面试官：mybatis的缓存机制是怎么样的？

MyBatis的缓存分为一级缓存和二级缓存，两种缓存的缓存粒度是一样的，都是对应一条sql查询语句，但是二者的生命周期是不一样的，一级缓存的生命周期是SqlSession对象的使用期间，随着SqlSession对象的死亡而消失；二级缓存的生命周期是同MyBatis应用一样长的；并且是首先查询二级缓存，然后再查询一起缓存正如大多数持久层框架一样，MyBatis 同样提供了一级缓存和二级缓存的支持。

1.缓存，其存储作用域为 Session，当 Session flush 或 close 之后，该Session中的所有 Cache 就将清空。

2. 二级缓存与一级缓存其机制相同，默认也是采用 PerpetualCache，HashMap存储，不同在于其存储作用域为 Mapper ，并且可自定义存储源，如 Ehcache。二级缓存的作用域是全局的，二级缓存在SqlSession关闭或提交之后才会生效。

在分析MyBatis的二级缓存之前，我们先简单看下MyBatis中一个关于二级缓存的类(其他相关的类和接口之前已经分析过)：org.apache.ibatis.mapping.MappedStatement：MappedStatement类在Mybatis框架中用于表示XML文件中一个sql语句节点，即一个、或者标签。Mybatis框架在初始化阶段会对XML配置文件进行读取，将其中的sql语句节点对象化为一个个MappedStatement对象3. 对于缓存数据更新机制，当某一个作用域(一级缓存Session/二级缓存Namespaces)的进行了 C/U/D 操作后，默认该作用域下所有 select 中的缓存将被clear。mybatis的一级缓存作用就是在同一个session过程中，将会对相同数据的存取不通过数据库，而是通过缓存机制提高效率。

看测试代码

importjava.io.IOException;importjava.io.InputStream;importjunit.framework.TestCase;importorg.apache.ibatis.io.Resources;importorg.apache.ibatis.session.SqlSession;importorg.apache.ibatis.session.SqlSessionFactory;importorg.apache.ibatis.session.SqlSessionFactoryBuilder;importorg.junit.Test;importcom.guowuxin.mybatis.mapper.UserMapper;importcom.guowuxin.mybatis.model.User;

/*用户表drop table if exists user;create table user(    id int primary key auto_increment,    username varchar(50) unique,    password varchar(100),    useraddress varchar(50));insert intouservalues(1,"guowuxin","guowuxin","beijing");insert intouservalues(1,"guoxiaoming","guowuxin","beijing");*/

//如果两次查询都是查询1 将只会发送一条sql语句到数据库，否则将是2条sql语句

public class TestClass extends TestCase{    public static SqlSessionFactory sqlSessionFactoy;    static{        String resource ="mybatis-config.xml";        try {            InputStream inputStream = Resources.getResourceAsStream(resource);            sqlSessionFactoy = new SqlSessionFactoryBuilder().build(inputStream);        } catch (IOException e) {            e.printStackTrace();        }    }    @Test    public void testgetUserById() {        SqlSession sqlSession = sqlSessionFactoy.openSession();        try {                  UserMapper mapper = sqlSession.getMapper(UserMapper.class); User u1=mapper.getUserById(1);                System.out.println(u1); User u2=mapper.getUserById(1);                System.out.println(u2);                sqlSession.commit();      } finally {            sqlSession.close();        }    }}

packagecom.guowuxin.mybatis.mapper;importcom.guowuxin.mybatis.model.User;//通过面向接口的mybatis编程方式，需要保证方法名和配置文件中的id名称一致publicinterfaceUserMapper{publicUser getUserById(int id);}packagecom.guowuxin.mybatis.model;publicclassUser{privateint id;privateint age;privateString userName;privateString userAddress;publicint getId() {returnid;    }publicvoidsetId(int id) {this.id = id;    }publicString getUserName() {returnuserName;    }publicvoidsetUserName(String userName) {this.userName = userName;    }publicString getUserAddress() {returnuserAddress;    }publicvoidsetUserAddress(String userAddress) {this.userAddress = userAddress;    }publicint getAge() {returnage;    }publicvoidsetAge(int age) {this.age = age;    }    @OverridepublicString toString()    {finalStringBuilder builder =newStringBuilder();        builder.append("id=").append(id).append(",");        builder.append("age=").append(age).append(",");        builder.append("userName=").append(userName).append(",");        builder.append("userAddress=").append(userAddress).append(".");returnbuilder.toString();    }}

log4j.rootLogger=DEBUG,Console��#Consolelog4j.appender.Console=org.apache.log4j.ConsoleAppenderlog4j.appender.Console.layout=org.apache.log4j.PatternLayoutlog4j.appender.Console.layout.ConversionPattern=%d%t%-5p(%c) -%m%nlog4j.logger.com.ibatis=DEBUGlog4j.logger.com.ibatis.common.jdbc.SimpleDataSource=DEBUGlog4j.logger.com.ibatis.common.jdbc.ScriptRunner=DEBUGlog4j.logger.com.ibatis.sqlmap.engine.impl.SqlMapClientDelegate=DEBUGlog4j.logger.java.sql.Connection=DEBUGlog4j.logger.java.sql.Statement=DEBUGlog4j.logger.java.sql.PreparedStatement=DEBUG

select * from user where id=#{id}

结果：………………..

2016-02-24 20:12:37,568 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:12:37,568 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==>  Preparing: select *from user whereid=? 2016-02-24 20:12:37,586 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==> Parameters: 1(Integer)id=1,age=0,userName=guowuxin,userAddress=beijing.id=1,age=0,userName=guowuxin,userAddress=beijing.2016-02-24 20:12:37,593 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Resetting autocommittotrueon JDBC Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:12:37,593 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:12:37,593 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Returned connection 3874052topool.

