前言:
什么是Mybatis缓存?
使用缓存可以减少Java Application与数据库的交互次数,从而提升程序的运行效率。比如,查询id=1的user对象,第一次查询出来之后,会自动将该对象保存到缓存中。下一次查询该对象时,就可以直接从缓存中获取,不需要发送SQL查询数据库了。
Mybatis缓存分类
一级缓存:SqlSession级别,默认开启,且不能关闭。
mybatis的一级缓存是SqlSession级别的缓存,在操作数据库时需要构造SqlSession对象,在对象中有一个HashMap用于存储缓存数据,不同的SqlSession之间缓存数据区域(HashMap)是互相不影响的。 一级缓存的作用域是SqlSession范围的,当在同一个SqlSession中执行两次相同的sql语句时,第一次执行完毕会将数据库中查询的数据写到缓存(内存)中,第二次查询时会从缓存中获取数据,不再去底层进行数据库查询,从而提高了查询效率。需要注意的是:如果SqlSession执行了DML操作(insert、update、delete),并执行commit()操作,mybatis则会清空SqlSession中的一级缓存,这样做的目的是为了保证缓存数据中存储的是最新的信息,避免出现脏读现象。 当一个SqlSession结束后该SqlSession中的一级缓存也就不存在了,Mybatis默认开启一级缓存,不需要进行任何配置。
二级缓存:Mapper级别,默认关闭,可以开启。
二级缓存是Mapper级别的缓存,使用二级缓存时,多个SqlSession使用同一个Mapper的sql语句去操作数据库,得到的数据会存在二级缓存区域,它同样是使用HashMapper进行数据存储,相比一级缓存SqlSession,二级缓存的范围更大,多个SqlSession可以共用二级缓存,二级缓存是跨SqlSession的。 二级缓存是多个SqlSession共享的,其作用域是mapper的同一个namespace,不同的SqlSession两次执行相同的namespace下的sql语句,且向sql中传递的参数也相同,即最终执行相同的sql语句,则第一次执行完毕会将数据库中查询的数据写到缓存(内存),第二次查询时会从缓存中获取数据,不再去底层数据库查询,从而提高查询效率。 Mybatis默认关闭二级缓存,可以在setting全局参数中配置开启二级缓存。 下面我们通过代码来学习如何使用MyBatis缓存。
代码:
首先来演示一级缓存,以查询User对象为例。
1.User实体类
package com.southwind.entity;
public class User{ private int id; private String username; private String password; private int age;
public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return 'User [id=' + id + ', username=' + username + ', password=' + password + ', age=' + age + ']'; }
}
2.UserDAO接口
package com.southwind.dao;
import com.southwind.entity.User;
public interface UserDAO { public User get(int id); }
3.UserDAO.xml
mapper namespace='com.southwind.dao.UserDAO'>
select id='get' parameterType='int' resultType='user'> select * from user where id = #{id} select>
mapper>
4.config.xml
configuration>
settings> setting name='logImpl' value='STDOUT_LOGGING'/> settings>
typeAliases> typeAlias type='com.southwind.entity.User' alias='user'/> typeAliases>
environments default='development'> environment id='development'> transactionManager type='JDBC' /> dataSource type='POOLED'> property name='driver' value='com.mysql.jdbc.Driver' /> property name='url' value='jdbc:mysql://localhost:3306/mybatis?useUnicode=true&characterEncoding=UTF-8' /> property name='username' value='root' /> property name='password' value='root' /> dataSource> environment> environments>
mappers> mapper resource='com/southwind/dao/StudentDAO.xml'/> mappers>
configuration>
5.Test测试类
package com.southwind.test;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.southwind.dao.UserDAO; import com.southwind.entity.User;
public class Test { public static void main(String[] args) { InputStream is = Test.class.getClassLoader().getResourceAsStream('config.xml'); SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder(); SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is); SqlSession sqlSession = sqlSessionFactory.openSession(); UserDAO userDAO = sqlSession.getMapper(UserDAO.class);
//一级缓存 User user2 = userDAO.get(22); System.out.println(user2); User user3 = userDAO.get(22); System.out.println(user3); } }
可以看到结果,执行了一次SQL语句,查询出两个对象,第一个对象是通过SQL查询的,并保存到缓存中,第二个对象是直接从缓存中获取的。
我们说过一级缓存是SqlSession级别的,所以SqlSession一旦关闭,缓存也就不复存在了,修改代码,再次测试。
package com.southwind.test;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.southwind.dao.UserDAO; import com.southwind.entity.User;
public class Test { public static void main(String[] args) { InputStream is = Test.class.getClassLoader().getResourceAsStream('config.xml'); SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder(); SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is); SqlSession sqlSession = sqlSessionFactory.openSession(); UserDAO userDAO = sqlSession.getMapper(UserDAO.class);
//一级缓存 User user2 = userDAO.get(22); System.out.println(user2); sqlSession.close(); sqlSession = sqlSessionFactory.openSession(); userDAO = sqlSession.getMapper(UserDAO.class); User user3 = userDAO.get(22); System.out.println(user3); } }
可以看到,执行了两次SQL。
在关闭SqlSession,一级缓存失效的情况下,可以启用二级缓存,实现提升效率的要求。
MyBatis可以使用自带的二级缓存,也可以使用第三方的ehcache二级缓存。
自带二级缓存
1.config.xml中配置开启二级缓存。
configuration>
settings> setting name='logImpl' value='STDOUT_LOGGING'/> setting name='cacheEnabled' value='true'/> settings>
typeAliases> typeAlias type='com.southwind.entity.User' alias='user'/> typeAliases>
environments default='development'> environment id='development'> transactionManager type='JDBC' /> dataSource type='POOLED'> property name='driver' value='com.mysql.jdbc.Driver' /> property name='url' value='jdbc:mysql://localhost:3306/mybatis?useUnicode=true&characterEncoding=UTF-8' /> property name='username' value='root' /> property name='password' value='root' /> dataSource> environment> environments>
mappers> mapper resource='com/southwind/dao/StudentDAO.xml'/> mappers>
configuration>
2.UserDAO.xml中配置二级缓存。
