Watch out for XmlPullParser.nextText()

来源：互联网发布：apache ab.exe打不开编辑：程序博客网时间：2024/05/22 12:47

转载自：http://android-developers.blogspot.com/2011/12/watch-out-for-xmlpullparsernexttext.html

Watch out for XmlPullParser.nextText()

[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:

KXmlParser, via XmlPullParserFactory.newPullParser().
ExpatPullParser, via Xml.newPullParser().

The implementation from Xml.newPullParser() had a bug where calls to nextText()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 to next() 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 the nextTag() 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 an END_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 uses nextText() 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.

0 0