七、MyBatis的延迟加载和缓存

一、延迟加载

在一对多查询时，希望延迟加载，比如在查询用户信息时，要求用户的账户信息能够延迟加载
开启延迟加载
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">
<configuration>
    <properties resource="jdbcConfig.properties"></properties>

    <!--开启Mybatis支持延迟加载-->
    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

    <typeAliases>
        <package name="com.liaoxiang.domain"></package>
    </typeAliases>

    <environments default="mysql">
        <environment id="mysql">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"></property>
                <property name="url" value="${jdbc.url}"></property>
                <property name="username" value="${jdbc.username}"></property>
                <property name="password" value="${jdbc.password}"></property>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <package name="com.liaoxiang.dao"></package>
    </mappers>
</configuration>

IUserDao.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.liaoxiang.dao.IUserDao">

    <!-- 定义User的resultMap-->
    <resultMap id="userAccountMap" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
        <!-- 配置user对象中accounts集合的映射 -->
        <collection property="accounts" ofType="account"
                    select="com.liaoxiang.dao.IAccountDao.findAccountByUid"
                    column="id">
        </collection>
    </resultMap>

    <!-- 查询所有 -->
    <select id="findAll" resultMap="userAccountMap">
        select * from user
    </select>

    <!-- 根据id 查询用户 -->
    <select id="findById" parameterType="INT" resultType="user">
        select * from user where id = #{uid}
    </select>

</mapper>

IAccountDao.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.liaoxiang.dao.IAccountDao">

    <!-- 定义封装account和user的resultMap -->
    <resultMap id="accountUserMap" type="account">
        <id property="id" column="id"></id>
        <result property="uid" column="uid"></result>
        <result property="money" column="money"></result>
        <!--
            一对一的关系映射：配置封装user 的内容
                select 属性指定的内容：查询用户的唯一标识
                column 属性指定的内容：用户根据id 查询时，所需要的参数的值，就根据uid 的值去查用户
        -->
        <association property="user" javaType="user"
                     select="com.liaoxiang.dao.IUserDao.findById"
                     column="uid" >
        </association>
    </resultMap>

    <!-- 查询所有账户 -->
    <select id="findAll" resultMap="accountUserMap">
        select * from account
    </select>

    <!-- 根据用户id查询账户列表 -->
    <select id="findAccountByUid" resultType="account">
        select * from account where uid = #{uid}
    </select>

</mapper>

测试方法：

@Test
public void testFindAll(){
    List<User> users = userDao.findAll();
    /*for(User user : users){
        System.out.println(user);
        //System.out.println(user.getAccounts());
        System.out.println("-----------------------------------------------------------");
    }*/
}

执行测试方法：

注释掉SqlMapConfig.xml里面的延迟加载设置：

在SpringBoot中配置mybatis的延迟加载：

mybatis:
  configuration:
    lazy-loading-enabled: true
  type-aliases-package: com.liaoxiang.model

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.liaoxiang.mapper.UserMapper">
    <resultMap id="BaseResultMap" type="user">
        <id column="user_id" property="userId"></id>
        <result column="user_name" property="userName"/>
        <result column="password" property="password"/>
        <result column="name" property="name"/>
        <result column="enabled" property="enabled"/>
    </resultMap>

    <resultMap id="includeRolesUserMap" type="com.liaoxiang.model.User" extends="BaseResultMap">
        <collection property="roles" ofType="com.liaoxiang.model.Role"
                    select="com.liaoxiang.mapper.RoleMapper.findRolesByUid"
                    column="user_id">
        </collection>
    </resultMap>

    <select id="findAllUser" resultMap="includeRolesUserMap">
        select * from user
    </select>

    <select id="findUserByUserName" resultMap="BaseResultMap" parameterType="String">
        select * from user where user_name = #{userName}
    </select>
</mapper>

RoleMapper.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.liaoxiang.mapper.RoleMapper">
    <resultMap id="BaseResultMap" type="role">
        <id column="role_id" property="roleId"></id>
        <result column="role_name" property="roleName"/>
        <result column="role_authority" property="roleAuthority"/>
    </resultMap>

    <select id="findRolesByUid" resultMap="BaseResultMap" parameterType="Integer">
        SELECT r.* FROM role r, user_role ur WHERE ur.rid=r.role_id AND ur.uid=#{uid}
    </select>

    <select id="findAllRole" resultMap="BaseResultMap">
        select * from role
    </select>

