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. MyBatis一级缓存基于PerpetualCache的HashMap本地缓存，其存储作用域为 Session，默认情况下，一级缓存是开启状态的。当 Session flush(); 或 close(); 之后，该Session中的所有 Cache 就将清空。
2.MyBatis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(Namespace)，并且可自定义存储源，如 Ehcache。
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。
程序测试一级缓存
usersMapper.xml配置：
<?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.entity.test009.usersMapper"><select id="myUsers" parameterType="int" resultType="Users">
select * from users where id=#{id}
</select>
<select id="myUpdate" parameterType="Users">
update users set username=#{username},password=#{password} where id=#{id}
</select></mapper>
写一个获得一级缓存session的工具类：
public class MyBatisUtils {public static SqlSession mySession() throws Exception{String resource = "conf.xml";
Reader reader = Resources.getResourceAsReader(resource);
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(reader);
SqlSession session = factory.openSession(true);
//session.clearCache();//删除缓存
return session;
}
}
测试程序代码
※第1种方法
@Test
public void cacheOne() throws Exception
{
SqlSession session = MyBatisUtils.mySession();
String statement ="com.entity.test009.usersMapper.myUsers"; users = session.selectOne(statement , 3);
System.out.println(users); users = session.selectOne(statement , 3);
System.out.println(users);
//清理session的缓存
session.clearCache();
System.out.println("-----------------------------------");
users = session.selectOne(statement , 3);
System.out.println(users);
}
※运行后的结果：
2015-05-04 22:49:43,323 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Preparing: select * from users where id=?
2015-05-04 22:49:43,361 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Parameters: 3(Integer)
2015-05-04 22:49:43,387 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - <== Total: 1
Users [id=3, username=张三, password=123456, age=99, sex=0]
Users [id=3, username=张三, password=123456, age=99, sex=0]
--------------------- //session.clearCache();后又一次查询了数据库，并非从缓存中读取数据
2015-05-04 22:49:43,388 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Preparing: select * from users where id=?
2015-05-04 22:49:43,389 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Parameters: 3(Integer)
2015-05-04 22:49:43,390 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - <== Total: 1
Users [id=3, username=张三, password=123456, age=99, sex=0]
※第2种方法
@Test
public void cacheOne() throws Exception
{
SqlSession session = MyBatisUtils.mySession();
String statement ="com.entity.test009.usersMapper.myUsers"; users = session.selectOne(statement , 3);
System.out.println(users); users = session.selectOne(statement , 3);
System.out.println(users);
/**
* 第2个方法，执行一个更新的操作
*/
session.update("com.entity.test009.usersMapper.myUpdate", new Users(3, "张三", "123456", null, null));
session.commit();
System.out.println("---------------------");
users = session.selectOne(statement , 3);
System.out.println(users);
}
※运行后的结果：
2015-05-04 22:55:38,685 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Preparing: select * from users where id=?
2015-05-04 22:55:38,724 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Parameters: 3(Integer)
2015-05-04 22:55:38,747 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - <== Total: 1
Users [id=3, username=张三, password=123456, age=99, sex=0]
Users [id=3, username=张三, password=123456, age=99, sex=0]
2015-05-04 22:55:38,748 [main] DEBUG [com.entity.test009.usersMapper.myUpdate] - ==> Preparing: update users set username=?,password=? where id=?
2015-05-04 22:55:38,749 [main] DEBUG [com.entity.test009.usersMapper.myUpdate] - ==> Parameters: 张三(String), 123456(String), 3(Integer)
2015-05-04 22:55:38,749 [main] DEBUG [com.entity.test009.usersMapper.myUpdate] - <== Updates: 1
---------------------//大家可以看出来，执行完更新后，一级缓存会被自动清理，然后下边又一次查询数据库
2015-05-04 22:55:38,750 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Preparing: select * from users where id=?
