TAG。确保在正确的END_TAG停止,使用depth来跟踪嵌套深度。如果当前元素含有内嵌元素,depth的值会到匹配的END_TAG才会为0,例如,考虑一下解析器是怎么跳过<author>元素的,它有两个元素,<name>和<uri>。第一次while循环时,<author>的下一个START_TAG是<name>,那么depth会增加到2.第二次循环,解析器遇见的tag就是END_TAG</name>,那么depth变成1.第三次循环,解析器遇到START_TAG<uri>,值又增加到2.第四次循环,解析器遇到END_TAG</uri>,值又变成1.在第五次循环时,遇到了END_TAG</author>。值就减少到0,那么<author>元素被成功跳过。使用XML数据下面的实例使用AsyncTask获取并解析XML feed。它脱离主线程执行,当处理完成后更新UI。
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 } }AsyncTask子类DownloadXmlTask的实现:doInBackground()执行loadXmlFromNetWork(),传递URL参数获取feed和处理feed,完成时返回一个字符串。onPostExecute()使用返回的字符串返回到UI。
// 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); }}在loadXmlFromNetwork()方法中的DownloadXmlTask做下面的事情:
// 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(); InputStream stream = conn.getInputStream(); }