mybatis之sql输出映射(七)

野性酷女 2022-06-10 03:10 156阅读 0赞

1. resultType  
使用要求  
使用resultType进行结果映射时，需要查询出的列名和映射的对象的属性名一致，才能映射成功。

如果查询的列名和对象的属性名全部不一致，那么映射的对象为空。  
如果查询的列名和对象的属性名有一个一致，那么映射的对象不为空，但是只有映射正确那一个属性才有值。

如果查询的sql的列名有别名，那么这个别名就是和属性映射的列名。  
注:对简单类型的结果映射也是有要求的，查询的列必须是一列，才能映射为简单类型。

UserMapper接口

public interface UserMapper {
        int findUserCount();
    }

UserMapper.xml

测试代码

@Test
        public void test_06(){
            SqlSession sqlSession = sqlSessionFactory.openSession();
            UserMapper userMapper =   sqlSession.getMapper(UserMapper.class);
            int count = userMapper.findUserCount();
            System.out.println(count);
        }

2. resultMap  
使用要求  
使用resultMap进行结果映射时，不需要查询的列名和映射的属性名必须一致。但是需要声明一个resultMap，来对列名和属性名进行映射。

对以下sql语句进行对象映射

Select id id\_,username username\_,sex sex\_ from user where id = 1;

UserMapper接口

public interface UserMapper {
        User findUserById(int id);
    }

UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    
    <mapper namespace="com.test.mapper.UserMapper">
        <resultMap type="com.test.domain.User" id="mapId">
            <id column="id_" property="id"/>
            <result column="username_" property="username" />
            <result column="sex_" property="sex" />
            <result column="birthday_" property="birthday" />
        </resultMap>
        //注：返回结果为resultMap，resultMap值为上面resultMap标签的id
        <select id="findUserById" parameterType="int" resultMap="mapId">
            SELECT id id_, username username_, sex sex_, birthday birthday_ FROM USER WHERE id =#{id}
        </select>

测试代码

@Test
        public void test_02() throws Exception{
            SqlSession sqlSession = sqlSessionFactory.openSession();
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            User user = mapper.findUserById(1);
            System.out.println(user);
        }

注：如果返回resultMap写成resultType并应用resultMap标签的id，会报如下错误

org.apache.ibatis.exceptions.PersistenceException: 
    ### Error building SqlSession.
    ### The error may exist in mapper/UserMapper.xml
    ### The error occurred while processing mapper_resultMap[mapId]
    ### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: org.apache.ibatis.builder.BuilderException: Error resolving class. Cause: org.apache.ibatis.type.TypeException: Could not resolve type alias 'mapId'. Cause: java.lang.ClassNotFoundException: Cannot find class: mapId
        at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:26)
        at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:82)
        at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:66)
        at com.test.test.Test.init(Test.java:109)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    //一下省略。。。