Mybatis缓存的机制解析与二级缓存的详细分析-----Mybatis框架

淡淡的烟草味﹌ 2024-03-02 08:03 23阅读 0赞

>     内存是临时存储数据的空间,断电后消失
>     硬盘:持久化的文件存储在硬盘当中,关机后文件仍旧保留
>     实际上,各大关系型数据库的数据都是存储在文件当中的
>     各大关系型数据库的数据都是存储在文件中的,mysql的信息通过orm映射成java对象
>     Mybatis的缓存机制,将SQL如select * from t_car where id = 1;
>     执行DQL语句的时候将查询结果放到缓存中,如果下次还是执行完全相同的DQL,就直接从缓存拿数据
>     目的在于提高查询效率和执行效率,减少查询数据库的次数
>     通过了减少IO的方式提速,IO:读文件和写文件
>     如果缓存读取的文件被修改了,那么mybatis就会自动清除缓存中保留的数据
>     缓存通常是我们程序开发中优化程序的重要手段,我们常用的缓存技术如下
>     字符串常量池(直接保存在字符串常量池,不需要重新创建)整数型常量池
>     线程池(创建多个线程放在里面)数据库连接池
>     在实际的生产环境下,多方面优化,缓存就是优化的一个方面,数据库表的索引的优化等等
>     缓存:提前把数据放到缓存中(内存),下一次用的时候,可以直接从缓存中取出来,提升了访问效率cache
>     mybatis提供了两级缓存
>     一级缓存,将数据存储到SqlSession中,二级缓存,将数据存储到SqlSessionFactory中
>     还可以集成其他第三方的缓存,比如EthCache(java开发的),MemCache(C语言开发的)
>     查询语句的信息存到SqlSession中(当前的一次会话),可以理解为一级缓存,如果存到SqlSessionFactory中,代表着一个环境(数据库environment)
>     如果执行的多个SQL都是一样的就直接从这个SqlSession中取信息,这就是一级缓存
>     缓存只针对DQL语句,DML语句必须连接数据库
>     默认情况下,Mybatis的二级缓存是开启的,只需要在对应的SqlMapper下添加标签即可
>     
>     除此以外,被查询出的结果对象还必须实现Serializable(序列化接口)
>     让它得以序列化才能实现
>     SqlSession对象被关闭或提交后,一级缓存的数据才会被写入到二级缓存,此时二级缓存才可用
>     当我们启用了二级缓存以后,二级缓存会引出一个缓存命中率的信息
> 
>     内存是临时存储数据的空间,断电后消失
>     硬盘:持久化的文件存储在硬盘当中,关机后文件仍旧保留
>     实际上,各大关系型数据库的数据都是存储在文件当中的
>     各大关系型数据库的数据都是存储在文件中的,mysql的信息通过orm映射成java对象
>     Mybatis的缓存机制,将SQL如select * from t_car where id = 1;
>     执行DQL语句的时候将查询结果放到缓存中,如果下次还是执行完全相同的DQL,就直接从缓存拿数据
>     目的在于提高查询效率和执行效率,减少查询数据库的次数
>     通过了减少IO的方式提速,IO:读文件和写文件
>     如果缓存读取的文件被修改了,那么mybatis就会自动清除缓存中保留的数据
>     缓存通常是我们程序开发中优化程序的重要手段,我们常用的缓存技术如下
>     字符串常量池(直接保存在字符串常量池,不需要重新创建)整数型常量池
>     线程池(创建多个线程放在里面)数据库连接池
>     在实际的生产环境下,多方面优化,缓存就是优化的一个方面,数据库表的索引的优化等等
>     缓存:提前把数据放到缓存中(内存),下一次用的时候,可以直接从缓存中取出来,提升了访问效率cache
>     mybatis提供了两级缓存
>     一级缓存,将数据存储到SqlSession中,二级缓存,将数据存储到SqlSessionFactory中
>     还可以集成其他第三方的缓存,比如EthCache(java开发的),MemCache(C语言开发的)
>     查询语句的信息存到SqlSession中(当前的一次会话),可以理解为一级缓存,如果存到SqlSessionFactory中,代表着一个环境(数据库environment)
>     如果执行的多个SQL都是一样的就直接从这个SqlSession中取信息,这就是一级缓存
>     缓存只针对DQL语句,DML语句必须连接数据库
>     默认情况下,Mybatis的二级缓存是开启的,只需要在对应的SqlMapper下添加标签即可
>     
>     除此以外,被查询出的结果对象还必须实现Serializable(序列化接口)
>     让它得以序列化才能实现
>     SqlSession对象被关闭或提交后,一级缓存的数据才会被写入到二级缓存,此时二级缓存才可用
>     当我们启用了二级缓存以后,二级缓存会引出一个缓存命中率的信息

