本文将介绍怎样开启 MyBatis 的二级缓存。MyBatis 要开启二级缓存很简单，过程如下：
1）在 mybatis-cfg.xml 文件的 <settings> 中配置 <setting name="cacheEnabled" value="true" />，启用 MyBatis 全局缓存配置。
2）在 Mapper XML 文件中添加 <cache /> 标签，该标签可以配置缓存策略、刷新时间、缓存数量等。
（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/demo2/UserMapper.xml" />
</mappers>
</configuration>上面配置文件使用 “<setting name="cacheEnabled" value="true" />” 开启 MyBatis 全局缓存。
（2）定义JavaBean实体 UserBean.java，代码如下：
package com.hxstrive.mybatis.cache.demo2;
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
}注意：上面定义的 UserBean 需要实现 java.io.Serializable 接口，因为缓存需要序列化和反序列化。
（3）定义 Mapper 接口和配置文件，如下：
a、UserMapper.java
package com.hxstrive.mybatis.cache.demo2;
import java.util.List;
public interface UserMapper {
List<UserBean> findAll();
}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.demo2.UserMapper"> <!-- 开启二级缓存 --> <cache /> <!-- 映射结果 --> <resultMap id="RESULT_MAP" type="com.hxstrive.mybatis.cache.demo2.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="findAll" resultMap="RESULT_MAP" useCache="true"> select `user_id`, `name`, `sex`, `age` from `user` </select> </mapper>
上面通过 “<cache />” 配置开启 MyBatis 的二级缓存，这里的 <cache> 不是必须的，因为在前面使用了 “<setting name="cacheEnabled" value="true" />” 开启全局缓存。
（4）客户端代码如下：
package com.hxstrive.mybatis.cache.demo2;
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;
import java.util.List;
public class CacheDemo {
public static void main(String[] args) throws Exception {
String cfgName = "com/hxstrive/mybatis/cache/demo2/mybatis-cfg.xml";
InputStream input = Resources.getResourceAsStream(cfgName);
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlFactory = factoryBuilder.build(input);
getUserInfo(sqlFactory.openSession());
getUserInfo(sqlFactory.openSession());
}
private static void getUserInfo(SqlSession sqlSession) {
System.out.println("查询用户信息：");
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
List<UserBean> userBeanList = userMapper.findAll();
for(UserBean userBean : userBeanList) {
System.out.println(userBean + " -- " + userBean.getName());
}
sqlSession.close();
}
}上面客户端代码中的 getUserInfo 方法将分别在不同的 SqlSession 中执行，这是因为 MyBatis 的二级缓存是 SqlSessionFactory 级别的。
运行客户端代码输出如下：
2020-09-07 13:11:03,400 DEBUG [org.apache.ibatis.logging.LogFactory] - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter. 2020-09-07 13:11:03,411 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 2020-09-07 13:11:03,411 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 2020-09-07 13:11:03,411 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 2020-09-07 13:11:03,412 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections. 查询用户信息： 2020-09-07 13:11:03,516 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo2.UserMapper]: 0.0 2020-09-07 13:11:03,530 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2020-09-07 13:11:04,058 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Created connection 1663411182. 2020-09-07 13:11:04,058 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6325a3ee] 2020-09-07 13:11:04,066 DEBUG [com.hxstrive.mybatis.cache.demo2.UserMapper.findAll] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@6325a3ee] 2020-09-07 13:11:04,067 DEBUG [com.hxstrive.mybatis.cache.demo2.UserMapper.findAll] - ==> Preparing: select `user_id`, `name`, `sex`, `age` from `user` 2020-09-07 13:11:04,199 DEBUG [com.hxstrive.mybatis.cache.demo2.UserMapper.findAll] - ==> Parameters: 2020-09-07 13:11:04,256 DEBUG [com.hxstrive.mybatis.cache.demo2.UserMapper.findAll] - <== Total: 3 com.hxstrive.mybatis.cache.demo2.UserBean@6737fd8f -- Helen com.hxstrive.mybatis.cache.demo2.UserBean@72b6cbcc -- 张小凡 com.hxstrive.mybatis.cache.demo2.UserBean@a7e666 -- 叶星云 2020-09-07 13:11:04,259 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6325a3ee] 2020-09-07 13:11:04,259 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6325a3ee] 2020-09-07 13:11:04,260 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Returned connection 1663411182 to pool. 查询用户信息： 2020-09-07 13:11:04,260 DEBUG [org.apache.ibatis.cache.decorators.LoggingCache] - Cache Hit Ratio [com.hxstrive.mybatis.cache.demo2.UserMapper]: 0.5 com.hxstrive.mybatis.cache.demo2.UserBean@6737fd8f -- Helen com.hxstrive.mybatis.cache.demo2.UserBean@72b6cbcc -- 张小凡 com.hxstrive.mybatis.cache.demo2.UserBean@a7e666 -- 叶星云
仔细查看上面的输出结果，第一次查询创建了一个连接 “Created connection 1663411182”，而第二次连接并没有创建连接，而是直接从缓存中获取，此时缓存命中率为 0.5（Cache Hit Ratio [com.hxstrive.mybatis.cache.demo2.UserMapper]: 0.5）。
