本文将介绍怎样使用 <cache> 标签指定二级缓存策略、以及缓存大小。如下：
<!-- evication LRU 最近最少使用--> <cache eviction="LRU" size="2" />
上面指定缓存策略为 LRU（最近最少使用），缓存大小为2个对象。
LRU是Least Recently Used的缩写，即最近最少使用，是一种常用的页面置换算法，选择最近最久未使用的页面予以淘汰。该算法赋予每个页面一个访问字段，用来记录一个页面自上次被访问以来所经历的时间 t，当须淘汰一个页面时，选择现有页面中其 t 值最大的，即最近最少使用的页面予以淘汰。
（1）MyBatis 配置文件 mybatis-cfg.xml 内容如下：
<?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>
<properties resource="database.properties"/>
<settings>
<!-- 全局映射器启用缓存 -->
<setting name="cacheEnabled" value="true" />
</settings>
<environments default="MySqlDatabase" >
<environment id="MySqlDatabase" >
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/hxstrive/mybatis/cache/demo3/UserMapper.xml" />
</mappers>
</configuration>（2）定义 JavaBean UserBean.java，代码如下：
package com.hxstrive.mybatis.cache.demo3;
import java.io.Serializable;
public class UserBean implements Serializable {
private Integer userId;
private String name;
private String sex;
private Integer age;
// 忽略 getter 和 setter 方法
}（3）定义 Mapper
a、UserMapper.java
package com.hxstrive.mybatis.cache.demo3;
import org.apache.ibatis.annotations.Param;
public interface UserMapper {
// 根据用户ID获取用户信息
UserBean getUserById(@Param("userId") int userId);
}b、UserMapper.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="com.hxstrive.mybatis.cache.demo3.UserMapper">
<!-- 开启二级缓存 -->
<!-- evication LRU 最近最少使用-->
<cache eviction="LRU"
size="2" />
<!-- 映射结果 -->
<resultMap id="RESULT_MAP" type="com.hxstrive.mybatis.cache.demo3.UserBean">
<id column="user_id" jdbcType="INTEGER" property="userId" />
<result column="name" jdbcType="VARCHAR" property="name" />
<result column="sex" jdbcType="VARCHAR" property="sex" />
<result column="age" jdbcType="INTEGER" property="age" />
</resultMap>
<!-- 查询所有用户信息 -->
<select id="getUserById" resultMap="RESULT_MAP" useCache="true">
select `user_id`, `name`, `sex`, `age`
from `user` where `user_id`=#{userId}
</select>
</mapper>（4）客户端代码：
package com.hxstrive.mybatis.cache.demo3;
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 java.io.InputStream;
public class CacheDemo {
public static void main(String[] args) throws Exception {
String cfgName = "com/hxstrive/mybatis/cache/demo3/mybatis-cfg.xml";
InputStream input = Resources.getResourceAsStream(cfgName);
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlFactory = factoryBuilder.build(input);
getUserById(sqlFactory, 1);
getUserById(sqlFactory, 2);
getUserById(sqlFactory, 3);
getUserById(sqlFactory, 3);
getUserById(sqlFactory, 2);
getUserById(sqlFactory, 1);
}
private static void getUserById(SqlSessionFactory sqlFactory, int userId) {
System.out.println("查询用户信息：");
SqlSession sqlSession = sqlFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
UserBean userBean = userMapper.getUserById(userId);
System.out.println(userBean + " -- " + userBean.getName());
sqlSession.close();
}
}运行上面客户端代码，输出结果如下：
2020-09-07 22:28:21,772 DEBUG [org.apache.ibatis.logging.LogFactory] - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter. 2020-09-07 22:28:21,790 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 2020-09-07 22:28:21,790 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 2020-09-07 22:28:21,790 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 2020-09-07 22:28:21,790 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 查询用户信息： 2020-09-07 22:28:22,345 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo3.UserMapper]: 0.0 2020-09-07 22:28:22,367 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2020-09-07 22:28:23,000 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Created connection 922872566. 2020-09-07 22:28:23,000 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,002 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,003 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Preparing: select `user_id`, `name`, `sex`, `age` from `user` where `user_id`=? 2020-09-07 22:28:23,036 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Parameters: 1(Integer) 2020-09-07 22:28:23,073 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - <== Total: 1 com.hxstrive.mybatis.cache.demo3.UserBean@76707e36 -- Helen 2020-09-07 22:28:23,096 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,097 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,098 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Returned connection 922872566 to pool. 