解析XML数据
发布时间：2020-12-16 05:52:43 所属栏目：百科 来源：网络整理
导读：可扩展标记语言(XML)是一个以机器可读的形式编码文档的规则的集合。XML是一种流行的 在网络上分享数据的 格式。频繁地更新他们的内容的 网站 ，比如新闻站点或博客，经常提供一个XML feed，以使外部的程序能够了解内容变化的最新情况。对于联网apps来说，上
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可扩展标记语言(XML)是一个以机器可读的形式编码文档的规则的集合。XML是一种流行的在网络上分享数据的格式。频繁地更新他们的内容的网站，比如新闻站点或博客，经常提供一个XML feed，以使外部的程序能够了解内容变化的最新情况。对于联网apps来说，上传和解析XML数据是一种常见的任务。这一节将解释如何解析XML文档并使用它们的数据。
选择一个Parser我们建议使用XmlPullParser，它是Android上解析XML的一种高效的和可维护的方式。在历史上，Android有这个接口的两种实现：
两种中的任何一种选择都挺好。这一节的例子使用了ExpatPullParser，通过Xml.newPullParser()。 分析Feed解析一个feed的第一步是确定你对哪些fields感兴趣。Parser提取那些fields的数据，并忽略其余的。 这里是一个在示例app中解析的feed的摘录。在feed中，到StackOverflow.com的每一个post以一个entry标签的形式出现，其包含了一些嵌套的标签：
<?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app为entry标签提取数据，及它的嵌套标签的标题，链接，和总结。
实例化Parser下一步是实例化一个parser，并启动解析过程。在这个代码片段中，一个parser被初始化为不处理命名空间，并使用提供的InputStream作为它的输入。它通过一个对nextTag()的调用来启动解析过程，并调用readFeed()方法，后者会提取并处理app感兴趣的数据：
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。它查找标记为“entry”的元素作为一个递归地feed处理的起点。如果一个标签不是一个entry标签，它就跳过它。一旦整个feed都被递归地处理完了，则readFeed()返回一个List，其中包含了它从feed中提取entries(包含嵌套的数据成员)。然后这个List被parser返回。
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解析一个XML feed的步骤如下：
这个代码片段显示了parser如何解析entries，titles，links和summaries。
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,"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,link);
}
// Processes title tags in the feed.
private String readTitle(XmlPullParser parser) throws IOException,XmlPullParserException {
parser.require(XmlPullParser.START_TAG,"title");
String title = readText(parser);
parser.require(XmlPullParser.END_TAG,"title");
return title;
}
// Processes link tags in the feed.
private String readLink(XmlPullParser parser) throws IOException,XmlPullParserException {
String link = "";
parser.require(XmlPullParser.START_TAG,"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,"link");
return link;
}
// Processes summary tags in the feed.
private String readSummary(XmlPullParser parser) throws IOException,"summary");
String summary = readText(parser);
parser.require(XmlPullParser.END_TAG,"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;
}
...
}
跳过你不关心的标签上面描述的XML解析的其中一步是paser跳过它不感兴趣的标签。这里是parser的skip()方法：
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;
}
}
}
这是它如何工作的：
这样的话，如果当前的元素具有嵌套的元素，则depth的值将一直为非0，直到parser消费掉了所有在最初的START_TAG和与其匹配的END_TAG之间的所有的events。比如，考虑parser如何跳过<author>元素，它具有2个嵌套的元素，<name>和<uri>：
消费XML数据例子应用程序在一个AsyncTask中获取并解析XML feed。这将把处理过程从主UI线程中分离。当处理过程完成时，app更新main activity (NetworkActivity)中的UI。 在下面所示的摘录中，loadPage()方法做了如下的事情：
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
}
}
下面所示的AsyncTask子类，DownloadXmlTask，实现了如下的AsyncTask方法：
// 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);
}
}
下面是方法loadXmlFromNetwork()，它会在DownloadXmlTask中调用。它做了如下的事情：
// 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();
return conn.getInputStream();
}
译自：
http://developer.android.com/training/basics/network-ops/xml.html
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