File _outputFile=new File("system_file.xml");
      OutputFormat outputFormat=new OutputFormat("XML","gb2312",true);
      FileWriter fileWriter=new FileWriter(_outputFile);
      XMLSerializer xmlSerializer=new XMLSerializer(
          fileWriter,outputFormat);
      xmlSerializer.asDOMSerializer();
      xmlSerializer.serialize(doc.getDocumentElement());
详细代码见
http://expert.csdn.net/Expert/topic/1386/1386721.xml?temp=.2296259