User user = new User();
            user.setUsername("root");
            User user1 = new User();
            user1.setIdentitycard("5555");
            user1.setId("99999");
            User user2 = new User();
            user2.setId("55");
            BeanUtils.copyProperties(user1,user ,new String[]{
   "id","username"});
            BeanUtils.copyProperties(user2,user ,new String[]{
   "username"});
            System.out.println(user.toString());

从user1复制东西到user实体类，第三个参数是不需要复制的字段名字