try{
  SAXBuiler builder=new SAXBuilder();
  Document doc=builder.build(new File(fileName));
  Element root=doc.getRootElement();
  Element book=new Element("book");
  book.addContent(new Element("name").setText("xml"));
  root.addContent(book);
  FileWriter fw=new FileWriter(fileName);
  XMLOutputter outputter=new XMLOutputter("",true,"UTF-8");
  outputter.output(doc,fw);
}catch(Exception e){}//fileName即是你这个文件的文件名,要全路径,比如D:/java_xml.xml