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Mybatis基础概念与高级应用小结

作者：元九

这篇文章主要介绍了Mybatis基础回顾与高级应用,本文内容有点小长，希望大家耐心阅读，此文结合实例代码给大家介绍的非常详细，需要的朋友可以参考下

Mybatis基础回顾与高级应用

数据库:mysql5.7

jdk:15

引入依赖

<!--引入依赖-->
    <dependencies>
        <!--mybatis坐标-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
        </dependency>
        <!--mysql驱动坐标-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.6</version>
            <scope>runtime</scope>
        </dependency>
        <!--单元测试坐标-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.22</version>
        </dependency>
    </dependencies>

User实体

@Data
public class User {
    private Integer id;
    private String username;
}

jdbc.properties

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://127.0.0.1/zdy_mybatis
jdbc.username=root
jdbc.password=root

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>
    <!--加载外部的properties文件-->
    <properties resource="jdbc.properties"></properties>
​
    <!--给实体类的全限定类名给别名-->
    <typeAliases>
        <!--给单独的实体起别名-->
     <!--   <typeAlias type="com.yun.pojo.User" alias="user"></typeAlias>-->
        <!--批量起别名：该包下所有的类的本身的类名：别名还不区分大小写-->
        <package name="com.yun.pojo"/>
    </typeAliases>
​
    <!--environments:运行环境-->
    <environments default="development">
        <environment id="development">
            <!--当前事务交由JDBC进行管理-->
            <transactionManager type="JDBC"></transactionManager>
            <!--当前使用mybatis提供的连接池-->
            <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>
​
    <!--引入映射配置文件-->
    <mappers>
        <mapper resource="UserMapper.xml"></mapper>
    </mappers>
</configuration>

案例一-查询用户

UserMapper.xml

<mapper namespace="user">
    <!--select-->
    <select id="findAll" resultType="com.yun.pojo.User">
        select * from user
    </select>
</mapper>
@Test
    public void test1() throws IOException {
        //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        //2.解析了配置文件，并创建了sqlSessionFactory工厂
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //3.生产sqlSession
        // 默认开启一个事务，但是该事务不会自动提交
        //在进行增删改操作时，要手动提交事务
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //4.sqlSession调用方法：查询所有selectList  查询单个：selectOne 添加：insert  修改：update 删除：delete
        List<User> users = sqlSession.selectList("user.findAll");
        users.forEach(item ->{
            System.out.println(item);
        });
        sqlSession.close();
​
    }

输出结果

User(id=1, username=Tom)
User(id=2, username=Jerry)

案例二-添加用户

UserMapper.xml

<!--parameterType:参数类型-->
<insert id="saveUser" parameterType="com.yun.pojo.User">
        insert into user Values (#{id},#{username})
</insert>
@Test
    public void test2() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        User user = new User();
        user.setId(3);
        user.setUsername("jack");
        sqlSession.insert("user.saveUser",user);
        sqlSession.commit();
        sqlSession.close();
    }

数据库结果:

案例三-编辑用户

UserMapper.xml

<update id="updateUser" parameterType="com.yun.pojo.User">
        update user set username = #{username} where id = #{id}
</update>
@Test
    public void test3() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        User user = new User();
        user.setId(3);
        user.setUsername("rose");
        sqlSession.update("user.updateUser",user);
        sqlSession.commit();
        sqlSession.close();
    }

数据库结果:

案例四-删除用户

UserMapper.xml

<delete id="deleteUser" parameterType="java.lang.Integer">
        delete from user where id =#{id}
</delete>
 @Test
    public void test4() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
​
        User user = new User();
        user.setId(3);
        sqlSession.delete("user.deleteUser",3);
        sqlSession.commit();
​
        sqlSession.close();
​
    }

数据库结果:

传统开发方式

public interface IUserDao {
​
    //查询所有用户
    List<User> findAll() throws IOException;
}

UserDaoImpl

public class UserDaoImpl implements IUserDao {
​
    @Override
    public List<User> findAll() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        List<User> users = sqlSession.selectList("user.findAll");
        sqlSession.close();
        return users;
    }
}
@Test
    public void test5() throws IOException {
        UserDaoImpl dao = new UserDaoImpl();
        List<User> users = dao.findAll();
        System.out.println(users);
    }

