修改配置信息，并保存到文件

public static void loadDemo() throws IOException {

        Properties prop = new Properties();
        FileInputStream fis = new FileInputStream("info.txt");
        prop.load(fis);
        prop.setProperty("zhangsan", "10");

        FileOutputStream fos=new FileOutputStream("info.txt");
        prop.store(fos, "haha");    //第二个参数是注释，

        fis.close();
        fos.close();
    }