>     package com.powernode.mybatis.Test;
>     
>     import com.powernode.mybatis.mappers.CarMapper;
>     import com.powernode.mybatis.pojo.Car;
>     import com.powernode.mybatis.utils.SqlSessionUtils;
>     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.Test;
>     import org.slf4j.Logger;
>     import org.slf4j.LoggerFactory;
>     
>     public class CarMapperTest
>     {
>         private static final Logger logger = LoggerFactory.getLogger(CarMapperTest.class);
>         @Test
>         public void TestselectById() throws Exception
>         {
>             SqlSession sqlSession = SqlSessionUtils.openSession();
>             CarMapper mapper = sqlSession.getMapper(CarMapper.class);
>             Car car = mapper.selectById(3L);
>             logger.info(car.toString());
>             //两个Mapper也只执行一次DQL语句,充分说明了DQL语句只会执行一次在一个SqlSession会话中
>             //和反射生成出的调用方法的对象无关
>             //增加一个sqlSession就不再是执行一次了,因为目前是在一级缓存SqlSession中调用的
>             SqlSession session = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml")).openSession();
>             CarMapper m = session.getMapper(CarMapper.class);
>             //不再执行SQL语句了
>             Car c = m.selectById(3L);
>             //不走缓存的情况:不是同一个SqlSession会话对象,不是同一条SQL(查询条件不一样)
>             //第一次DQL和第二次DQL之间以下两个操作任意出现一个都会清空缓存
>             //1>执行了SQLSession的clearCache方法(手动清空缓存)
>             //2>或执行了DML类型的语句,不管是那张表,都会清空缓存
>             logger.info(c.toString());
>             SqlSessionUtils.close(sqlSession);
>         }
>         @Test
>         public void sessionTest()
>         {
>             SqlSession sqlSession = SqlSessionUtils.openSession();
>             CarMapper mapper = sqlSession.getMapper(CarMapper.class);
>             Car car = mapper.selectById(3L);
>             logger.info(car.toString());
>             //手动清除Sql缓存
>     //        sqlSession.clearCache();
>             //因为执行了DML语句,会清空我们的缓存,不管是不是同一张表,他都会清空缓存
>             mapper.insertClazz(1002,"高三三班");
>             CarMapper mapper1 = sqlSession.getMapper(CarMapper.class);
>             Car car1 = mapper1.selectById(3L);
>             logger.info(car1.toString());
>         }
>         @Test
>         public void selectByIdSecondCache() throws Exception
>         {
>             SqlSessionFactory sqb = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml"));
>             SqlSession sqlSession = sqb.openSession();
>             CarMapper mapper = sqlSession.getMapper(CarMapper.class);
>             Car car = mapper.selectByIdSecondCache(3L);
>             logger.info(car.toString());
>             sqlSession.close();
>             SqlSession sqlSession1 = sqb.openSession();
>             CarMapper mapper1 = sqlSession1.getMapper(CarMapper.class);
>             Car car1 = mapper1.selectByIdSecondCache(3L);
>             logger.info(car1.toString());
>         }
>     }
> 
>     package com.powernode.mybatis.Test;
>     
>     import com.powernode.mybatis.mappers.CarMapper;
>     import com.powernode.mybatis.pojo.Car;
>     import com.powernode.mybatis.utils.SqlSessionUtils;
>     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.Test;
>     import org.slf4j.Logger;
>     import org.slf4j.LoggerFactory;
>     
>     public class CarMapperTest
>     {
>         private static final Logger logger = LoggerFactory.getLogger(CarMapperTest.class);
>         @Test
>         public void TestselectById() throws Exception
>         {
>             SqlSession sqlSession = SqlSessionUtils.openSession();
>             CarMapper mapper = sqlSession.getMapper(CarMapper.class);
>             Car car = mapper.selectById(3L);
>             logger.info(car.toString());
>             //两个Mapper也只执行一次DQL语句,充分说明了DQL语句只会执行一次在一个SqlSession会话中
>             //和反射生成出的调用方法的对象无关
>             //增加一个sqlSession就不再是执行一次了,因为目前是在一级缓存SqlSession中调用的
>             SqlSession session = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml")).openSession();
>             CarMapper m = session.getMapper(CarMapper.class);
>             //不再执行SQL语句了
>             Car c = m.selectById(3L);
>             //不走缓存的情况:不是同一个SqlSession会话对象,不是同一条SQL(查询条件不一样)
>             //第一次DQL和第二次DQL之间以下两个操作任意出现一个都会清空缓存
>             //1>执行了SQLSession的clearCache方法(手动清空缓存)
>             //2>或执行了DML类型的语句,不管是那张表,都会清空缓存
>             logger.info(c.toString());
>             SqlSessionUtils.close(sqlSession);
>         }
>         @Test
>         public void sessionTest()
>         {
>             SqlSession sqlSession = SqlSessionUtils.openSession();
>             CarMapper mapper = sqlSession.getMapper(CarMapper.class);
>             Car car = mapper.selectById(3L);
>             logger.info(car.toString());
>             //手动清除Sql缓存
>     //        sqlSession.clearCache();
>             //因为执行了DML语句,会清空我们的缓存,不管是不是同一张表,他都会清空缓存
>             mapper.insertClazz(1002,"高三三班");
>             CarMapper mapper1 = sqlSession.getMapper(CarMapper.class);
>             Car car1 = mapper1.selectById(3L);
>             logger.info(car1.toString());
>         }
>         @Test
>         public void selectByIdSecondCache() throws Exception
>         {
>             SqlSessionFactory sqb = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml"));
>             SqlSession sqlSession = sqb.openSession();
>             CarMapper mapper = sqlSession.getMapper(CarMapper.class);
>             Car car = mapper.selectByIdSecondCache(3L);
>             logger.info(car.toString());
>             sqlSession.close();
>             SqlSession sqlSession1 = sqb.openSession();
>             CarMapper mapper1 = sqlSession1.getMapper(CarMapper.class);
>             Car car1 = mapper1.selectByIdSecondCache(3L);
>             logger.info(car1.toString());
>         }
>     }

