JDOM issue: The Content already has an existing parent－Jemmy Walker

　　漫不經心的憂傷，踹到JDOM來。拿既有的Element當作一份新產出XML Document怎麼處理呢？假設Element值是<book>漫不經心</book>，若要以<book>為根元素，其寫法如下：

XMLOutputter outputter = new XMLOutputter();
fos = new FileOutputStream(filename);
Document doc = new Document(element.detach());
outputter.output(doc, fos);
fos.flush();
fos.close();

　　而若想在<book>之上再有個<root>作為根元素，則寫法如下：

XMLOutputter outputter = new XMLOutputter();
fos = new FileOutputStream(filename);
Element root = new Element("root");
Document doc = new Document(root);
root.addContent(element.detach());
outputter.output(doc, fos);
fos.flush();
fos.close();

　　沒辦法將Element物件當作內容被加到addContent，需用detach複製自己。否則常有以下的Exception：

org.jdom.IllegalAddException: The Content already has an existing parent "root"