[This post is by Jesse Wilson from the Dalvik team. —Tim Bray]
Using XmlPullParser is an efficient and maintainable way to parse XML on Android. Historically Android has had two implementations of this interface:
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KXmlParser, via XmlPullParserFactory.newPullParser().
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ExpatPullParser, via Xml.newPullParser().
The implementation from Xml.newPullParser() had a bug where calls tonextText() didn’t always advance to the END_TAG as the documentation promised it would. As a consequence, some apps may be working around the bug with extra calls tonext() or nextTag():
public void parseXml(Reader reader) throws XmlPullParserException, IOException { XmlPullParser parser = Xml.newPullParser(); parser.setInput(reader); parser.nextTag(); parser.require(XmlPullParser.START_TAG, null, "menu"); while (parser.nextTag() == XmlPullParser.START_TAG) { parser.require(XmlPullParser.START_TAG, null, "item"); String itemText = parser.nextText(); parser.nextTag(); // this call shouldn’t be necessary! parser.require(XmlPullParser.END_TAG, null, "item"); System.out.println("menu option: " + itemText); } parser.require(XmlPullParser.END_TAG, null, "menu"); } public static void main(String[] args) throws Exception { new Menu().parseXml(new StringReader("<?xml version='1.0'?>" + "<menu>" + " <item>Waffles</item>" + " <item>Coffee</item>" + "</menu>")); }In Ice Cream Sandwich we changed Xml.newPullParser() to return a KxmlParser and deleted our ExpatPullParser class. This fixes thenextTag() bug. Unfortunately, apps that currently work around the bug may crash under Ice Cream Sandwich:
org.xmlpull.v1.XmlPullParserException: expected: END_TAG {null}item (position:START_TAG <item>@1:37 in java.io.StringReader@40442fa8) at org.kxml2.io.KXmlParser.require(KXmlParser.java:2046) at com.publicobject.waffles.Menu.parseXml(Menu.java:25) at com.publicobject.waffles.Menu.main(Menu.java:32)The fix is to call nextTag() after a call to nextText() only if the current position is not anEND_TAG:
while (parser.nextTag() == XmlPullParser.START_TAG) { parser.require(XmlPullParser.START_TAG, null, "item"); String itemText = parser.nextText(); if (parser.getEventType() != XmlPullParser.END_TAG) { parser.nextTag(); } parser.require(XmlPullParser.END_TAG, null, "item"); System.out.println("menu option: " + itemText); }The code above will parse XML correctly on all releases. If your application usesnextText() extensively, use this helper method in place of calls to nextText():
private String safeNextText(XmlPullParser parser) throws XmlPullParserException, IOException { String result = parser.nextText(); if (parser.getEventType() != XmlPullParser.END_TAG) { parser.nextTag(); } return result; }Moving to a single XmlPullParser simplifies maintenance and allows us to spend more energy on improving system performance.