打印结果:

[User(id=1, username=Tom), User(id=2, username=Jerry)]

代理开发方式(使用JDK动态代理产生代理对象,由代理对象执行并且操作)

Mapper接口开发需要遵行以下规范:

mapper.xml文件中的namespace与mapper接口的全限定名相同;
2. mapper接口方法名和mapper.xml中定义的每个statement的id相同
3. mapper接口方法的输入参数类型和mapper.xml中定义的每个sql的parameterType的类型相同
4. mapper接口方法的输出参数类型和mapper.xml中定义的每个sql的resultType的类型相同

根据上述的规范修改UserMapper.xml

<mapper namespace="com.yun.dao.IUserDao">
​
    <!--select-->
    <select id="findAll" resultType="com.yun.pojo.User">
        select * from user
    </select>
</mapper>
public interface IUserDao {
​
    //查询所有用户
    List<User> findAll() throws IOException;
}
@Test
public void test6() throws IOException {
    InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
    SqlSession sqlSession = sqlSessionFactory.openSession();
​
    IUserDao mapper = sqlSession.getMapper(IUserDao.class);
    List<User> all = mapper.findAll();
    all.forEach(item ->{
        System.out.println(all);
    });
}

输出结果:

User(id=1, username=Tom)
User(id=2, username=Jerry)

动态sql语句 if标签

public interface IUserDao {
​
    //多条件组合查询：演示if
    public List<User> findByCondition(User user);
}
<!--抽取sql片段-->
<sql id="selectUser">
    select * from user
</sql>
​
<!--多条件组合查询：演示if-->
<select id="findByCondition" parameterType="user" resultType="user">
   <include refid="selectUser"></include>
   <where>
       <if test="id !=null">
           and id = #{id}
       </if>
       <if test="username !=null">
           and username = #{username}
       </if>
   </where>
</select>
@Test
    public void test7() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
​
        IUserDao mapper = sqlSession.getMapper(IUserDao.class);
​
        User user1 = new User();
        user1.setId(1);
        user1.setUsername("Tom");
​
        List<User> all = mapper.findByCondition(user1);
        for (User user : all) {
            System.out.println(user);
        }
    }

输出结果:

User(id=1, username=Tom)

动态sql语句 foreach标签

public interface IUserDao {
​
    //多值查询：演示foreach
    public List<User> findByIds(int[] ids);
}
<!--多值查询：演示foreach-->
<select id="findByIds" parameterType="list" resultType="user">
   <include refid="selectUser"></include>
   <where>
       <foreach collection="array" open="id in (" close=")" item="id" separator=",">
           #{id}
       </foreach>
   </where>
</select>
@Test
public void test8() throws IOException {
   InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
   SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
   SqlSession sqlSession = sqlSessionFactory.openSession();
​
   IUserDao mapper = sqlSession.getMapper(IUserDao.class);
​
   int[] arr = {1,2};
​
   List<User> all = mapper.findByIds(arr);
   for (User user : all) {
       System.out.println(user);
   }
}

输出结果:

User(id=1, username=Tom)
User(id=2, username=Jerry)

Mybatis复杂映射

一对一

User实体

@Data
public class User {
​
    private Integer id;
​
    private String username;
​
    //该用户所具有的订单信息
    private List<Order> orders;
​
    //该用户所具有的角色信息
    private List<Role> roles;
}

Order实体

@Data
public class Order {
​
    private Integer id;
​
    private String orderTime;
​
    private BigDecimal total;
​
    //表明该订单属于哪个用户
    private User user;
}

Role实体

@Data
public class Role {
​
    private Integer id;
​
    private String roleName;
}
public interface IOrderMapper {
​
    /**
     * 查询订单的同时还查询该订单所属的用户
     * @return
     */
    public List<Order> findOrderAndUser();
}
<resultMap id="orderMap" type="com.yun.pojo.Order">
    <result property="id" column="id"></result>
    <result property="orderTime" column="order_time"></result>
    <result property="total" column="total"></result>
​
    <association property="user" javaType="com.yun.pojo.User">
        <result property="id" column="uid"></result>
        <result property="username" column="username"></result>
    </association>
</resultMap>
​
<!--resultMap:手动来配置实体属性与表字段的映射关系-->
<select id="findOrderAndUser" resultMap="orderMap">
    select * from orders o,user u  where o.uid = u.id
</select>
@Test
public void test1() throws IOException {
    //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
    InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
    //2.解析了配置文件，并创建了sqlSessionFactory工厂
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    //3.生产sqlSession
    SqlSession sqlSession = sqlSessionFactory.openSession();
    IOrderMapper mapper = sqlSession.getMapper(IOrderMapper.class);
    List<Order> orderAndUser = mapper.findOrderAndUser();
    orderAndUser.forEach(order -> {
        System.out.println(order);
    });
}

