public static void main(String[] args) {
        FileInputStream fis = null;
        FileOutputStream fos = null;
        try {
            File file = new File("D://test.txt");

            fis = new FileInputStream(file);
            /**
             * new FileOutputStream(file)
             * new FileOutputStream(file, true)   追加模式
             * 如果不选择追加模式，就会清空原文件内容，类似的有FileWriter(内部维护的是FileOutputStream)等。。。
             */
            fos = new FileOutputStream(file);

            byte[] b = new byte[1024];
            int index = 0;
            while ((index = fis.read(b)) >= 0) {
                System.out.println(new String(b, 0, index));
            }
            //  "\r\n" 为换行符
            fos.write("\r\ntest".getBytes());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (fis != null && fos != null) {
                    fos.close();
                    fis.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }