缓存即为存在内存中的临时数据.将用户经常查询的数据存放在缓存(内存)中,用户去查询数据就不用去每次去数据库中查询,而是去缓存中查询,从而提高了查询的效率,解决了高并发系统的性能问题.MyBatis提供了两种缓存机制：一级缓存（本地）和二级缓存（全局）
1、默认情况下，只有一级缓存( SqlSession级别的缓存也称为本地缓存)开启。
2、二级缓存需要手动开启和配置，他是基于namespace级别的缓存。
3、为了提高扩展性。MyBatis定义了缓存接口Cache。我们可以通过实现Cache接口来自定义二级缓存

1. 基础环境

1.1 数据库准备

USE mybatis;
# 创建一个名称为tb_book的表
CREATE TABLE  tb_book( id INT PRIMARY KEY AUTO_INCREMENT,name VARCHAR(255),price double,author VARCHAR(40)
);
# 插入3条数据
INSERT INTO tb_book(name,price,author) VALUES('Java基础入门',45.0,'Liy');
INSERT INTO tb_book(name,price,author) VALUES('Java基础案例教程',48.0,'Tom');
INSERT INTO tb_book(name,price,author) VALUES('JavaWeb程序设计任务教程',50.0,'Jack');

1.2 Idea环境搭建

1.2.1 创建一个cache的module

1.2.2 在pom.xml中引入所需要的包

mybatis, mysql-connector-java, junit, log4j, lombok

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><parent><artifactId>mybatis-series</artifactId><groupId>com.biem</groupId><version>1.0-SNAPSHOT</version></parent><modelVersion>4.0.0</modelVersion><artifactId>cache</artifactId><properties><maven.compiler.source>8</maven.compiler.source><maven.compiler.target>8</maven.compiler.target></properties><dependencies><dependency><groupId>org.mybatis</groupId><artifactId>mybatis</artifactId></dependency><dependency><groupId>junit</groupId><artifactId>junit</artifactId><scope>test</scope></dependency><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId></dependency><dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.16</version></dependency></dependencies>
</project>

1.2.3 创建包：com.biem.pojo, com.biem.mapper, com.biem.util

1.2.4 在resource下添加配置文件

1.2.4.1 添加配置文件jdbc.properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC
jdbc.username=root
jdbc.password=root

1.2.4.2 添加配置文件log4j.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd"><log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/"><appender name="STDOUT" class="org.apache.log4j.ConsoleAppender"><param name="Encoding" value="UTF-8"/><layout class="org.apache.log4j.PatternLayout"><param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS} %m (%F:%L) \n"/></layout></appender><logger name="java.sql"><level value="debug"/></logger><logger name="org.apache.ibatis"><level value="info"/></logger><root><level value="debug"/><appender-ref ref="STDOUT"/></root>
</log4j:configuration>

1.2.4.3 添加MyBatis的核心配置类mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><!--引入properties文件--><properties resource="jdbc.properties"></properties><!--将下划线映射为驼峰--><settings><setting name="mapUnderscoreToCamelCase" value="true"/></settings><!--设置类型别名--><typeAliases><package name="com.biem.pojo"/></typeAliases><environments default="development"><environment id="development"><transactionManager type="JDBC"/><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><!-- 引入映射文件 --><mappers><package name="com.biem.mapper"/></mappers></configuration>

1.2.4.4 创建映射文件所在的目录

1.2.5 添加工具类com.biem.util.MyBatisUtil.java

package com.biem.util;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 java.io.IOException;
import java.io.InputStream;/*** ClassName: MybatisUtil* Package: com.biem.util* Description:** @Create 2023/4/5 22:23* @Version 1.0*/
public class MyBatisUtil {//利用static(静态)属于类不属于对象,且全局唯一private static SqlSessionFactory sqlSessionFactory = null;//利用静态块在初始化类时实例化sqlSessionFactorystatic {InputStream is= null;try {is = Resources.getResourceAsStream("mybatis-config.xml");sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);} catch (IOException e) {e.printStackTrace();throw new ExceptionInInitializerError(e);}}/*** openSession 创建一个新的SqlSession对象* @return SqlSession对象*/public static SqlSession openSession(boolean autoCommit){return sqlSessionFactory.openSession(autoCommit);}public static SqlSession openSession(){return sqlSessionFactory.openSession();}/*** 释放一个有效的SqlSession对象* @param session 准备释放SqlSession对象*/public static void closeSession(SqlSession session){if(session != null){session.close();}}
}