mapper namespace='com.southwind.dao.UserDAO'> cache>cache> select id='get' parameterType='int' resultType='user'> select * from user where id = #{id} select>
mapper>
3.User实现Serializable接口。
package com.southwind.entity;
import java.io.Serializable;
public class User implements Serializable{ private int id; private String username; private String password; private int age;
public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return 'User [id=' + id + ', username=' + username + ', password=' + password + ', age=' + age + ']'; }
}
4.测试。
package com.southwind.test;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.southwind.dao.UserDAO; import com.southwind.entity.User;
public class Test { public static void main(String[] args) { InputStream is = Test.class.getClassLoader().getResourceAsStream('config.xml'); SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder(); SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is); SqlSession sqlSession = sqlSessionFactory.openSession(); UserDAO userDAO = sqlSession.getMapper(UserDAO.class);
//一级缓存 User user2 = userDAO.get(22); System.out.println(user2); sqlSession.close(); sqlSession = sqlSessionFactory.openSession(); userDAO = sqlSession.getMapper(UserDAO.class); User user3 = userDAO.get(22); System.out.println(user3); } }
可以看到,执行了一次SQL,查询出两个对象。
ehcache二级缓存 1.添加ehcache依赖jar包。
project xmlns='http://maven./POM/4.0.0' xmlns:xsi='http://www./2001/XMLSchema-instance' xsi:schemaLocation='http://maven./POM/4.0.0 http://maven./maven-v4_0_0.xsd'> modelVersion>4.0.0modelVersion> groupId>com.southwindgroupId> artifactId>ehcacheartifactId> packaging>warpackaging> version>0.0.1-SNAPSHOTversion> name>ehcacheMaven Webappname> url>http://maven.url> dependencies>
dependency> groupId>org.mybatisgroupId> artifactId>mybatisartifactId> version>3.4.5version> dependency>
dependency> groupId>mysqlgroupId> artifactId>mysql-connector-javaartifactId> version>5.1.25version> dependency>
dependency> groupId>org.mybatisgroupId> artifactId>mybatis-ehcacheartifactId> version>1.0.0version> dependency>
dependencies> build> finalName>ehcachefinalName> build> project>
2.添加ehcache.xml。
ehcache xmlns:xsi='http://www./2001/XMLSchema-instance' xsi:noNamespaceSchemaLocation='../config/ehcache.xsd'> diskStore/> defaultCache maxElementsInMemory='1000' maxElementsOnDisk='10000000' eternal='false' overflowToDisk='false' timeToIdleSeconds='120' timeToLiveSeconds='120' diskExpiryThreadIntervalSeconds='120' memoryStoreEvictionPolicy='LRU'> defaultCache> ehcache>
3.config.xml中配置开启二级缓存。
configuration>
settings> setting name='logImpl' value='STDOUT_LOGGING'/> setting name='cacheEnabled' value='true'/> settings>
typeAliases> typeAlias type='com.southwind.entity.User' alias='user'/> typeAliases>
environments default='development'> environment id='development'> transactionManager type='JDBC' /> dataSource type='POOLED'> property name='driver' value='com.mysql.jdbc.Driver' /> property name='url' value='jdbc:mysql://localhost:3306/mybatis?useUnicode=true&characterEncoding=UTF-8' /> property name='username' value='root' /> property name='password' value='root' /> dataSource> environment> environments>
mappers> mapper resource='com/southwind/dao/StudentDAO.xml'/> mappers>
configuration>
4.UserDAO.xml中配置二级缓存。
mapper namespace='com.southwind.dao.UserDAO'> cache type='org.mybatis.caches.ehcache.EhcacheCache' > property name='timeToIdleSeconds' value='3600'/> property name='timeToLiveSeconds' value='3600'/> property name='memoryStoreEvictionPolicy' value='LRU'/> cache> select id='get' parameterType='int' resultType='user'> select * from user where id = #{id} select>
mapper>
5.User不需要实现Serializable接口。
package com.southwind.entity;
public class User{ private int id; private String username; private String password; private int age;
public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return 'User [id=' + id + ', username=' + username + ', password=' + password + ', age=' + age + ']'; }
}
6.测试。
package com.southwind.test;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.southwind.dao.UserDAO; import com.southwind.entity.User;
public class Test { public static void main(String[] args) { InputStream is = Test.class.getClassLoader().getResourceAsStream('config.xml'); SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder(); SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is); SqlSession sqlSession = sqlSessionFactory.openSession(); UserDAO userDAO = sqlSession.getMapper(UserDAO.class);
//一级缓存 User user2 = userDAO.get(22); System.out.println(user2); sqlSession.close(); sqlSession = sqlSessionFactory.openSession(); userDAO = sqlSession.getMapper(UserDAO.class); User user3 = userDAO.get(22); System.out.println(user3); } }
同样执行一次SQL,查询出两个对象。
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