MyBatis一级缓存和二级缓存

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前言：

MyBatis中一级缓存和二级缓存默认都是开启的；

一级缓存是Session级别的缓存，二级缓存是SqlSessionFactory级别的缓存

一级缓存：

package cn.et.lesson05;import java.io.InputStream;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;/** * 缓存: * 数据交换的缓冲区;当要读取数据时,会首先从内存中查找需要的数据,如果找到了就直接执行; * 找不到的话再从数据库中找,从数据库中找出来后返回数据并将其缓存于内存中; *  * 缓存算法:FIFO 先进先出算法   内存不足时 先存储进去数据  会先被剔除 * LRU最近使用次数最少算法内存不足时 最近使用次数最少被剔除 * LFU使用次数最少算法 内存不足时  使用次数最少的被剔除 *  * @author Administrator * */public class TestMybatis {private static SqlSessionFactory getSqlSessionFactory() {//因为需要的mybatis.xml文件 不在同一层目录  所以这里才使用 cn.et.lesson01.TestMybatisInputStream is = TestMybatis.class.getResourceAsStream("mybatis.xml");SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);return factory;}/** * 一级缓存 * @param args */public static void main(String[] args) {SqlSessionFactory factory = getSqlSessionFactory();//打开第一个SqlSessionSqlSession sqlSession1 = factory.openSession();//获取第一个EmpMapper实例EmpMapper dm = sqlSession1.getMapper(EmpMapper.class);//发起第一次sql查询Emp emp = dm.selectEmp("7369");//获取第二个EmpMapper实例EmpMapper dm1 = sqlSession1.getMapper(EmpMapper.class);//发起第二次sql查询Emp emp1 = dm1.selectEmp("7369");System.out.println(dm==dm1);System.out.println(emp == emp1);System.out.println("\r-----------------分割线----------------\r");//打开第二个SqlSessionSqlSession sqlSession2 = factory.openSession();//通过sqlSession2获取EmpMapper实例EmpMapper em = sqlSession2.getMapper(EmpMapper.class);Emp selectEmp = em.selectEmp("7369");}}

Run:

从运行结果可以很明显看出一级缓存是Session级别的缓存并且存放在Session中

因此当不同的Session发起相同的select语句时发起了两次查询

2017-06-16 20:42:00 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Opening JDBC Connection2017-06-16 20:42:00 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] Created connection 1448851814.2017-06-16 20:42:00 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Setting autocommit to false on JDBC Connection [oracle.jdbc.driver.T4CConnection@565bb966]2017-06-16 20:42:00 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] ==>  Preparing: select * from emp where empno = ? 2017-06-16 20:42:00 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] ==> Parameters: 7369(String)2017-06-16 20:42:00 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] <==      Total: 1falsetrue-----------------分割线----------------2017-06-16 20:42:00 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Opening JDBC Connection2017-06-16 20:42:00 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] Created connection 342983234.2017-06-16 20:42:00 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Setting autocommit to false on JDBC Connection [oracle.jdbc.driver.T4CConnection@14718242]2017-06-16 20:42:00 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] ==>  Preparing: select * from emp where empno = ? 2017-06-16 20:42:00 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] ==> Parameters: 7369(String)2017-06-16 20:42:00 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] <==      Total: 1

二级缓存：

实体类必须实现Serializable

emp_mapper.xml

<?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="cn.et.lesson05.EmpMapper"><!--<cache></cache> 启用二级缓存  属性默认  --><cache></cache><select id="selectEmp" resultType="cn.et.lesson05.Emp">select * from emp where empno = #{0}</select></mapper>

Test

/** * 二级缓存 * @param args */public static void main(String[] args) {SqlSessionFactory factory = getSqlSessionFactory();SqlSession session = factory.openSession();EmpMapper dm = session.getMapper(EmpMapper.class);Emp emp = dm.selectEmp("7369");//开启二级缓存后  关闭session时   //session中一级缓存的数据会自动发往二级缓存session.close();//打开第二个SqlSessionSqlSession session2 = factory.openSession();//通过sqlSession2获取EmpMapper实例EmpMapper em = session2.getMapper(EmpMapper.class);Emp selectEmp = em.selectEmp("7369");}

Run

2017-06-16 21:28:05 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Opening JDBC Connection2017-06-16 21:28:05 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] Created connection 110640533.2017-06-16 21:28:05 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Setting autocommit to false on JDBC Connection [oracle.jdbc.driver.T4CConnection@6983d95]2017-06-16 21:28:05 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] ==>  Preparing: select * from emp where empno = ? 2017-06-16 21:28:05 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] ==> Parameters: 7369(String)2017-06-16 21:28:05 DEBUG [cn.et.lesson05.EmpMapper.selectEmp] <==      Total: 12017-06-16 21:28:05 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Resetting autocommit to true on JDBC Connection [oracle.jdbc.driver.T4CConnection@6983d95]2017-06-16 21:28:05 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] Closing JDBC Connection [oracle.jdbc.driver.T4CConnection@6983d95]2017-06-16 21:28:05 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] Returned connection 110640533 to pool.2017-06-16 21:28:05 DEBUG [cn.et.lesson05.EmpMapper] Cache Hit Ratio [cn.et.lesson05.EmpMapper]: 0.5

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