运行结果:

Order(id=1, orderTime=2022-05-01, total=1000.00, user=User(id=1, username=Tom, orders=null, roles=null))
Order(id=2, orderTime=2022-05-10, total=2000.00, user=User(id=2, username=Jerry, orders=null, roles=null))
Order(id=3, orderTime=2022-05-20, total=3000.00, user=User(id=2, username=Jerry, orders=null, roles=null))

一对多

public interface IUserMapper {
​
    /**
     * 查询所有用户信息,同时查询出每个用户关联的订单信息
     * @return
     */
    public List<User> findAll();
}
<resultMap id="userMap" type="com.yun.pojo.User">
    <id property="id" column="id"></id>
    <result property="username" column="username"></result>
​
    <collection property="orders" ofType="com.yun.pojo.Order">
        <id property="id" column="oid"></id>
        <result property="orderTime" column="order_time"></result>
        <result property="total" column="total"></result>
    </collection>
</resultMap>
​
<!--resultMap:手动来配置实体属性与表字段的映射关系-->
<select id="findAll" resultMap="userMap">
    select u.*,o.id oid,o.order_time,o.total,o.uid from user u left join orders o on u.id = o.uid
</select>
@Test
    public void test2() throws IOException {
        //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        //2.解析了配置文件，并创建了sqlSessionFactory工厂
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //3.生产sqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        IUserMapper mapper = sqlSession.getMapper(IUserMapper.class);
        List<User> users = mapper.findAll();
        users.forEach(user -> {
            System.out.println(user.getUsername());
            System.out.println(user.getOrders());
        });
    }

运行结果:

Tom
[Order(id=1, orderTime=2022-05-01, total=1000.00, user=null)]
Jerry
[Order(id=2, orderTime=2022-05-10, total=2000.00, user=null), Order(id=3, orderTime=2022-05-20, total=3000.00, user=null)]

多对多

public interface IUserMapper {
​
    /**
     * 查询所有用户信息,同事查询出每个用户关联的角色信息
     * @return
     */
    public List<User> findUserAndRole();
}
<resultMap id="userAndRoleMap" type="com.yun.pojo.User">
    <result property="id" column="userId"></result>
    <result property="username" column="username"></result>
​
    <collection property="roles" ofType="com.yun.pojo.Role">
        <result property="id" column="roleId"></result>
        <result property="roleName" column="rolename"></result>
    </collection>
</resultMap>
​
<select id="findUserAndRole" resultMap="userAndRoleMap">
    SELECT * FROM USER u
    LEFT JOIN sys_user_role sur ON u.id = sur.user_id
    LEFT JOIN sys_role sr ON sur.role_id = sr.id
</select>
@Test
public void test3() throws IOException {
    //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
    InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
    //2.解析了配置文件，并创建了sqlSessionFactory工厂
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    //3.生产sqlSession
    SqlSession sqlSession = sqlSessionFactory.openSession();
    IUserMapper mapper = sqlSession.getMapper(IUserMapper.class);
    List<User> users = mapper.findUserAndRole();
    users.forEach(user -> {
        System.out.println(user.getUsername());
        System.out.println(user.getRoles());
    });
}

运行结果:

Tom
[Role(id=null, roleName=董事长), Role(id=null, roleName=经理)]
Jerry
[Role(id=null, roleName=董事长), Role(id=null, roleName=经理)]

