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 configuration

PUBLIC "-//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 mapper

PUBLIC "-//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 mapper

PUBLIC "-//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();

           }

       }