1.2.6 实体类com.biem.pojo.Book.java

package com.biem.pojo;import lombok.*;/*** ClassName: Book* Package: com.biem.pojo* Description:** @Create 2023/5/18 7:58* @Version 1.0*/
@Getter
@Setter
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class Book {private Integer id;private String name;private Integer price;private String author;
}

1.2.7 mapper接口

package com.biem.mapper;/*** ClassName: BookMapper* Package: com.biem.mapper* Description:** @Create 2023/5/18 8:01* @Version 1.0*/
public interface BookMapper {}

1.2.7 mapper映射文件

<?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.biem.mapper.BookMapper"><!-- sql语句要和接口的方法名保持一致 --></mapper>

2. 一级缓存（本地）

MyBatis的一级缓存是SqlSession级别的缓存。
如果同一个SqlSession对象多次执行完全相同的SQL语句，在第一次执行完成后，MyBatis会将查询的结果写入到一级缓存中。此后如果程序没有执行插入、更新、删除操作。当第二次执行相同的查询语句时，MyBatis会直接读取一级缓存中的数据，而不在去数据库查询，从而提高了数据库的查询效率。
测试案例1：
从数据库tb_book查询id为1的图书信息的时候，当第一次查询后会将结果写入MyBatis的一级缓存，当程序第二次查询id为1的图书信息，MyBatis会直接从一级缓存中读取。

2.1 接口添加方法

package com.biem.mapper;import com.biem.pojo.Book;public interface BookMapper {public Book findBookById(Integer id);
}

2.2 映射文件添加实现

<?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.biem.mapper.BookMapper"><!-- sql语句要和接口的方法名保持一致 --><!--public Book findBookById(Integer id);--><select id="findBookById" parameterType="Integer" resultType="Book">select * from tb_book where id=#{id}</select>
</mapper>

2.3 测试方法

package com.biem.test;import com.biem.mapper.BookMapper;
import com.biem.pojo.Book;
import com.biem.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;/*** ClassName: UserTest* Package: com.biem.test* Description:** @Create 2023/5/18 8:44* @Version 1.0*/
public class UserTest {@Testpublic void findBookById(){SqlSession session = MyBatisUtil.openSession();BookMapper mapper = session.getMapper(BookMapper.class);Book book = mapper.findBookById(1);System.out.println("book = " + book);session.close();}@Testpublic void testCache1(){SqlSession session = MyBatisUtil.openSession();BookMapper mapper1 = session.getMapper(BookMapper.class);Book book1 = mapper1.findBookById(1);System.out.println("book1 = " + book1);BookMapper mapper2 = session.getMapper(BookMapper.class);Book book2 = mapper2.findBookById(1);System.out.println("book1 = " + book2);session.close();}
}

2.4 结果分析

findBookById输出如下：

DEBUG 05-18 09:07:47,909 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:07:47,986 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:07:48,047 <==      Total: 1 (BaseJdbcLogger.java:137)
book = Book(id=1, name=Java基础入门, price=45, author=Liy)

testCache1输出如下

DEBUG 05-18 09:07:48,056 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:07:48,057 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:07:48,059 <==      Total: 1 (BaseJdbcLogger.java:137)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)

从上述输出可以看出testCache1只是查询了一次，第二次查询的时候并没有连接数据库
testCache1fail输出如下

DEBUG 05-18 09:14:30,875 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:14:30,953 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:14:31,015 <==      Total: 1 (BaseJdbcLogger.java:137)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)
DEBUG 05-18 09:14:31,059 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:14:31,060 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:14:31,065 <==      Total: 1 (BaseJdbcLogger.java:137)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)

3. 二级缓存（全局）

3.1 开启缓存前配置

3.1.1 实体类实现Serializable 接口

package com.biem.pojo;import lombok.*;import java.io.Serializable;@Getter
@Setter
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class Book implements Serializable {private Integer id;private String name;private Integer price;private String author;
}