Mybatis注解开发

Mybasits常用注解:
@Insert: 实现新增
@Update: 实现更新
@Delete: 实现删除
@Select: 实现查询
@Result: 实现结果集封装;他代替的是标签<resultMap>,该注解中可以使用单个@Result注解,也可以使用@Result集合,
         使用格式:@Results({@Result(),@Result()}) 或 @Results(@Result())
@Results: 可以与@Result一起使用,封装多个结果集
@One: 实现一对一结果集封装
@Many: 实现一对多结果集封装

测试案例

public interface IUserMapper {
​
    //添加用户
    @Insert("insert into user values(#{id},#{username})")
    public void addUser(User user);
​
    //更新用户
    @Update("update user set username = #{} where id = #{id}")
    public void updateUser(User user);
​
    //查询用户
    @Select("select * from user")
    public List<User> getAllUser();
​
    //删除用户
    @Delete("delete from user where id=#{id}")
    public void delUser(Integer id);
}
private IUserMapper mapper;
​
    @Before
    public void before() throws IOException {
        //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        //2.解析了配置文件，并创建了sqlSessionFactory工厂
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //3.生产sqlSession                                     true:事务自动提交
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        mapper = sqlSession.getMapper(IUserMapper.class);
    }
​
    @Test
    public void addUser(){
        User user = new User();
        user.setId(3);
        user.setUsername("jack");
        mapper.addUser(user);
    }
​
    @Test
    public void updateUser(){
        User user = new User();
        user.setId(3);
        user.setUsername("rose");
        mapper.updateUser(user);
    }
​
    @Test
    public void getAllUser(){
        List<User> userList = mapper.getAllUser();
        userList.forEach(item -> {
            System.out.println(item);
        });
    }
​
    @Test
    public void delUser(){
        mapper.delUser(3);
    }

运行结果:

Mybatis注解实现复杂映射开发

一对一

public interface IOrderMapper {
​
    /**
     * 查询订单的同时还查询该订单所属的用户
     * @return
     */
    @Results({
            @Result(property = "id",column = "id"),
            @Result(property = "orderTime",column = "order_time"),
            @Result(property = "total",column = "total"),
            @Result(property = "user",column = "uid",javaType = User.class,
                    one = @One(select = "com.yun.mapper.IUserMapper.getUserById")),
    })
    @Select("select * from orders")
    public List<Order> findOrderAndUser();
}
​
public interface IUserMapper {
    @Select("select * from user where id = #{id}")
    public User getUserById(Integer id);
}
@Test
    public void oneToOne(){
        List<Order> orderAndUser = orderMapper.findOrderAndUser();
        orderAndUser.forEach(item -> {
            System.out.println(item);
        });
    }

运行结果:

Order(id=1, orderTime=2022-05-01, total=1000.00, user=User(id=1, username=Tom, orders=null, roles=null))
Order(id=2, orderTime=2022-05-10, total=2000.00, user=User(id=2, username=Jerry, orders=null, roles=null))
Order(id=3, orderTime=2022-05-20, total=3000.00, user=User(id=2, username=Jerry, orders=null, roles=null))

一对多

public interface IUserMapper {
​
    /**
     * 查询所有用户信息,同时查询出每个用户关联的订单信息
     * @return
     */
    @Select("select * from user")
    @Results({
            @Result(property = "id",column = "id"),
            @Result(property = "username",column = "username"),
            @Result(property = "orders",column = "id",javaType = List.class,
                    many = @Many(select = "com.yun.mapper.IOrderMapper.getOrderByUid"))
    })
    public List<User> findAll();
}
​
public interface IOrderMapper {
    @Select("select * from orders where uid = #{uid}")
    public List<Order> getOrderByUid(Integer uid);
}
@Test
    public void oneToMore(){
        List<User> users = mapper.findAll();
        users.forEach(item -> {
            System.out.println(item);
        });
​
    }

运行结果:

User(id=1, username=Tom, orders=[Order(id=1, orderTime=null, total=1000.00, user=null)], roles=null)
User(id=2, username=Jerry, orders=[Order(id=2, orderTime=null, total=2000.00, user=null), Order(id=3, orderTime=null, total=3000.00, user=null)], roles=null)

