Java XML SAX 解析注意

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 ;        }    } 

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。