MyBatis的缓存

一、缓存介绍

1.为什么使用缓存？

首次访问时，查询数据库，并将数据存储到内存中；再次访问时直接访问缓存，减少IO、硬盘读写次数、提高效率

2.Mybatis中的一级缓存和二级缓存

(1)一级缓存

它指的是mybatis中的SqlSession对象的缓存。当我们执行完查询之后，查询的结果会同时存在在SqlSession为我们提供的一块区域中。当我们再次查询同样的数据，mybatis会先去SqlSession中查询是否有，有的话直接拿出来使用。当SqlSession对象消失时，Mybatis的一级缓存也就消失了。

(2)二级缓存

它指的是Mybatis中SqlSessionFactory对象的缓存，由同一个SqlSessioFactory对象创建的SqlSession共享其缓存。

 

1、一级缓存代码实现 

1.mapper

 public User findUserById(Integer id);

    
    
        select  * from  user where id=#{id}
    

2.测试一级缓存

public class MyBatisTest {
    private InputStream inputStream;
    private SqlSession sqlSession;
    private SqlSessionFactory sqlSessionFactory;
    @Before
    public void before() throws IOException {
        String rescource = "mybatis-config.xml";
        //加载配置文件
        inputStream =Resources.getResourceAsStream(rescource);
        //创建sqlSessionFactory
        sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
        //获得数据的会话实例
        sqlSession=sqlSessionFactory.openSession();
    }

    @Test
    public void testFirstCache(){
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        UserMapper userMapper = sqlSession1.getMapper(UserMapper.class);
        User userParam1 = userMapper.findUserById(41);//执行查询
        System.out.println("================第一次查询=============");
        System.out.println("第一次查询"+userParam1);
        User userParam2= userMapper.findUserById(41);//不执行查询
        System.out.println("================第二次查询=============");
        System.out.println("第二次查询"+userParam2);

        SqlSession sqlSession2= sqlSessionFactory.openSession();
        UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
        User userParam3= userMapper2.findUserById(41);//执行查询
        System.out.println("================第三次查询=============");
        System.out.println("第三次查询"+userParam3);


    }


    @After
    public void close() throws IOException {
        /**
         * 关闭资源
         */
      //  sqlSession.close();
     //   inputStream.close();
    }

}

3.结果

 2、一级缓存的分析

一级缓存是SqlSession范围的缓存，当调用SqlSession的commit()，close()等方法时，就会清空一级缓存。

 

1. 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 的用户信息，如果没有，从数据库查询用户信息。 得到用户信息，将用户信息存储到一级缓存中。

2. 如果sqlSession去执行 commit操作（执行插入、更新、删除），清空 SqlSession 中的一级缓存，这样做的目的为了让缓存中存储的是最新的信息，避免脏读。

3. 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的用户信息，缓存中有，直接从缓存中获取用户信息。

3、测试清空一级缓存

 清空缓存前

public class MyBatisTest {
    private InputStream inputStream;
    private SqlSession sqlSession;
    private SqlSessionFactory sqlSessionFactory;
    @Before
    public void before() throws IOException {
        String rescource = "mybatis-config.xml";
        //加载配置文件
        inputStream =Resources.getResourceAsStream(rescource);
        //创建sqlSessionFactory
        sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
        //获得数据的会话实例
        sqlSession=sqlSessionFactory.openSession();
    }

    @Test
    public void testFirstCache(){
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        UserMapper userMapper = sqlSession1.getMapper(UserMapper.class);
        User userParam1 = userMapper.findUserById(41);//执行查询
        System.out.println("================第一次查询=============");
        System.out.println("第一次查询"+userParam1);
        User userParam2= userMapper.findUserById(41);//不执行查询
        System.out.println("================第二次查询=============");
        System.out.println("第二次查询"+userParam2);
//        sqlSession1.commit();

//        SqlSession sqlSession2= sqlSessionFactory.openSession();
//        UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
        User userParam3= userMapper.findUserById(41);
        System.out.println("================第三次查询=============");
        System.out.println("第三次查询"+userParam3);


    }


    @After
    public void close() throws IOException {
        /**
         * 关闭资源
         */
       // sqlSession.close();
       // inputStream.close();
    }

