MyBatis之缓存机制

约定不等于承诺〃 2024-03-22 18:43 37阅读 0赞

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 ####

![在这里插入图片描述][552e4be211634bbda57d93e2d08a77d8.png]

#### 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 ####

![在这里插入图片描述][c1e95b28ff5a4d5eaff4992fe003c317.png]

#### 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 configuration
            PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
            "http://mybatis.org/dtd/mybatis-3-config.dtd">
    <configuration>
        
        <properties resource="jdbc.properties"></properties>
    
        
        <settings>
            <setting name="mapUnderscoreToCamelCase" value="true"/>
        </settings>
    
        
        <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>
    
        
        <mappers>
            <package name="com.biem.mapper"/>
        </mappers>
        
    </configuration>

##### 1.2.4.4 创建映射文件所在的目录 #####

![在这里插入图片描述][a428c68852bb46c58b739c23aa20831e.png]

#### 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sqlSessionFactory
        static {
        
            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 ####

## 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 mapper
            PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
            "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="com.biem.mapper.BookMapper">
        
        
        <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 {
        
        @Test
        public void findBookById(){
        
            SqlSession session = MyBatisUtil.openSession();
            BookMapper mapper = session.getMapper(BookMapper.class);
            Book book = mapper.findBookById(1);
            System.out.println("book = " + book);
            session.close();
        }
    
        @Test
        public 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

#### 3.1.3 开启当前mapper下的二级缓存BookMapper.xml ####

BookMapper.xml添加

### 3.2 添加测试方法UserTest.java ###

@Test
        public 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();
        }
    
        @Test
        public 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)

[552e4be211634bbda57d93e2d08a77d8.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/94241472fb694fc59d799ad4a2c8a340.png
[c1e95b28ff5a4d5eaff4992fe003c317.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/f02e7b8526264b89a7f22f6795d24316.png
[a428c68852bb46c58b739c23aa20831e.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/dff45de0e12146dcadb9442956b6fb0c.png