3.1.2 mybatis-config.xml中开启二级缓存全局配置

mybatis-config.xml中添加

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

3.1.3 开启当前mapper下的二级缓存BookMapper.xml

BookMapper.xml添加

<cache></cache>

3.2 添加测试方法UserTest.java

    @Testpublic void testCache2_0(){SqlSession session1 = MyBatisUtil.openSession();BookMapper mapper1 = session1.getMapper(BookMapper.class);Book book1 = mapper1.findBookById(1);System.out.println("book1 = " + book1);SqlSession session2 = MyBatisUtil.openSession();BookMapper mapper2 = session2.getMapper(BookMapper.class);Book book2 = mapper2.findBookById(1);System.out.println("book1 = " + book2);session1.close();session2.close();}@Testpublic void testCache2_1(){SqlSession session1 = MyBatisUtil.openSession();BookMapper mapper1 = session1.getMapper(BookMapper.class);Book book1 = mapper1.findBookById(1);System.out.println("book1 = " + book1);session1.close();SqlSession session2 = MyBatisUtil.openSession();BookMapper mapper2 = session2.getMapper(BookMapper.class);Book book2 = mapper2.findBookById(1);System.out.println("book1 = " + book2);session2.close();}

3.3结果分析

testCache2_1进行了一次查询，结果如下：

DEBUG 05-18 09:40:00,228 Cache Hit Ratio [com.biem.mapper.BookMapper]: 0.0 (LoggingCache.java:60)
DEBUG 05-18 09:40:02,793 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:40:02,867 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:40:02,944 <==      Total: 1 (BaseJdbcLogger.java:137)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)
WARN  05-18 09:40:02,960 As you are using functionality that deserializes object streams, it is recommended to define the JEP-290 serial filter. Please refer to https://docs.oracle.com/pls/topic/lookup?ctx=javase15&id=GUID-8296D8E8-2B93-4B9A-856E-0A65AF9B8C66 (SerialFilterChecker.java:46)
DEBUG 05-18 09:40:02,968 Cache Hit Ratio [com.biem.mapper.BookMapper]: 0.5 (LoggingCache.java:60)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)

testCache2_2进行了一次查询，结果如下：

DEBUG 05-18 09:41:38,669 Cache Hit Ratio [com.biem.mapper.BookMapper]: 0.0 (LoggingCache.java:60)
DEBUG 05-18 09:41:41,356 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:41:41,433 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:41:41,504 <==      Total: 1 (BaseJdbcLogger.java:137)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)
WARN  05-18 09:41:41,523 As you are using functionality that deserializes object streams, it is recommended to define the JEP-290 serial filter. Please refer to https://docs.oracle.com/pls/topic/lookup?ctx=javase15&id=GUID-8296D8E8-2B93-4B9A-856E-0A65AF9B8C66 (SerialFilterChecker.java:46)
DEBUG 05-18 09:41:41,529 Cache Hit Ratio [com.biem.mapper.BookMapper]: 0.5 (LoggingCache.java:60)
book2 = Book(id=1, name=Java基础入门, price=45, author=Liy)
DEBUG 05-18 09:41:41,531 Cache Hit Ratio [com.biem.mapper.BookMapper]: 0.6666666666666666 (LoggingCache.java:60)
book3 = Book(id=1, name=Java基础入门, price=45, author=Liy)

testCache2_0和testCache1一样的内容，输出结果如下：

DEBUG 05-18 09:39:11,470 Cache Hit Ratio [com.biem.mapper.BookMapper]: 0.0 (LoggingCache.java:60)
DEBUG 05-18 09:39:14,368 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:39:14,501 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:39:14,567 <==      Total: 1 (BaseJdbcLogger.java:137)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)
DEBUG 05-18 09:39:14,580 Cache Hit Ratio [com.biem.mapper.BookMapper]: 0.0 (LoggingCache.java:60)
DEBUG 05-18 09:39:14,656 ==>  Preparing: select * from tb_book where id=? (BaseJdbcLogger.java:137)
DEBUG 05-18 09:39:14,656 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:137)
DEBUG 05-18 09:39:14,659 <==      Total: 1 (BaseJdbcLogger.java:137)
book1 = Book(id=1, name=Java基础入门, price=45, author=Liy)

testCache2_0依然进行了两次查询，session.close()执行之前并没有刷新缓存。

备注：如果使用annotation方式，仅需要在***Mapper.class上添加@CacheNamespace(blocking = true)

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