public void Loadfile(String xmlfile)
	{
		File file = new File(xmlfile);
		if (!file.exists())
			return;
		
		Document doc = null;
		try 
		{
			DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
			DocumentBuilder db = dbf.newDocumentBuilder();
			doc = db.parse(file);
			doc.getDocumentElement().normalize();
		} 
		catch (Exception e) 
		{
			throw new RuntimeException(e);
		}
		
		NodeList nodeLst = doc.getChildNodes();
	
		List<Node> roots = new ArrayList<Node>();
		for (int i = 0; i < nodeLst.getLength(); i++) {
			Node n = nodeLst.item(i);
			if(n.getNodeType()==Node.ELEMENT_NODE) {
				roots.add(n);
			}
		}
		
		if(roots.size()==0) {
			throw new RuntimeException("Found No top net in this file");
		}
		else if(roots.size()>1) {
			throw new RuntimeException("Found multiple top net in this file");
		}
		
		this.root = new XmlNode(roots.get(0));
		vistDFS(root);
	}