MyBatis的缓存！！！！

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
Mybatis中的一级缓存和二级缓存？
- 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。
- 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（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;/*** <p>Project: mybatis - UserMapper</p>* <p>Powered by scl On 2023-12-22 15:52:05</p>* <p>描述：<p>** @author 孙臣龙 [1846080280@qq.com]* @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：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.by.mapper.UserMapper"><select id="getUserById" parameterType="int" resultType="user">select *from userwhere id = #{id}</select><delete id="deleteUserById" parameterType="int">deletefrom userwhere id = #{id};</delete>
</mapper>

（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;/*** <p>Project: mybatis - MyBatisTest</p>* <p>Powered by scl On 2023-12-18 11:44:53</p>* <p>描述：<p>** @author 孙臣龙 [1846080280@qq.com]* @version 1.0* @since 17*/
public class MyBatisTestCache {private InputStream inputStream;private SqlSession sqlSession;@Beforepublic 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*/@Testpublic 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*/@Testpublic 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*/@Testpublic void testFirstNoCache2() throws IOException {String resource = "mybatis-config.xml";InputStream inputStream = Resources.getResourceAsStream(resource);// 创建sqlSessionFActorySqlSessionFactory 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);}@Afterpublic 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：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.by.mapper.UserMapper"><!--局部开启二级缓存--><cache></cache><select id="getUserById" parameterType="int" resultType="user">select *from userwhere id = #{id}</select><delete id="deleteUserById" parameterType="int">deletefrom userwhere id = #{id};</delete>
</mapper>

（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;/*** <p>Project: mybatis - MyBatisTest</p>* <p>Powered by scl On 2023-12-18 11:44:53</p>* <p>描述：<p>** @author 孙臣龙 [1846080280@qq.com]* @version 1.0* @since 17*/
public class MyBatisTestCache2 {private InputStream inputStream;private SqlSession sqlSession;@Beforepublic 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*/@Testpublic 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*/@Testpublic 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*/@Testpublic void testSecondNoCache2() throws IOException {String resource = "mybatis-config.xml";InputStream inputStream1 = Resources.getResourceAsStream(resource);InputStream inputStream2 = Resources.getResourceAsStream(resource);//工厂1SqlSessionFactory sessionFactory1 = new SqlSessionFactoryBuilder().build(inputStream1);SqlSession sqlSession1 = sessionFactory1.openSession();UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);//工厂2SqlSessionFactory 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);}@Afterpublic 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：<cache></cache>
        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

<settings><!-- 开启二级缓存的支持 --><setting name="cacheEnabled" value="true"/>
</settings>

2.配置相关的Mapper映射文件开启局部缓存（必须有）

 <!-- 开启二级缓存的支持 --><cache></cache>