2015-05-04 22:55:38,750 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Parameters: 3(Integer)
2015-05-04 22:55:38,751 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - <== Total: 1
Users [id=3, username=张三, password=123456, age=99, sex=0]
※第3种方法
@Test
public void cacheOne() throws Exception
{
SqlSession session = MyBatisUtils.mySession();
String statement ="com.entity.test009.usersMapper.myUsers"; users = session.selectOne(statement , 3);
System.out.println(users); users = session.selectOne(statement , 3);
System.out.println(users); /**
* 第3个方法，并非同一个session对象。关闭session后，重新再开一个session2。这个时候和刚才的不一样，所以就不会用session刚才的缓存了
*/
session.close();
SqlSession session2 = MyBatisUtils.mySession(); System.out.println("---------------------");
users = session2.selectOne(statement , 3);
System.out.println(users);
}
※运行后的结果：
2015-05-04 23:06:25,518 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Preparing: select * from users where id=?
2015-05-04 23:06:25,557 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Parameters: 3(Integer)
2015-05-04 23:06:25,580 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - <== Total: 1
Users [id=3, username=张三, password=123456, age=99, sex=0]
Users [id=3, username=张三, password=123456, age=99, sex=0]
---------------------//重新获取session，这个session2重新从数据库查询一条数据出来
2015-05-04 23:06:25,794 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Preparing: select * from users where id=?
2015-05-04 23:06:25,794 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Parameters: 3(Integer)
2015-05-04 23:06:25,795 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - <== Total: 1
Users [id=3, username=张三, password=123456, age=99, sex=0]
二级缓存测试（默认关闭状态）
首先在usersMapper.xml开启
<?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.entity.test009.usersMapper">
<cache/>
<select id="myUsers" parameterType="int" resultType="Users">
select * from users where id=#{id}
</select><select id="myUpdate" parameterType="Users">
update users set username=#{username},password=#{password} where id=#{id}
</select></mapper>
然后新建一个创建工厂的工具类，返回一个factory，这样创建出来的session就不一样。
public class CopyOfMyBatisUtils {public static SqlSessionFactory factory() throws Exception{String resource = "conf.xml";
Reader reader = Resources.getResourceAsReader(resource);
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(reader);
return factory;
}
}
测试代码：
@Test
public void cacheTwo() throws Exception
{ String statement ="com.entity.test009.usersMapper.myUsers"; SqlSessionFactory factory = CopyOfMyBatisUtils.factory();
SqlSession session1 = factory.openSession();
SqlSession session2 = factory.openSession(); users = session1.selectOne(statement , 6);
session1.commit();//需要提交事物，否则二级缓存无效
System.out.println(users); System.out.println("****************************"); users = session1.selectOne(statement , 6);
session2.commit();
System.out.println(users); }
运行结果：
2015-05-04 23:45:21,750 [main] DEBUG [com.entity.test009.usersMapper] - Cache Hit Ratio [com.entity.test009.usersMapper]: 0.0
2015-05-04 23:45:22,035 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Preparing: select * from users where id=?
2015-05-04 23:45:22,079 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - ==> Parameters: 6(Integer)
2015-05-04 23:45:22,100 [main] DEBUG [com.entity.test009.usersMapper.myUsers] - <== Total: 1
Users [id=6, username=张3, password=654321, age=99, sex=0]
****************************
2015-05-04 23:45:22,109 [main] DEBUG [com.entity.test009.usersMapper] - Cache Hit Ratio 缓存命中率 [com.entity.test009.usersMapper]: 0.5 百分比
Users [id=6, username=张3, password=654321, age=99, sex=0]
补充说明：
映射语句文件中的所有select语句将会被缓存。
映射语句文件中的所有insert，update和delete语句会刷新缓存。
缓存会使用Least Recently Used（LRU，最近最少使用的）算法来收回。
根据时间表（比如no Flush Interval，没有刷新间隔），缓存不会以任何时间顺序来刷新。
缓存会存储列表集合或对象（无论查询方法返回什么）的1024个引用。
缓存会被视为是read/write（可读/可写）的缓存，意味着对象检索不是共享的，而且可以安全地被调用者修改，而不干扰其他调用者或线程所做的潜在修改
<cache eviction="FIFO" //回收策略为先进先出flushInterval="60000" //自动刷新时间60ssize="512" //最多缓存512个引用对象readOnly="true" //只读
/>