如果取得2个不同的id…………………..

2016-02-24 20:12:57,655 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Created connection 3874052.2016-02-24 20:12:57,656 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:12:57,656 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==>  Preparing: select *from user whereid=? 2016-02-24 20:12:57,664 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==> Parameters: 1(Integer)id=1,age=0,userName=guowuxin,userAddress=beijing.2016-02-24 20:12:57,679 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:12:57,679 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==>  Preparing: select *from user whereid=? 2016-02-24 20:12:57,679 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==> Parameters: 2(Integer)id=2,age=0,userName=guoxiaoming,userAddress=beijing.2016-02-24 20:12:57,679 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Resetting autocommittotrueon JDBC Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:12:57,679 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:12:57,679 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Returned connection 3874052topool.

二级缓存机制指的则是在不同的session之间都可以共享相同的数据内容。

importjava.io.IOException;importjava.io.InputStream;importorg.apache.ibatis.io.Resources;importorg.apache.ibatis.session.SqlSession;importorg.apache.ibatis.session.SqlSessionFactory;importorg.apache.ibatis.session.SqlSessionFactoryBuilder;importorg.junit.Test;importcom.guowuxin.mybatis.mapper.UserMapper;importcom.guowuxin.mybatis.model.User;importjunit.framework.TestCase;publicclassTestTwoCacheextendsTestCase{publicstaticSqlSessionFactory sqlSessionFactory;static{        String resource ="mybatis-config.xml";try{            InputStream inputStream = Resources.getResourceAsStream(resource);            sqlSessionFactory =newSqlSessionFactoryBuilder().build(inputStream);        }catch(IOException e) {            e.printStackTrace();        }    }    @TestpublicvoidtestCache() {        SqlSession sqlSession1 = sqlSessionFactory.openSession();      SqlSession sqlSession2 = sqlSessionFactory.openSession();try{                  UserMapper mapper1 = sqlSession1.getMapper(UserMapper.class);                  User u1=mapper1.getUserById(1);                System.out.println(u1);                sqlSession1.commit();                UserMapper mapper2 = sqlSession2.getMapper(UserMapper.class);                User u2=mapper2.getUserById(1);                System.out.println(u2);                sqlSession2.commit();      }finally{            sqlSession1.close();            sqlSession2.close();      }    }}

在没有开启二级缓存之前，将会执行两次sql语句。

2016-02-24 20:28:10,786 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - PooledDataSource forcefully closed/removed all connections.2016-02-24 20:28:10,850 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Openning JDBC Connection2016-02-24 20:28:11,011 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Created connection 3874052.2016-02-24 20:28:11,012 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:28:11,013 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==>  Preparing: select *from user whereid=? 2016-02-24 20:28:11,030 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==> Parameters: 1(Integer)id=1,age=0,userName=guowuxin,userAddress=beijing.2016-02-24 20:28:11,040 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Openning JDBC Connection2016-02-24 20:28:11,154 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Created connection 29420695.2016-02-24 20:28:11,154 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@1c0ec97]2016-02-24 20:28:11,154 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==>  Preparing: select *from user whereid=? 2016-02-24 20:28:11,154 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==> Parameters: 1(Integer)id=1,age=0,userName=guowuxin,userAddress=beijing.2016-02-24 20:28:11,154 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Resetting autocommittotrueon JDBC Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:28:11,154 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3b1d04]2016-02-24 20:28:11,154 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Returned connection 3874052topool.2016-02-24 20:28:11,154 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Resetting autocommittotrueon JDBC Connection [com.mysql.jdbc.JDBC4Connection@1c0ec97]2016-02-24 20:28:11,154 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1c0ec97]2016-02-24 20:28:11,154 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Returned connection 29420695topool.------------------------

在实体类映射文件中加入

select * from user where id=#{id}

2016-02-25 10:15:11,162 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Created connection 22522451.2016-02-25 10:15:11,164 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@157aa53]2016-02-25 10:15:11,164 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==>  Preparing: select *from user whereid=? 2016-02-25 10:15:11,182 mainDEBUG(com.guowuxin.mybatis.mapper.UserMapper.getUserById) - ==> Parameters: 1(Integer)id=1,age=0,userName=guowuxin,userAddress=beijing.2016-02-25 10:15:11,196 mainDEBUG(org.apache.ibatis.cache.decorators.LoggingCache) - Cache Hit Ratio [com.guowuxin.mybatis.mapper.UserMapper]: 0.5id=1,age=0,userName=guowuxin,userAddress=beijing.2016-02-25 10:15:11,196 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Resetting autocommittotrueon JDBC Connection [com.mysql.jdbc.JDBC4Connection@157aa53]2016-02-25 10:15:11,196 mainDEBUG(org.apache.ibatis.transaction.jdbc.JdbcTransaction) - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@157aa53]2016-02-25 10:15:11,197 mainDEBUG(org.apache.ibatis.datasource.pooled.PooledDataSource) - Returned connection 22522451topool.

就会发现cache命中，只执行了一次sql语句———————————————如果二级缓存想要命中实现，则必须要将上一次sqlSession commit之后才能生效，不然将不会命中，原因：两个不同的session必须提交前面一个session才能缓存生效的原因是因为mybatis的缓存会被一个transactioncache类包装住，所有的cache.putObject全部都会被暂时存到一个map里，等事务提交以后，这个map里的缓存对象才会被真正的cache类执行putObject操作。这么设计的原因是为了防止事务执行过程中出异常导致回滚，如果get到object后直接put进缓存，万一发生回滚，就很容易导致mybatis缓存被脏读

参考文章：http://my.oschina.net/KingPan/blog/280167#OSC_h1_1