mybatis练习，不知道哪里理解错了

Question

这是我的sql映射文件中的语句，主要是其中的where和if关键字，我的理解是如果传入对象user中的age属性不为空，那么就是执行select * from user，也就是应该把整个user表查出来，但是实际啥都没查出来，当创建User对象时再给age设置值，那么就执行select * from user where age=#{age}，实际输出符合预期，就是第一种情况为啥输出为空？这个不明白

    <select id="selectByCondition" resultType="user" parameterType="user">
        select * from user
        <where>
            <if test="age!=null">
                and age=#{age}
            </if>
        </where>
    </select>

user类如下

public class User {
    private int id;
    private String name;
    private int age;

测试

public class ProxyTest {
    public static void main(String[] args) throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("MybatisConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User user = new User();
        //user.setAge(18);
        List<User> users = mapper.selectByCondition(user);
        for (User user1 : users) {
            System.out.println(user1.getName());
        }
    }
}

Answer 1