用sax读xml文件时常会出现这个异常，一般上网可以看到的原因是:

1.BOM

   在java中测试inputstream中是否有bom可以用apache commons IO 的org.apache.commons.io.input.BOMInputStream，如果你的项目有引入IO的情况下

   BOM的基础知识可以参考:http://www.unicode.org/faq/utf_bom.html

   也贴一个过滤BOM的小方法

private static InputStream checkForUtf8BOMAndDiscardIfAny(InputStream inputStream) throws IOException {
    PushbackInputStream pushbackInputStream = new PushbackInputStream(new BufferedInputStream(inputStream), 3);
    byte[] bom = new byte[3];
    if (pushbackInputStream.read(bom) != -1) {
        if (!(bom[0] == (byte) 0xEF && bom[1] == (byte) 0xBB && bom[2] == (byte) 0xBF)) {
            pushbackInputStream.unread(bom);
        }
    }
    return pushbackInputStream; 
}

2.非良构的xml内容,例如在<?xml前出现非法的字符

   可以采用正则从<?xml开始截取内容

3.今天我发现的是因为:Accept-Encoding启用了压缩传输,可以试着把Accept-Encoding设成:identity,同时也要注意是否采用Chunked Transfer Encoding(分段传输)

后话,有人说直接把流传给sax parser可以过滤掉。当然我用的sax,jaxp发现不会过滤掉。文档测试的环境:java 7 + xerces sax。全部异常栈

org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Content is not allowed in prolog.
	at org.apache.xerces.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
	at org.apache.xerces.util.ErrorHandlerWrapper.fatalError(Unknown Source)
	at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
	at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
	at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
	at org.apache.xerces.impl.XMLScanner.reportFatalError(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentScannerImpl$PrologDispatcher.dispatch(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
	at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
	at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
	at org.apache.xerces.jaxp.SAXParserImpl.parse(Unknown Source)