一级缓存与二级缓存

1.理解mybatis的缓存

　　如大部分的持久化框架，mybatis也提供了一级缓存与二级缓存。

　　一级缓存：基于perpetualcache的hashmap本地缓存，其存储作用为session，当session进行flush或者close之后，将该session中的cache进行清空。

　　二级缓存：机制与一级缓存类似，默认采用perpetualcache，hashmap存储，不同在于存储作用域为Mapper（namespace），并且可以自定义存储。

　　对于存储数据更新机制，当某一个作用域进行了C/U/D之后，默认该作用域下所有select中的缓存将被clear。

一：一级缓存

1.需求，目录结构

　　根据id查询对应的用户记录对象。

　　

2.准备数据

CREATE TABLE c_user(
    id INT PRIMARY KEY auto_increment,
    name VARCHAR(20),
    age INT
);
INSERT INTO c_user(name,age) VALUES("TOM",12);
INSERT INTO c_user(NAME,age) VALUES("BOB",18);

3.创建实体类

package com.cao.bean;

public class CUser {
    private int id;
    private String name;
    private int age;
    public CUser() {}
    public CUser(int id,String name,int age) {
        this.id=id;
        this.name=name;
        this.age=age;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setSex(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "CUser [id=" + id + ", name=" + name + ", age=" + age + "]";
    }
    

}

4.映射文件

<?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="procedure">
        <select id="getUser" parameterType="int" resultType="com.cao.bean.CUser">
            select * from c_user where id=#{id}
        </select>
        <update id="updateUser" parameterType="com.cao.bean.CUser">
            update c_user set
            name=#{name},age=#{age} where id=#{id}
        </update>
    </mapper>

5.配置文件

<?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>
  <environments default="development">
    <environment id="development">
      <transactionManager type="JDBC">
        <property name="" value=""/>
      </transactionManager>
      <dataSource type="UNPOOLED">
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://127.0.0.1:3308/mybatis"/>
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
      </dataSource>
    </environment>
  </environments>

   <mappers>
      <mapper resource="com/jun/sql/config/user.xml"/>
   </mappers>

</configuration>

6.测试类

package com.jun.main;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.Map;

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 com.cao.bean.CUser;
public class MainTest {
    /**
     * 方式一
     * @throws Exception
     */
    @Test
    public void test1() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/cao/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession=sqlSessionFactory.openSession(true);                //true后是自动提交
        
        String statement="procedure.getUser";
        
        CUser cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        //再跑一次
        cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        sqlSession.close();
    }
    
    
}

7.效果

　　说明一级缓存是开着的。

　　

8.调用方法进行清空缓存

　　sqlSession.clearCache();

package com.jun.main;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.Map;

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 com.cao.bean.CUser;
public class MainTest {
    /**
     * 方式一
     * @throws Exception
     */
    @Test
    public void test1() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/cao/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession=sqlSessionFactory.openSession(true);                //true后是自动提交
        
        String statement="procedure.getUser";
        
        CUser cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        //再跑一次
        cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        //清空缓存
        sqlSession.clearCache();
        cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        
        sqlSession.close();
    }
    
    
}

9.效果

　　

10.增删改后清空缓存

package com.jun.main;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.Map;

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 com.cao.bean.CUser;
public class MainTest {
    /**
     * 方式一
     * @throws Exception
     */
    @Test
    public void test1() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/cao/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession=sqlSessionFactory.openSession(true);                //true后是自动提交
        
        String statement="procedure.getUser";
        
        CUser cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        //再跑一次
        cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        //清空缓存
//        sqlSession.clearCache();
        //执行增删改操作
        String statement2="procedure.updateUser";
        int update=sqlSession.update(statement2, new CUser(1,"JACK",15));
        sqlSession.commit();
        System.out.println(update);
        //再次查询
        cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        
        sqlSession.close();
    }
    
    
}

11.效果

　　

12.关闭session清空缓存

二：二级缓存

1.添加一个<>cache>在映射文件中

　　<cache/>

2.新建对象（这个对象必须实现序列化接口Serializable）

　　注意点。

package com.cao.bean;

import java.io.Serializable;

public class CUser implements Serializable{
    private int id;
    private String name;
    private int age;
    public CUser() {}
    public CUser(int id,String name,int age) {
        this.id=id;
        this.name=name;
        this.age=age;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setSex(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "CUser [id=" + id + ", name=" + name + ", age=" + age + "]";
    }
    

}

 3.在映射文件中加一个cache标签

<?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="procedure">
        <cache></cache>
        <select id="getUser" parameterType="int" resultType="com.cao.bean.CUser">
            select * from c_user where id=#{id}
        </select>
        <update id="updateUser" parameterType="com.cao.bean.CUser">
            update c_user set
            name=#{name},age=#{age} where id=#{id}
        </update>
    </mapper>

4.测试类

package com.jun.main;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.Map;

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 com.cao.bean.CUser;
public class MainTest {
    /**
     * 一
     * @throws Exception
     */
    @Test
    public void test1() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/cao/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession=sqlSessionFactory.openSession(true);                //true后是自动提交
        
        String statement="procedure.getUser";
        
        CUser cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        //再跑一次
        cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        //清空缓存
//        sqlSession.clearCache();
        //执行增删改操作
        String statement2="procedure.updateUser";
        int update=sqlSession.update(statement2, new CUser(1,"JACK",15));
        sqlSession.commit();
        System.out.println(update);
        //再次查询
        cuser=sqlSession.selectOne(statement, 1);
        System.out.println(cuser);
        
        sqlSession.close();
    }
    /**
     * 二
     * @throws Exception
     */
    @Test
    public void test2() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/cao/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession1=sqlSessionFactory.openSession();            
        SqlSession sqlSession2=sqlSessionFactory.openSession();
        
        String statement="procedure.getUser";
        
        CUser cuser=sqlSession1.selectOne(statement, 1);
        sqlSession1.commit();
        System.out.println(cuser);
        //再跑一次
        cuser=sqlSession2.selectOne(statement, 1);
        sqlSession1.commit();
        System.out.println(cuser);
        
        
        sqlSession1.close();
    }
    
    
}

5.效果

　　

6.注意点

　　如果第一个session没有提交，则没有进行二级缓存。

　　当然，如果第一个session是自动提交，第二个session也没有进行二次缓存。

　　所以，想实现二级缓存，需要前面的session已经提交过，并且相同的提交sql。

三：回收策略

1.策略

　　LRU:最近最少使用，移除长时间不使用的对象

　　FIFO:先进先出

　　SOFT:软引用，移除基于垃圾回收状态和软引用规则的对象

　　WEAK:弱引用，更积极的移除基于垃圾收集器状态和软引用规则的对象

2.默认

　　LRU。