</mapper>

执行测试方法，查看日志

@RunWith(SpringRunner.class)
@SpringBootTest
public class UserTest {
    @Autowired
    private UserService userService;
    @Test
    public void test1(){
        List<User> users = userService.findAll();
        /*for (User user : users) {
            System.out.println(user);
        }*/
    }
}

注释掉延迟加载执行同样的查询所有user的方法时，同时查询了role

二、缓存

1、一级缓存

@Test
public void testFirstLevelCache(){
    User user1 = userDao.findById(41);
    User user2 = userDao.findById(41);
    System.out.println(user1 == user2);
}

两次获取的对象为同一个对象，两次查询只发送了一次sql语句

清除一级缓存

@Test
    public void testFirstLevelCache(){
        User user1 = userDao.findById(41);
        //1、清空缓存
        // 关闭sqlSession
        //sqlSession.close();
        //再次获取SqlSession对象
        //sqlSession = factory.openSession();
        //2、此方法也可以清空缓存
        sqlSession.clearCache();
        userDao = sqlSession.getMapper(IUserDao.class);
        User user2 = userDao.findById(41);
        System.out.println(user1 == user2);
    }

两次获取的对象不再相等，且查询发送两次sql语句

一级缓存时SqlSession范围的缓存，当调用SqlSession的修改，添加，删除，commit()，close()等方法时，就会清空一级缓存。

2、二级缓存

配置开启二级缓存
1、在SqlMapConfig.xml中配置
2、在IUserDao.xml中：<cache/>
3、在查询语句中：useCache="true"

<configuration>
    <!-- 配置数据库properties-->
    <properties resource="jdbcConfig.properties"></properties>
	
	<!-- 配置缓存 -->
    <settings>
    	<!--默认值就为true -->
        <setting name="cacheEnabled" value="true"/>
    </settings>
    
    <!--使用typeAliases配置别名，它只能配置domain中类的别名 -->
    <typeAliases>
        <package name="com.liaoxiang.domain"></package>
    </typeAliases>

    <!--配置环境-->
    <environments default="mysql">
        <!-- 配置mysql的环境-->
        <environment id="mysql">
            <!-- 配置事务 -->
            <transactionManager type="JDBC"></transactionManager>

            <!--配置连接池-->
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"></property>
                <property name="url" value="${jdbc.url}"></property>
                <property name="username" value="${jdbc.username}"></property>
                <property name="password" value="${jdbc.password}"></property>
            </dataSource>
        </environment>
    </environments>
    <!-- 配置映射文件的位置 -->
    <mappers>
        <package name="com.liaoxiang.dao"></package>
    </mappers>
</configuration>

IUserDao.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.liaoxiang.dao.IUserDao">
    <!--开启user支持二级缓存-->
    <cache/>

    <!-- 查询所有 -->
    <select id="findAll" resultType="user">
        select * from user
    </select>

    <!-- 根据id查询用户 开启二级缓存-->
    <select id="findById" parameterType="INT" resultType="user" useCache="true">
        select * from user where id = #{uid}
    </select>

    <!-- 更新用户信息-->
    <update id="updateUser" parameterType="user">
        update user set username=#{username},address=#{address} where id=#{id}
    </update>
</mapper>

测试

@Test
public void testFirstLevelCache(){
    // 创建第一个 sqlSession1
    SqlSession sqlSession1 = factory.openSession();
    IUserDao dao1 = sqlSession1.getMapper(IUserDao.class);
    User user1 = dao1.findById(41);
    System.out.println(user1);
    sqlSession1.close();//一级缓存消失
    // 创建第二个 sqlSession2
    SqlSession sqlSession2 = factory.openSession();
    IUserDao dao2 = sqlSession2.getMapper(IUserDao.class);
    User user2 = dao2.findById(41);
    System.out.println(user2);
    sqlSession2.close();//一级缓存消失
    System.out.println(user1 == user2);
}

二级缓存中存放的是数据而不是对象，当第二次查询时，会从二级缓存中获取数据，创建一个新的对象，所以是两个不同的对象
其他博客参考：https://blog.youkuaiyun.com/Lotus_dong/article/details/116334317

springboot整和mybatis后，一级缓存不起作用。。。
发送一个查询请求，在service中连续查询两次，发现执行了两次sql语句

public User findUserByUserName(String userName) {
    User user = userMapper.findUserByUserName(userName);
    User user1 = userMapper.findUserByUserName(userName);
    System.out.println(user == user1);
    return user;
}

在service类上加上@Transactional注解后，此时一级缓存能够生效，但是在使用postman发送两个相同查询的时候，每次都只查询一次，两次查询也会执行两次sql语句，原因有空再分析！！