java xml SAX 解析注意 什么时候可以把解析值赋给对象一般从网上看到的sax解析，都是在Handler中的characters方法进行对象数据的赋值。示例代码如下：
PRivate TransportFile parseXML(String xml) {SAXParserFactory saxfac = SAXParserFactory.newInstance();try { SAXParser saxparser = saxfac.newSAXParser(); InputStream is = new ByteArrayInputStream(xml.getBytes()); MySAXHandler handler = new MySAXHandler(); saxparser.parse(is, handler); return handler.getData();} catch (ParserConfigurationException e) { e.printStackTrace();} catch (SAXException e) { e.printStackTrace();} catch (FileNotFoundException e) { e.printStackTrace();} catch (IOException e) { e.printStackTrace();}return null;}private class MySAXHandler extends DefaultHandler{String currentTagName = "";TransportFile mData = null ;@Overridepublic void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException { currentTagName = qName ; if("file".equals(qName)){ mData = new TransportFile(); }}@Overridepublic void characters(char[] ch, int start, int length) throws SAXException { String str = new String(ch,start,length); if("guid".equals(currentTagName)){ mData.guid = str; }else if("name".equals(currentTagName)){ mData.name = str; }else if("type".equals(currentTagName)){ mData.type = str; }else if("length".equals(currentTagName)){ mData.length = Long.parseLong(str); }else if("index".equals(currentTagName)){ mData.index = Integer.parseInt(str); }else if("count".equals(currentTagName)){ mData.count = Integer.parseInt(str); }else if("data".equals(currentTagName)){ mData.data = Base64.decode(str); }}@Overridepublic void endElement(String uri, String localName, String qName) throws SAXException { currentTagName = "";}public TransportFile getData(){ return mData ;}}
普通的使用场景中上述代码没有问题，但是当xml中某一标签的内容很长时，就会引发上述代码的bug。实践发现sax解析每次也就解析1k左右的数据，超出部分其实是要分段多次解析的。所以问题来了，如果在characters方法中解析，那么其他几段的数据仍然会不断覆盖最终返回对象中的数据，导致数据丢失。因此，对于赋值给最终传回对象的数据，在characters阶段只能不断拼接，解析必须在endElement时才可以完成。否则当数据内容比较大的时候，网上代码中的bug就会凸显出来。顺便贴上我的代码：
private class MySAXHandler extends DefaultHandler{ String currentTagName = ""; TransportFile mData = null ; @Override public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException { currentTagName = qName ; mStringBuilder = new StringBuilder(); if("file".equals(qName)){ mData = new TransportFile(); } } private StringBuilder mStringBuilder; @Override public void characters(char[] ch, int start, int length) throws SAXException { mStringBuilder.append(ch, start, length); } @Override public void endElement(String uri, String localName, String qName) throws SAXException { String str = mStringBuilder.toString(); if("guid".equals(currentTagName)){ mData.guid = str; }else if("name".equals(currentTagName)){ mData.name = str; }else if("type".equals(currentTagName)){ mData.type = str; }else if("length".equals(currentTagName)){ mData.length = Long.parseLong(str); }else if("index".equals(currentTagName)){ mData.index = Integer.parseInt(str); }else if("count".equals(currentTagName)){ mData.count = Integer.parseInt(str); }else if("data".equals(currentTagName)){ mData.data = Base64.decode(str); } currentTagName = ""; } public TransportFile getData(){ return mData ; } }
characters方法参数注意ch是当前解析到的字符数组，并不是精确的标签内的内容。下面是解析第一个标签时characters中 ch 、 start、length输出：
===========characters ch: <?xml version='1.0' encoding='utf-8' standalone='yes' ?><file><guid>678c6f92-d617-40af-bb87-a80c3b2be91f</guid><name>0CAQLTZGO.jpg</name><type>image</type><length>71374</length><index>0</index><count>1</count><data>/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAYEBQYFBAYGBQYHBwYIChAKCgkJChQODwwQFxQYGBcUFhYaHSUfGhsjHBYWICwgIyYnKSopGR8tMC0oMCUoKSj/2wBDAQcHBwoIChMKChMoGhYaKCgoKCgoK.....===========characters start:31===========characters length:36
真正当前需要的数据是ch数组从start开始的length个字符。