MyBatis的缓存！！！！
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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
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Mybatis中的一级缓存和二级缓存？
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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。
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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。
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1.一级缓存（默认开启）
(1) 首先在UserMapper接口定义两个方法：
package com.by.mapper;
import com.by.pojo.User;
import org.apache.ibatis.annotations.Param;
import org.junit.Test;
import java.util.List;
/**
* Project: mybatis - UserMapper
* Powered by scl On 2023-12-22 15:52:05
* 描述：
*
* @author 孙臣龙 [[email protected]]
* @version 1.0
* @since 17
*/
public interface UserMapper {
User getUserById(Integer id);
void deleteUserById(Integer id);
}
（2）在UserMapper.xml文件中实现这两个方法：
delete
from user
where id = #{id};
（3）测试类：
/*
* Copyright (c) 2020, 2023, All rights reserved.
*
*/
package com.by;
import com.by.mapper.UserMapper;
import com.by.pojo.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;
/**
* Project: mybatis - MyBatisTest
* Powered by scl On 2023-12-18 11:44:53
* 描述：
*
* @author 孙臣龙 [[email protected]]
* @version 1.0
* @since 17
*/
public class MyBatisTestCache {
private InputStream inputStream;
private SqlSession sqlSession;
@Before
public void init() throws IOException {
加载配置文件
//String resource = "mybatis-config.xml";
//inputStream = Resources.getResourceAsStream(resource);
//
创建sqlSessionFActory
//SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//
获得数据的绘画实例
//sqlSession = sessionFactory.openSession();
}
/**
* 一级缓存
*
* @throws IOException
*/
@Test
public void testFirstGoCache() {
UserMapper userMapper1 = sqlSession.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession.getMapper(UserMapper.class);
System.out.println("=========One===========");
User user1 = userMapper1.getUserById(42); // 执行sql语句
System.out.println(user1);
System.out.println("=========Two===========");
User user2 = userMapper2.getUserById(42); // 不执行sql语句，走缓存
System.out.println(user2);
}
/**
* 一级缓存,不走缓存，同一个sqlSession，中间执行了增删改
*
* @throws IOException
*/
@Test
public void testFirstNoCache() {
UserMapper userMapper1 = sqlSession.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession.getMapper(UserMapper.class);
System.out.println("=========One===========");
User user1 = userMapper1.getUserById(42); // 执行sql语句
System.out.println(user1);
System.out.println("========同一个sqlsession之间执行增删改========");
userMapper1.deleteUserById(41);
sqlSession.commit();
System.out.println("=========Two===========");
User user2 = userMapper2.getUserById(42); // 执行sql语句
System.out.println(user2);
}
/**
* 一级缓存，不走缓存，不同的sqlSession
*
* @throws IOException
*/
@Test
public void testFirstNoCache2() throws IOException {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
// 创建sqlSessionFActory
SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession1 = sessionFactory.openSession();
UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
// 获得数据的绘画实例
SqlSession sqlSession2 = sessionFactory.openSession();
UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
System.out.println("=========One===========");
User user1 = userMapper1.getUserById(42); // 执行sql语句
System.out.println(user1);
System.out.println("=========Two===========");
User user2 = userMapper2.getUserById(42); // 执行sql语句
System.out.println(user2);
}
@After
public void close() throws IOException {
//inputStream.close();
//sqlSession.close();
}
}
2.二级缓存（需要手动开启）
（1）在UserMapper接口中创建两个方法：同上
（2）在UserMapper.xml文件中实现这两个方法：
delete
from user
where id = #{id};
（3）测试类：
/*
* Copyright (c) 2020, 2023, All rights reserved.
*
*/
package com.by;
import com.by.mapper.UserMapper;
import com.by.pojo.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
/**
* Project: mybatis - MyBatisTest
* Powered by scl On 2023-12-18 11:44:53
* 描述：
*
* @author 孙臣龙 [[email protected]]
* @version 1.0
* @since 17
*/
public class MyBatisTestCache2 {
private InputStream inputStream;
private SqlSession sqlSession;
@Before
public void init() throws IOException {
加载配置文件
//String resource = "mybatis-config.xml";
//inputStream = Resources.getResourceAsStream(resource);
//
创建sqlSessionFActory
//SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//
获得数据的绘画实例
//sqlSession = sessionFactory.openSession();
}
/**
* 二级缓存
*
* @throws IOException
*/
@Test
public void testSecondGoCache() {
UserMapper userMapper1 = sqlSession.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession.getMapper(UserMapper.class);
System.out.println("=========One===========");
User user1 = userMapper1.getUserById(42); // 执行sql语句
System.out.println(user1);
sqlSession.commit();
System.out.println("=========Two===========");
User user2 = userMapper2.getUserById(42); // 不执行sql语句，走缓存
System.out.println(user2);
}
/**
* 二级缓存,不走缓存，两个sql之间执行增删改
*
* @throws IOException
*/
@Test
public void testSecondNoCache() {
UserMapper userMapper1 = sqlSession.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession.getMapper(UserMapper.class);
System.out.println("=========One===========");
User user1 = userMapper1.getUserById(42); // 执行sql语句
System.out.println(user1);
sqlSession.commit();
System.out.println("************增删改**************");
userMapper1.deleteUserById(41);
sqlSession.commit();
System.out.println("=========Two===========");
User user2 = userMapper2.getUserById(42); // 执行sql语句
System.out.println(user2);
}
/**
* 二级缓存,不走缓存，不同的sqlSessionFactory
*
* @throws IOException
*/
@Test
public void testSecondNoCache2() throws IOException {
String resource = "mybatis-config.xml";
InputStream inputStream1 = Resources.getResourceAsStream(resource);
InputStream inputStream2 = Resources.getResourceAsStream(resource);
//工厂1
SqlSessionFactory sessionFactory1 = new SqlSessionFactoryBuilder().build(inputStream1);
SqlSession sqlSession1 = sessionFactory1.openSession();
UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
//工厂2
SqlSessionFactory sessionFactory2 = new SqlSessionFactoryBuilder().build(inputStream2);
SqlSession sqlSession2 = sessionFactory2.openSession();
UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
System.out.println("=========One===========");
User user1 = userMapper1.getUserById(42); // 执行sql语句
System.out.println(user1);
sqlSession1.commit();
System.out.println("=========Two===========");
User user2 = userMapper2.getUserById(42); // 执行sql语句
System.out.println(user2);
}
@After
public void close() throws IOException {
//inputStream.close();
//sqlSession.close();
}
}
总结：
1、一级缓存
范围：一级缓存范围是sqlSession
配置：默认开启
走缓存：同一个sqlsession执行同一条sql
不走缓存：不同sqlSession 或 两次查询之间执行了增删改
2、二级缓存
范围：二级缓存范围是sqlSessionFactory
配置：
走缓存：同一个sqlSessionFactrory，sqlsession执行commit或close
不走缓存：不同sqlSessionFactrory 或 两次查询之间执行了增删改
开启缓存：
1.在SqlMapConfig.xml 文件开启二级缓存，默认开启，可以省略
2.配置相关的Mapper映射文件 开启局部缓存（必须有）