>     package com.powernode.mybatis.pojo;
>     
>     import java.io.Serializable;
>     
>     public class Car implements Serializable
>     {
>         //这是一个封装汽车相关信息的普通的java类,数据库表中的字段应该和属性一一对应
>         //为什么使用包装类,因为当我们查值的时候,返回值有可能是null
>         //避免数据不兼容
>         private Long id;
>         private String carNum;
>         private String brand;
>         private Double guidePrice;
>         private String produceTime;
>         private String carType;
>         public Car(){};
>     
>         public Car(Long id, String carNum, String brand, Double guidePrice, String produceTime, String carType) {
>             this.id = id;
>             this.carNum = carNum;
>             this.brand = brand;
>             this.guidePrice = guidePrice;
>             this.produceTime = produceTime;
>             this.carType = carType;
>         }
>     
>         public Long getId() {
>             return id;
>         }
>     
>         public void setId(Long id) {
>             this.id = id;
>         }
>     
>         public String getCarNum() {
>             return carNum;
>         }
>     
>         public void setCarNum(String carNum) {
>             this.carNum = carNum;
>         }
>     
>         public String getBrand() {
>             return brand;
>         }
>     
>         public void setBrand(String brand) {
>             this.brand = brand;
>         }
>     
>         public Double getGuidePrice() {
>             return guidePrice;
>         }
>     
>         public void setGuidePrice(Double guidePrice) {
>             this.guidePrice = guidePrice;
>         }
>     
>         public String getProduceTime() {
>             return produceTime;
>         }
>     
>         public void setProduceTime(String produceTime) {
>             this.produceTime = produceTime;
>         }
>     
>         public String getCarType() {
>             return carType;
>         }
>     
>         public void setCarType(String carType) {
>             this.carType = carType;
>         }
>     
>         @Override
>         public String toString() {
>             return "Car{" +
>                     "id=" + id +
>                     ", carNum='" + carNum + '\'' +
>                     ", brand='" + brand + '\'' +
>                     ", guidePrice=" + guidePrice +
>                     ", produceTime='" + produceTime + '\'' +
>                     ", carType='" + carType + '\'' +
>                     '}';
>         }
>     }
> 
>     package com.powernode.mybatis.pojo;
>     
>     import java.io.Serializable;
>     
>     public class Car implements Serializable
>     {
>         //这是一个封装汽车相关信息的普通的java类,数据库表中的字段应该和属性一一对应
>         //为什么使用包装类,因为当我们查值的时候,返回值有可能是null
>         //避免数据不兼容
>         private Long id;
>         private String carNum;
>         private String brand;
>         private Double guidePrice;
>         private String produceTime;
>         private String carType;
>         public Car(){};
>     
>         public Car(Long id, String carNum, String brand, Double guidePrice, String produceTime, String carType) {
>             this.id = id;
>             this.carNum = carNum;
>             this.brand = brand;
>             this.guidePrice = guidePrice;
>             this.produceTime = produceTime;
>             this.carType = carType;
>         }
>     
>         public Long getId() {
>             return id;
>         }
>     
>         public void setId(Long id) {
>             this.id = id;
>         }
>     
>         public String getCarNum() {
>             return carNum;
>         }
>     
>         public void setCarNum(String carNum) {
>             this.carNum = carNum;
>         }
>     
>         public String getBrand() {
>             return brand;
>         }
>     
>         public void setBrand(String brand) {
>             this.brand = brand;
>         }
>     
>         public Double getGuidePrice() {
>             return guidePrice;
>         }
>     
>         public void setGuidePrice(Double guidePrice) {
>             this.guidePrice = guidePrice;
>         }
>     
>         public String getProduceTime() {
>             return produceTime;
>         }
>     
>         public void setProduceTime(String produceTime) {
>             this.produceTime = produceTime;
>         }
>     
>         public String getCarType() {
>             return carType;
>         }
>     
>         public void setCarType(String carType) {
>             this.carType = carType;
>         }
>     
>         @Override
>         public String toString() {
>             return "Car{" +
>                     "id=" + id +
>                     ", carNum='" + carNum + '\'' +
>                     ", brand='" + brand + '\'' +
>                     ", guidePrice=" + guidePrice +
>                     ", produceTime='" + produceTime + '\'' +
>                     ", carType='" + carType + '\'' +
>                     '}';
>         }
>     }

