一文彻底搞懂Mybatis系列（十二）之MyBatis多对一映射延迟加载（association和lazyLoadingEnabled）

在association标签中配置fetchType="lazy"，是局部的设置，
只对当前的association关联的sal语句起作用。

StuMapper接口

public interface StuMapper {
  public Stu selectBySidStep1(Integer sid);
}

ClazzMapper接口

public interface ClazzMapper {
    public Clazz selectByCidStep2(Integer cid);
}

<?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.powernode.mybatis.mapper.StuMapper">
    <!--分步骤查询-->
    <resultMap id="selectBySidStepMap" type="Stu">
        <id property="sid" column="sid"/>
        <result property="name" column="name"/>
        <association property="clazz"
                     select="com.powernode.mybatis.mapper.ClazzMapper.selectByCidStep2"
                     column="cid"
                     fetchType="lazy"
        />
    </resultMap>
    <select id="selectBySidStep1" resultMap="selectBySidStepMap">
        select s.sid,s.name,s.cid from t_stu s
        where s.sid = #{sid}
    </select>
</mapper>

测试类 情况一：调用学生类对象的toString()方法

@Test
    public void testAdvanceMappingStep(){
        SqlSession sqlSession = SqlSessionUtil.openSqlSession();
        StuMapper mapper = sqlSession.getMapper(StuMapper.class);
        Stu stu = mapper.selectBySidStep1(1);
        System.out.println(stu.toString());

运行结果，我们发现，两个接口对应的sql都被执行了

StuMapper.selectBySidStep1 - ==>  Preparing: select s.sid,s.name,s.cid from t_stu s where s.sid = ?
StuMapper.selectBySidStep1 - ==> Parameters: 1(Integer)
StuMapper.selectBySidStep1 - <==      Total: 1
ClazzMapper.selectByCidStep2 - ==>  Preparing: select * from t_clazz where cid = ?
ClazzMapper.selectByCidStep2 - ==> Parameters: 1000(Integer)
ClazzMapper.selectByCidStep2 - <==      Total: 1
Stu{sid=1, name='张三', clazz=Clazz{cid=1000, name='高三一班'}}

测试类 情况二：调用学生类对象的getName()方法

@Test
    public void testAdvanceMappingStep(){
        SqlSession sqlSession = SqlSessionUtil.openSqlSession();
        StuMapper mapper = sqlSession.getMapper(StuMapper.class);
        Stu stu = mapper.selectBySidStep1(1);
        System.out.println(stu.getName());

运行结果，我们发现，只执行了一条sql，说明延迟加载生效了

StuMapper.selectBySidStep1 - ==>  Preparing: select s.sid,s.name,s.cid from t_stu s where s.sid = ?
StuMapper.selectBySidStep1 - ==> Parameters: 1(Integer)
StuMapper.selectBySidStep1 - <==      Total: 1
张三

测试类 情况三：先调用学生类对象的getName()方法，再调用班级属性方法getClazz()

@Test
    public void testAdvanceMappingStep(){
        SqlSession sqlSession = SqlSessionUtil.openSqlSession();
        StuMapper mapper = sqlSession.getMapper(StuMapper.class);
        Stu stu = mapper.selectBySidStep1(1);
        System.out.println(stu.getName());
        System.out.println(stu.getClazz());

运行结果，我们发现，只有在调用的时候，sql语句才会被执行，再次验证了延迟加载的配置效果

StuMapper.selectBySidStep1 - ==>  Preparing: select s.sid,s.name,s.cid from t_stu s where s.sid = ?
StuMapper.selectBySidStep1 - ==> Parameters: 1(Integer)
StuMapper.selectBySidStep1 - <==      Total: 1
张三
ClazzMapper.selectByCidStep2 - ==>  Preparing: select * from t_clazz where cid = ?
ClazzMapper.selectByCidStep2 - ==> Parameters: 1000(Integer)
ClazzMapper.selectByCidStep2 - <==      Total: 1
Clazz{cid=1000, name='高三一班'}

具体配置如下：

<?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">
<configuration>
    <properties resource="jdbc.properties"/>
    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>
    <typeAliases>
        <package name="com.powernode.mybatis.pojo" />
    </typeAliases>
    <environments default="dev">
        <environment id="dev">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <package name="com.powernode.mybatis.mapper"/>
    </mappers>
</configuration>