多对多

public interface IUserMapper {
    
    /**
    * 查询所有用户信息,同事查询出每个用户关联的角色信息
    * @return
    */
    @Select("select * from user")
    @Results({
            @Result(property = "id",column = "id"),
            @Result(property = "username",column = "username"),
            @Result(property = "roles",column = "id",javaType = List.class,
                    many = @Many(select ="com.yun.mapper.IRoleMapper.getAll"))
    })
    public List<User> findUserAndRole();
}
​
public interface IRoleMapper {
​
    @Select("select * from sys_role sr,sys_user_role sur where sr.id = sur.role_id and sur.user_id = #{uid}")
    public List<Role> getAll(Integer uid);
}
@Test
public void moreToMore(){
    
    List<User> users = mapper.findUserAndRole();
    users.forEach(item -> {
        System.out.println(item);
    });
}

运行结果:

User(id=1, username=Tom, orders=null, roles=[Role(id=1, roleName=董事长), Role(id=2, roleName=经理)])
User(id=2, username=Jerry, orders=null, roles=[Role(id=1, roleName=董事长), Role(id=2, roleName=经理)])

Mybatis缓存

基础概念

缓存就是内存中的数据,常常来自对数据库查询结果的保存,使用缓存,我们可以避免频繁的与数据库进行交互,进而提高响应速度.

一级缓存是SqlSession,在操作数据库时需要构造sqlSession对象,在对象中有一个数据结构(HashMap)用于存储缓存数据.不同的sqlSession之间互不影响.

二级缓存是mapper级别的缓存,多个sqlSession去操作同一个mapper的sql语句,多个sqlSession可以共用二级缓存,二级缓存是跨sqlSession的.

一级缓存

demo

public class CacheTest {
​
    private IUserMapper mapper;
    private SqlSession sqlSession;
​
    @Before
    public void before() throws IOException {
        //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        //2.解析了配置文件，并创建了sqlSessionFactory工厂
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //3.生产sqlSession                          true:事务自动提交
        sqlSession = sqlSessionFactory.openSession(true);
        mapper = sqlSession.getMapper(IUserMapper.class);
    }
​
    @Test
    public void test1() {
​
        //第一次查询id为1的用户
        User user1 = mapper.getUserById(1);
​
        //第二次查询id为1的用户
        User user2 = mapper.getUserById(1);
​
        System.out.println(user1 == user2);
    }
}

返回结果为 true ;测试一级缓存是默认开启的

结论:第一次发起查询用户id为1的用户信息,先去找缓存中是否有id为1的用户,如果没有,则从数据库查询用户信息,得到用户信息并且将用户信息存储到一级缓存中,第二次发起查询用户id为1的用户信息,先去找缓存中是否有id为1的用户,如果缓存中有,直接从缓存中获取用户信息;

现在我们变换一下上面的demo

public class CacheTest {
​
    private IUserMapper mapper;
    private SqlSession sqlSession;
​
    @Before
    public void before() throws IOException {
        //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        //2.解析了配置文件，并创建了sqlSessionFactory工厂
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //3.生产sqlSession                          true:事务自动提交
        sqlSession = sqlSessionFactory.openSession();
        mapper = sqlSession.getMapper(IUserMapper.class);
    }
​
    @Test
    public void test1() {
​
        //第一次查询id为1的用户
        User user1 = mapper.getUserById(1);
​
        User user = new User();
        user.setId(1);
        user.setUsername("Lucy");
        mapper.updateUser(user);
​
        sqlSession.commit();
​
        //第二次查询id为1的用户
        User user2 = mapper.getUserById(1);
​
        System.out.println(user1 == user2);
    }
}

返回结果为 false

结论:做增删改操作,并进行了事物的提交,就会刷新以及缓存;或者还可以通过 sqlSession.clearCache()清楚缓存;这样做的目的就是为了让缓存中存储的是最新的信息,避免脏读;

二级缓存

二级缓存的原理和一级缓存原理一样,第一次查询会将数据放入缓存中,然后第二次查询则会直接从缓存中获取,但是一级缓存是基于sqlSession的,而二级缓存是基于mapper文件的namespace,也就是说,多个sqlSession可以共享一个mapper中的二级缓存,并且如果两个mapper的namespace相同,即使是两个mapper,那么这两个mapper执行sql查询到的数据也将存在相同的二级缓存区域中.

