Mybatis基础概念与高级应用小结

2022-11-13 10:11:05 概念高级小结

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 {

    
    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 {

    
    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 {

    
    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 {

    
    @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 {

    
    @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 {
    
    
    @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!

到此这篇关于Mybatis基础回顾与高级应用的文章就介绍到这了,更多相关Mybatis高级应用内容请搜索以前的文章或继续浏览下面的相关文章希望大家以后多多支持！

相关文章

Mybatis基础概念与高级应用小结

目录

Mybatis基础回顾与高级应用

引入依赖

jdbc.properties

sqlMapConfig.xml

案例一-查询用户

案例二-添加用户

案例三-编辑用户

案例四-删除用户

传统开发方式

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)

动态sql语句 if标签

Mybatis复杂映射

一对一

一对多

多对多

Mybatis注解开发

Mybasits常用注解:

Mybatis注解实现复杂映射开发

Mybatis缓存

基础概念

一级缓存

返回结果为 true ;测试一级缓存是默认开启的

返回结果为 false

二级缓存

如何使用二级缓存

返回结果为 false