读文件问题

File file = new File("E:\\a\\update.log");

		StringBuffer sb = new StringBuffer();

		String s = new String();

		try {

			BufferedReader read = new BufferedReader(new FileReader(file));

			BufferedWriter writer = new BufferedWriter(new FileWriter(file));

			while((s = read.readLine())!=null){

				sb.append(s);     \\没有读到原来文件里的内容，

				sb.append("\r\n");\\运行后文件里只有一行"=========" ,哪里错了？

			}

			read.close();

			sb.append("==========\r\n");

			writer.write(sb.toString());

			writer.close();

			

		} catch (FileNotFoundException e) {

			// TODO Auto-generated catch block

			e.printStackTrace();

		} catch (IOException ee){

			ee.printStackTrace();

		}