如何在MyBatis上通过<if>语句使用update语句

<update id="updateUser" parameterType="User">
  UPDATE user_table
  <set>
    <if test="username != null">
      username = #{username},
    </if>
    <if test="password != null">
      password = #{password},
    </if>
    <if test="email != null">
      email = #{email},
    </if>
  </set>
  WHERE id = #{id}
</update>

User user = new User();
user.setId(1);
user.setUsername("newUsername");

SqlSession sqlSession = sqlSessionFactory.openSession();
try {
  UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
  userMapper.updateUser(user);
  sqlSession.commit();
} finally {
  sqlSession.close();
}