}

清空缓存后

public class MyBatisTest {
    private InputStream inputStream;
    private SqlSession sqlSession;
    private SqlSessionFactory sqlSessionFactory;
    @Before
    public void before() throws IOException {
        String rescource = "mybatis-config.xml";
        //加载配置文件
        inputStream =Resources.getResourceAsStream(rescource);
        //创建sqlSessionFactory
        sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
        //获得数据的会话实例
        sqlSession=sqlSessionFactory.openSession();
    }

    @Test
    public void testFirstCache(){
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        UserMapper userMapper = sqlSession1.getMapper(UserMapper.class);
        User userParam1 = userMapper.findUserById(41);//执行查询
        System.out.println("================第一次查询=============");
        System.out.println("第一次查询"+userParam1);
        User userParam2= userMapper.findUserById(41);//不执行查询
        System.out.println("================第二次查询=============");
        System.out.println("第二次查询"+userParam2);
        sqlSession1.commit();

//        SqlSession sqlSession2= sqlSessionFactory.openSession();
//        UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
        User userParam3= userMapper.findUserById(41);
        System.out.println("================第三次查询=============");
        System.out.println("第三次查询"+userParam3);


    }


    @After
    public void close() throws IOException {
        /**
         * 关闭资源
         */
      //  sqlSession.close();
     //   inputStream.close();
    }

}

 

 二、二级缓存

1.pojo

注意：当我们在使用二级缓存时，所缓存的类一定要实现java.io.Serializable接口，这种就可以使用序列化方式来保存对象。  

public class User implements Serializable {
    private Integer id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;
//get/set省略...

2.开启二级缓存

 (1)在SqlMapConfig.xml 文件开启二级缓存

(2)配置相关的Mapper映射文件

(3)mapper

 public User findUserById2(Integer id);

    
    
    
 
        select  * from  user where id=#{id}
    

3.测试

public class MyBatisTest {
    private InputStream inputStream;
    private SqlSession sqlSession;
    private SqlSessionFactory sqlSessionFactory;
    @Before
    public void before() throws IOException {
        String rescource = "mybatis-config.xml";
        //加载配置文件
        inputStream =Resources.getResourceAsStream(rescource);
        //创建sqlSessionFactory
        sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
        //获得数据的会话实例
        sqlSession=sqlSessionFactory.openSession();
    }
 @Test
    public void testSecondUserById(){
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
        User user1 = userMapper1.findUserById2(41);//执行查询
        System.out.println("第一次查询:" + user1);
        sqlSession1.commit();//二级缓存在sqlSession.commit()或者sqlSession.close()之后生效

        SqlSession sqlSession2 = sqlSessionFactory.openSession();
        UserMapper userMapper2= sqlSession2.getMapper(UserMapper.class);
        User user2 =userMapper2.findUserById2(41);//不执行查询
        System.out.println("第二次查询:" + user2);
    }

    @After
    public void close() throws IOException {
        /**
         * 关闭资源
         */
//        sqlSession.close();
//        inputStream.close();
    }

}

4.结果

 5.二级缓存分析

二级缓存是mapper映射级别的缓存，多个SqlSession去操作同一个Mapper映射的sql语句，多个SqlSession可以共用二级缓存，二级缓存是跨SqlSession的。

二级缓存结构图:

6.测试清空二级缓存 

    @Test
    public void testSecondUserById(){
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
        User user1 = userMapper1.findUserById2(41);//执行查询
        System.out.println("第一次查询:" + user1);
        sqlSession1.commit();//二级缓存在sqlSession.commit()或者sqlSession.close()之后生效

        SqlSession sqlSession2 = sqlSessionFactory.openSession();
        UserMapper userMapper2= sqlSession2.getMapper(UserMapper.class);
        User user2 =userMapper2.findUserById2(41);//不执行查询
        System.out.println("第二次查询:" + user2);
        sqlSession2.commit();

        SqlSession sqlSession3 = sqlSessionFactory.openSession();
        UserMapper userMapper3= sqlSession3.getMapper(UserMapper.class);
        User user3 =userMapper3.findUserById2(41);//不执行查询
        System.out.println("第三次查询:" + user3);
        sqlSession3.commit();
        sqlSession3.close();
    }

结果