>     <?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"/>
>         <settings>
>     
>             <setting name="lazyLoadingEnabled" value="true"/>
>             <setting name="mapUnderscoreToCamelCase" value="true"/>
>             <setting name="logImpl" value="SLF4J"/>
>             <setting name="cacheEnabled" value="true"/>
>         </settings>
>         <typeAliases>
>             <package name="com.powernode.mybatis.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.powernode.mybatis.mappers"/>
>         </mappers>
>     </configuration>
> 
>     <?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"/>
>         <settings>
>     
>             <setting name="lazyLoadingEnabled" value="true"/>
>             <setting name="mapUnderscoreToCamelCase" value="true"/>
>             <setting name="logImpl" value="SLF4J"/>
>             <setting name="cacheEnabled" value="true"/>
>         </settings>
>         <typeAliases>
>             <package name="com.powernode.mybatis.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.powernode.mybatis.mappers"/>
>         </mappers>
>     </configuration>

>     package com.powernode.mybatis.mappers;
>     
>     import com.powernode.mybatis.pojo.Car;
>     import org.apache.ibatis.annotations.Param;
>     
>     public interface CarMapper
>     {
>         Car selectByIdSecondCache(Long id);
>         //根据ID获取Car信息
>         Car selectById(Long id);
>         int insertClazz(@Param("cid") Integer cid,@Param("cname") String cname);
>     }
> 
>     package com.powernode.mybatis.mappers;
>     
>     import com.powernode.mybatis.pojo.Car;
>     import org.apache.ibatis.annotations.Param;
>     
>     public interface CarMapper
>     {
>         Car selectByIdSecondCache(Long id);
>         //根据ID获取Car信息
>         Car selectById(Long id);
>         int insertClazz(@Param("cid") Integer cid,@Param("cname") String cname);
>     }

>     <?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.powernode.mybatis.mappers.CarMapper">
>     
>         <cache></cache>
>         <select id="selectById" resultType="Car">
>             select * from t_car where id = #{id}
>         </select>
>         <insert id="insertClazz">
>             insert into t_clazz values(#{cid},#{cname})
>         </insert>
>         <select id="selectByIdSecondCache" resultType="Car">
>             select * from t_car where id = #{id}
>         </select>
>     </mapper>
> 
>     <?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.powernode.mybatis.mappers.CarMapper">
>     
>         <cache></cache>
>         <select id="selectById" resultType="Car">
>             select * from t_car where id = #{id}
>         </select>
>         <insert id="insertClazz">
>             insert into t_clazz values(#{cid},#{cname})
>         </insert>
>         <select id="selectByIdSecondCache" resultType="Car">
>             select * from t_car where id = #{id}
>         </select>
>     </mapper>