MyBatis高级映射之-一对一（一对多）关联映射

在hibernate中可以进行一对一，多对一，一对多，多对多，MyBatis中也可以实现这种映射，但是映射就显得比较麻烦了，下面看一个一对一的例子，学了hibernate都知道其实一对一跟一对多的原理其实是一致的，所以也是一对多的例子：

首先配置SqlMapConfig.xml：

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd"><!-- 配置管理器 --><configuration> <!-- 别名 --> <typeAliases> <typeAlias type="org.leadfar.Person" alias="Person"/> <typeAlias type="org.leadfar.IdCard" alias="IdCard"/> </typeAliases> <!-- 配置数据库连接信息 --> <environments default="development"> <environment id="development"> <transactionManager type="JDBC" /> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://localhost:3306/mybatis" /> <property name="username" value="root" /> <property name="password" value="root" /> </dataSource> </environment> </environments> <!-- 映射文件定位 --> <mappers> <mapper resource="org/leadfar/PersonMapper.xml" /> <mapper resource="org/leadfar/IdCardMapper.xml" /> </mappers></configuration>然后是PersonMapper.xml,IdCardMapper.xml

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="org.leadfar.Person"><!-- 设置keyProperty 跟useGeneratedKeys可以获取自增长的ID 只支持具有自增长方式的那种数据库，如,mysql,sql server(oracle不支持)--> <insert id="insert" parameterType="Person" keyProperty="id" useGeneratedKeys="true"> insert into t_person(id,t_name,age,birthday) values(#{id},#{name},#{age},#{birthday}) </insert> <!-- 当实体类属性跟数据库字段不一致时映射结果集 --><resultMap type="Person" id="result_person"> <result property="name" column="t_name"/></resultMap> <select id="selectOne" parameterType="int" resultMap="result_person"> select * from t_person where id=#{id} </select> </mapper>

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="org.leadfar.IdCard"><!-- 设置keyProperty 跟useGeneratedKeys可以获取自增长的ID 只支持具有自增长方式的那种数据库，如,mysql,sql server(oracle不支持)--> <insert id="insert" parameterType="Person" keyProperty="id" useGeneratedKeys="true"> insert into t_idcard(number,person_id) values(#{idCard.number},#{id}) </insert> <!-- 通过关联对象ID另外发出一条SQL语句来查询关联对象， 这样的查询如果查询对象是一个集合的话会出现N+1问题， 通过一条SQL语句查询对象列表即1，再通过每条记录关联的对象Id查询关联对象即N <resultMap type="IdCard" id="idCardMap"> <association property="person" column="person_id" javaType="Person" select="org.leadfar.Person.selectOne"/> </resultMap> <select id="selectIdCard" parameterType="int" resultMap="idCardMap"> select * from t_IdCard where id=#{id} </select> --> <!—下面通过连接查询可以解决N+1问题--!> <resultMap type="IdCard" id="idCardMap"> <id property="id" column="i_id"/> <result property="number" column="i_number"/> <association property="person" column="person_id" javaType="Person" resultMap="personMap"/> </resultMap> <resultMap type="Person" id="personMap"> <id property="id" column="p_id"/> <result property="name" column="p_name"/> <result property="age" column="p_age"/> <result property="birthday" column="p_birthday"/> </resultMap> <select id="selectIdCard" parameterType="int" resultMap="idCardMap"> select i.id as i_id, i.number as i_number, p.id as p_id, p.t_name as p_name, p.age as p_age, p.birthday as p_birthday from t_IdCard i left outer join t_person p on i.person_id = p.id where i.id=#{id} </select> </mapper> 分别测试N+1的那个映射，和不会出现N+1的映射: private SqlSessionFactory factory;public TestAssociation(){ //读取主配置文件的读取器 try { Reader reader =Resources.getResourceAsReader("SqlMapConfig.xml"); System.out.println("fuq"+reader); //基于reader通过SqlSessionFactoryBuilder构建工厂 factory=new SqlSessionFactoryBuilder().build(reader); System.out.println("factory"+factory); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } } public void testFindById() { //通过factory获得数据库连接会话 SqlSession session=factory.openSession(); try{ IdCard idCard = (IdCard)session.selectOne(IdCard.class.getName()+".selectIdCard",3); System.out.println("IdCard====="+idCard.getNumber()); System.out.println("Name====="+idCard.getPerson().getName()); session.commit(); }catch(Exception e){ session.rollback(); e.printStackTrace(); }finally{ session.close(); } } public void testFindById2() { //通过factory获得数据库连接会话 SqlSession session=factory.openSession(); try{ IdCard idCard = (IdCard)session.selectOne(IdCard.class.getName()+".selectIdCard",3); System.out.println("IdCard====="+idCard.getNumber()); System.out.println("Name====="+idCard.getPerson().getName()); session.commit(); }catch(Exception e){ session.rollback(); e.printStackTrace(); }finally{ session.close(); } }