*Mapper.xml中代码
<select id="query" parameterType="int" resultType="User">
select username , password from user where id=#{id}
</select>
主要测试代码
User user2 = userMapper.query(7);
System.out.println(user2);
System.out.println(user2.getName() + " " + user2.getPwd());
控制台输出user2为null,报错空指针异常
原因:实体类中属性(name, pwd)和表中字段(username, password)不一致导致
解决办法:关联实体类
<select id="query" parameterType="int" resultType="User">
select username as name, password as pwd from user where id=#{id}
</select>