Extensible Markup Language (XML) is a set of rules for encoding documents in machine-readable form. XML is a popular format for sharing data on the internet. Websites that frequently update their content, such as news sites or blogs, often provide an XML feed so that external programs can keep abreast of content changes. Uploading and parsing XML data is a common task for network-connected apps. This lesson explains how to parse XML documents and use their data.
Choose a Parser
We recommend XmlPullParser, which is an efficient and maintainable way to parse XML on Android. Historically Android has had two implementations of this interface:
-
KXmlParser via
XmlPullParserFactory.newPullParser(). -
ExpatPullParser, via Xml.newPullParser().
Either choice is fine. The example in this section uses ExpatPullParser, via Xml.newPullParser().
Analyze the Feed
The first step in parsing a feed is to decide which fields you're interested in. The parser extracts data for those fields and ignores the rest.
Here is an excerpt from the feed that's being parsed in the sample app. Each post to StackOverflow.com appears in the feed as an entry tag that contains several nested tags:
The sample app extracts data for the entry tag and its nested tags title, link, and summary.
Instantiate the Parser
The next step is to instantiate a parser and kick off the parsing process. In this snippet, a parser is initialized to not process namespaces, and to use the provided InputStream as its input. It starts the parsing process with a call to nextTag() and invokes the readFeed() method, which extracts and processes the data the app is interested in:
Read the Feed
The readFeed() method does the actual work of processing the feed. It looks for elements tagged "entry" as a starting point for recursively processing the feed. If a tag isn't an entry tag, it skips it. Once the whole feed has been recursively processed, readFeed() returns a List containing the entries (including nested data members) it extracted from the feed. This List is then returned by the parser.
Parse XML
The steps for parsing an XML feed are as follows:
- As described in Analyze the Feed, identify the tags you want to include in your app. This example extracts data for the
entry tag and its nested tags title, link, and summary. - Create the following methods:
- A "read" method for each tag you're interested in. For example,
readEntry(), readTitle(), and so on. The parser reads tags from the input stream. When it encounters a tag named entry, title, link or summary, it calls the appropriate method for that tag. Otherwise, it skips the tag. - Methods to extract data for each different type of tag and to advance the parser to the next tag. For example:
- For the
title and summary tags, the parser calls readText(). This method extracts data for these tags by calling parser.getText(). - For the
link tag, the parser extracts data for links by first determining if the link is the kind it's interested in. Then it uses parser.getAttributeValue() to extract the link's value. - For the
entry tag, the parser calls readEntry(). This method parses the entry's nested tags and returns an Entry object with the data members title, link, and summary.
- A helper
skip() method that's recursive. For more discussion of this topic, see Skip Tags You Don't Care About.
This snippet shows how the parser parses entries, titles, links, and 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, 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; } ... }
Skip Tags You Don't Care About
One of the steps in the XML parsing described above is for the parser to skip tags it's not interested in. Here is the parser's skip() method:
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; } } }
This is how it works:
- It throws an exception if the current event isn't a
START_TAG. - It consumes the
START_TAG, and all events up to and including the matching END_TAG. - To make sure that it stops at the correct
END_TAG and not at the first tag it encounters after the original START_TAG, it keeps track of the nesting depth.
Thus if the current element has nested elements, the value of depth won't be 0 until the parser has consumed all events between the original START_TAG and its matching END_TAG. For example, consider how the parser skips the <author> element, which has 2 nested elements, <name> and <uri>:
- The first time through the
while loop, the next tag the parser encounters after <author> is the START_TAG for <name>. The value for depth is incremented to 2. - The second time through the
while loop, the next tag the parser encounters is the END_TAG </name>. The value for depth is decremented to 1. - The third time through the
while loop, the next tag the parser encounters is the START_TAG <uri>. The value for depth is incremented to 2. - The fourth time through the
while loop, the next tag the parser encounters is the END_TAG </uri>. The value for depth is decremented to 1. - The fifth time and final time through the
while loop, the next tag the parser encounters is the END_TAG </author>. The value for depth is decremented to 0, indicating that the <author> element has been successfully skipped.
Consume XML Data
The example application fetches and parses the XML feed within an AsyncTask. This takes the processing off the main UI thread. When processing is complete, the app updates the UI in the main activity (NetworkActivity).
In the excerpt shown below, the loadPage() method does the following:
- Initializes a string variable with the URL for the XML feed.
- If the user's settings and the network connection allow it, invokes
new DownloadXmlTask().execute(url). This instantiates a new DownloadXmlTask object (AsyncTask subclass) and runs its execute() method, which downloads and parses the feed and returns a string result to be displayed in the 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 } }
The AsyncTask subclass shown below, DownloadXmlTask, implements the following AsyncTask methods:
-
doInBackground() executes the method loadXmlFromNetwork(). It passes the feed URL as a parameter. The method loadXmlFromNetwork() fetches and processes the feed. When it finishes, it passes back a result string. -
onPostExecute() takes the returned string and displays it in the 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); } }
Below is the method loadXmlFromNetwork() that is invoked from DownloadXmlTask. It does the following:
- Instantiates a
StackOverflowXmlParser. It also creates variables for a List of Entry objects (entries), and title, url, and summary, to hold the values extracted from the XML feed for those fields. - Calls
downloadUrl(), which fetches the feed and returns it as an InputStream. - Uses
StackOverflowXmlParser to parse the InputStream. StackOverflowXmlParser populates a List of entries with data from the feed. - Processes the
entries List, and combines the feed data with HTML markup. - Returns an HTML string that is displayed in the main activity UI by the
AsyncTask method onPostExecute().