如何使用二级缓存

首先在全局配置文件sqlMapconfig.xml文件加入如下代码

<settings>
        <setting name="cacheEnabled" value="true"/>
</settings>

注意,该配置需要放在properties标签下,具体顺序,可百度了解

其次,在xxxMapper.xml文件中开启缓存(如果当前操作时基于注解开发的话,使用注解@CacheNamespace)

<cache></cache>

demo2

public class CacheTest {
​
    private IUserMapper mapper;
    private SqlSession sqlSession;
    private SqlSessionFactory sqlSessionFactory;
​
    @Before
    public void before() throws IOException {
        //1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流
        InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml");
        //2.解析了配置文件，并创建了sqlSessionFactory工厂
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //3.生产sqlSession
        sqlSession = sqlSessionFactory.openSession();
        mapper = sqlSession.getMapper(IUserMapper.class);
    }
​
    @Test
    public void test2() {
​
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        SqlSession sqlSession2 = sqlSessionFactory.openSession();
        SqlSession sqlSession3 = sqlSessionFactory.openSession();
​
        IUserMapper mapper1 = sqlSession1.getMapper(IUserMapper.class);
        IUserMapper mapper2 = sqlSession2.getMapper(IUserMapper.class);
        IUserMapper mapper3 = sqlSession3.getMapper(IUserMapper.class);
​
        User user1 = mapper1.getUserById(1);
        //清空一级缓存
        sqlSession1.close();
        User user2 = mapper2.getUserById(1);
​
        System.out.println(user1 == user2);
    }
}

运行结果为 false

结论:通过debug断点显示,实际上,第二次查询则会直接从缓存中获取用户信息了,不过二级缓存缓存的不是对象,而是缓存的对象中的数据,所以查询结果为false;

注意,二级缓存底层还是HashMap结构,所以 po类需要实现序列化接口 ;因为二级缓存数据存储介质多种多样,不一定只存在内存中,有可能存在硬盘中,如果我们要在取这个缓存的话,就需要反序列化了,所以mybatis中的pojo都去实现Serializable接口;

变换一下demo2

@Test
    public void test2() {
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        SqlSession sqlSession2 = sqlSessionFactory.openSession();
        SqlSession sqlSession3 = sqlSessionFactory.openSession();
        IUserMapper mapper1 = sqlSession1.getMapper(IUserMapper.class);
        IUserMapper mapper2 = sqlSession2.getMapper(IUserMapper.class);
        IUserMapper mapper3 = sqlSession3.getMapper(IUserMapper.class);
        User user1 = mapper1.getUserById(1);
        //清空一级缓存
        sqlSession1.close();
        User user = new User();
        user.setId(1);
        user.setUsername("Tom");
        mapper3.updateUser(user);
        sqlSession3.commit();
        User user2 = mapper2.getUserById(1);
        System.out.println(user1 == user2);
    }

返回结果为 false

结论:做增删改操作,并进行了事物的提交,就会刷新以及缓存;这样做的目的就是为了让缓存中存储的是最新的信息,避免脏读;

此外mybatis中还可以配置useCache和flushCache等配置项;

useCache

是用来设置是否禁用二级缓存的,在statement中设置useCache=false可以禁用当前select语句的二级缓存,即每次查询都会会发出sql去查询,默认情况是true,即该sql使用二级缓存,例如

<select id="findAll" useCache = "false" resultMap="userMap">
        select u.*,o.id oid,o.order_time,o.total,o.uid from user u left join orders o on u.id = o.uid
</select>

使用sql注解方式可以使用@Options(useCache = false)的方式

flushCache

在mapper的同一个namespace中,如果有其他的insert,update,delete操作数据后需要刷新缓存,如果不执行刷新缓存会出现脏读,设置statememt配置中的

flushCache = "true"属性,默认情况下为true,即刷新缓存,如果改成false则不会刷新,使用缓存时如果手动修改数据库表中的查询数据会出现脏读 例如

<select id="findAll" flushCache = "true" useCache = "false" resultMap="userMap">
        select u.*,o.id oid,o.order_time,o.total,o.uid from user u left join orders o on u.id = o.uid
</select>

一般下执行完commit操作都需要刷新缓存,flushCache=true表示刷新缓存,这样可以避免数据库脏读,所以我们不用设置,默认即可

至此,mybatis基础概念及应用回顾完成!

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