查询用户信息： 2020-09-07 22:28:23,098 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo3.UserMapper]: 0.0 2020-09-07 22:28:23,098 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2020-09-07 22:28:23,098 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Checked out connection 922872566 from pool. 2020-09-07 22:28:23,098 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,099 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,099 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Preparing: select `user_id`, `name`, `sex`, `age` from `user` where `user_id`=? 2020-09-07 22:28:23,099 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Parameters: 2(Integer) 2020-09-07 22:28:23,101 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - <== Total: 1 com.hxstrive.mybatis.cache.demo3.UserBean@27a5f880 -- 张小凡 2020-09-07 22:28:23,101 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,102 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,102 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Returned connection 922872566 to pool. 查询用户信息： 2020-09-07 22:28:23,102 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo3.UserMapper]: 0.0 2020-09-07 22:28:23,102 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2020-09-07 22:28:23,102 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Checked out connection 922872566 from pool. 2020-09-07 22:28:23,103 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,103 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,103 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Preparing: select `user_id`, `name`, `sex`, `age` from `user` where `user_id`=? 2020-09-07 22:28:23,103 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Parameters: 3(Integer) 2020-09-07 22:28:23,106 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - <== Total: 1 com.hxstrive.mybatis.cache.demo3.UserBean@15d0c81b -- 叶星云 2020-09-07 22:28:23,106 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,107 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,107 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Returned connection 922872566 to pool. 查询用户信息： 2020-09-07 22:28:23,110 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo3.UserMapper]: 0.25 com.hxstrive.mybatis.cache.demo3.UserBean@2833cc44 -- 叶星云 查询用户信息： 2020-09-07 22:28:23,111 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo3.UserMapper]: 0.4 com.hxstrive.mybatis.cache.demo3.UserBean@33f88ab -- 张小凡 查询用户信息： 2020-09-07 22:28:23,111 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo3.UserMapper]: 0.3333333333333333 2020-09-07 22:28:23,112 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2020-09-07 22:28:23,112 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Checked out connection 922872566 from pool. 2020-09-07 22:28:23,112 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,112 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,113 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Preparing: select `user_id`, `name`, `sex`, `age` from `user` where `user_id`=? 2020-09-07 22:28:23,113 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - ==> Parameters: 1(Integer) 2020-09-07 22:28:23,115 DEBUG [com.hxstrive.mybatis.cache.demo3.UserMapper.getUserById] - <== Total: 1 com.hxstrive.mybatis.cache.demo3.UserBean@536aaa8d -- Helen 2020-09-07 22:28:23,115 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,116 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3701eaf6] 2020-09-07 22:28:23,116 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Returned connection 922872566 to pool.
根据上面的结果可以得知，如下规律：
来自数据库 —— com.hxstrive.mybatis.cache.demo3.UserBean@76707e36 -- Helen（被缓存了，然后被淘汰了） 来自数据库 —— com.hxstrive.mybatis.cache.demo3.UserBean@27a5f880 -- 张小凡（被缓存了） 来自数据库 —— com.hxstrive.mybatis.cache.demo3.UserBean@15d0c81b -- 叶星云（被缓存了） 从缓存获取 —— com.hxstrive.mybatis.cache.demo3.UserBean@2833cc44 -- 叶星云 从缓存获取 —— com.hxstrive.mybatis.cache.demo3.UserBean@33f88ab -- 张小凡 来自数据库 —— com.hxstrive.mybatis.cache.demo3.UserBean@536aaa8d -- Helen（没有命中，从数据库读取）
说明 MyBatis 只缓存了两个对象。如果你将上面客户端调用 getUserById 的顺序进行调整，如下：
getUserById(sqlFactory, 1); —— 从数据库查询，然后缓存，缓存 [1] getUserById(sqlFactory, 2); —— 缓存未命中，从数据库查询，然后缓存，缓存 [1, 2] getUserById(sqlFactory, 3); —— 缓存未命中，从数据库查询，然后将 ID=1 淘汰，添加当前对象到缓存，缓存 [2, 3] getUserById(sqlFactory, 1); —— 缓存未命中，从数据库查询，然后将 ID=2 淘汰，添加当前对象到缓存，缓存 [3, 1] getUserById(sqlFactory, 2); —— 缓存未命中，从数据库查询，然后将 ID=3 淘汰，添加当前对象到缓存，缓存 [1, 2] getUserById(sqlFactory, 3); —— 缓存未命中，从数据库查询，然后将 ID=1 淘汰，添加当前对象到缓存，缓存 [2, 3]
你会发现上面6次调用 getUserById 缓存命中率为0，所以调用顺序对缓存很重要。更多缓存知识需要读者去探索……
