org.dom4j.DocumentException: Error on line 1 of document: 前言中不允许有内容

下面是大致的异常栈:

org.dom4j.DocumentException: Error on line 1 of document  : 前言中不允许有内容。 Nested exception: 前言中不允许有内容。
	at org.dom4j.io.SAXReader.read(SAXReader.java:482)
	at org.dom4j.DocumentHelper.parseText(DocumentHelper.java:278)
	at com.apobates.parser.RssParser.build(RssParser.java:38)
	at com.apobates.machine.reader.Reader.mainParser(Reader.java:57)
	at com.apobates.machine.reader.Reader.load(Reader.java:37)
	at com.apobates.test.ParserEntityTest.main(ParserEntityTest.java:41)
Nested exception: 
org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; 前言中不允许有内容。
	at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
	at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLScanner.reportFatalError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
	at org.dom4j.io.SAXReader.read(SAXReader.java:465)
	at org.dom4j.DocumentHelper.parseText(DocumentHelper.java:278)
	at com.apobates.parser.RssParser.build(RssParser.java:38)
	at com.apobates.machine.reader.Reader.mainParser(Reader.java:57)
	at com.apobates.machine.reader.Reader.load(Reader.java:37)
	at com.apobates.test.ParserEntityTest.main(ParserEntityTest.java:41)
Nested exception: org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; 前言中不允许有内容。
	at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
	at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLScanner.reportFatalError(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(Unknown Source)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
	at org.dom4j.io.SAXReader.read(SAXReader.java:465)
	at org.dom4j.DocumentHelper.parseText(DocumentHelper.java:278)
	at com.apobates.parser.RssParser.build(RssParser.java:38)
	at com.apobates.machine.reader.Reader.mainParser(Reader.java:57)
	at com.apobates.machine.reader.Reader.load(Reader.java:37)
	at com.apobates.test.ParserEntityTest.main(ParserEntityTest.java:41)

上面的异常发生在我用HttpGet把 http://answers.microsoft.com/en-us/feed/f/ie的响应放到一个字符串中,在RssParser的build方法中运行下面代码:

Document doc=DocumentHelper.parseText(responseText);

时抛出的,在控制台上print出来的字符串另存为xml并没有发现xml是非良构的,我就开始找“前言中不允许有内容”，并没有找到xml中存在前言的概念，不过在google中却发现这个异常提示与之接近：Content is not allowed in prolog.

翻译一下是说：内容是不允许在序言。这下有头绪了。下面看一看xml序言有哪些内容：

The prolog refers to the information that appears before the start tag of the document or root element. It includes information that applies to the document as a whole, such as character encoding, document structure, and style sheets.

上面的定义来自MSDN:http://msdn.microsoft.com/en-us/library/vstudio/ms256037(v=vs.100).aspx

answers的feed xml的序言有：

en-us

xml version肯定没问题，后面是rss，好像发现跟名称空间有点关系，我用正则替换掉item中的a10：发现问题解决了，最离谱的是如果用下面的代码也没问题：

		SAXReader xmlReader = new SAXReader();
		List rs=new ArrayList();
		try {
			Document doc=xmlReader.read(new URL("http://answers.microsoft.com/en-us/feed/f/ie"));
			
			List list = doc.selectNodes("//item");
                       //ETC
		} catch (MalformedURLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (DocumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

dom4J对DocumentHelper.parseText要求过严了，一直想在这行代码之前关闭掉名称空间检查，苦读几天api，也没发现