MyBatis学习（四）

缓存

存在内存中的临时数据，将用户经常查询的数据放在缓存中，用户就不用从磁盘去查询，提高了查询效率

一级缓存

• 一级缓存（默认打开），在sqlSession从创建到close之间有效
• 官方文档：select会缓存，增删改会刷新缓存
• 验证：开启日志，在一次sqlsession中查询两次记录，并且比较。再两次查询中间修改一次数据，再查询。

代码（核心配置、bean、dao、utils、TEST)

mybatis-config.xml

<?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>

    <!--引入外部配置文件-->
    <properties resource="db.properties">
    </properties>

    <!--配置日志-->
    <settings>
<!--        <setting name="logImpl" value="LOG4J"/>-->
                <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

    <typeAliases>
        <package name="com.wangqi.bean"/>
    </typeAliases>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>


    <mappers>
        <!--mappers配置方式一-->
        <mapper class="com.wangqi.dao.UserMapper"/>
<!--        <mapper resource="com/wangqi/dao/StudentMapper.xml"/>-->
<!--        <mapper resource="com/wangqi/dao/TeacherMapper.xml"/>-->

        <!--mappers配置方式二和三，下面两种必须要求.xml和.java同名且在一个目录下
        <mapper class="com.wangqi.dao.UserMapper"/>
        <package name="com.wangqi.dao"/>-->
    </mappers>
</configuration>

User.java

package com.wangqi.bean;

public class User {
    private int id;
    private String name;
    private String pwd;

    public User() {
    }

    public User(int id, String name, String pwd) {
        this.id = id;
        this.name = name;
        this.pwd = pwd;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", pwd='" + pwd + '\'' +
                '}';
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPwd() {
        return pwd;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }
}
UserMapper.java
```java
package com.wangqi.dao;

import com.wangqi.bean.User;
import org.apache.ibatis.annotations.Param;

public interface UserMapper {

    User queryUserById(@Param("id") int id);

    int updateUser(User user);
}

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.wangqi.dao.UserMapper">
    <select id="queryUserById" resultType="User">
        select * from user where id = #{id}
    </select>

    <update id="updateUser" parameterType="User">
        update user set id = #{id}, name = #{name}, pwd = #{pwd} where id = 1
    </update>
</mapper>

MybatisUtils.java

package com.wangqi.utils;


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.IOException;
import java.io.InputStream;

public class MybatisUtils {

    static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
    }
}

Test.java

package com.wangqi.dao;

import com.wangqi.bean.User;
import com.wangqi.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

public class UserMapperTest {
    @Test
    public void Test() {
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);

        User user = mapper.queryUserById(2);
        int i = mapper.updateUser(new User(1,"小里","321"));
        User user2 = mapper.queryUserById(2);
        System.out.println(user == user2);

        sqlSession.close();
    }
}

运行结果

二级缓存

• 基于namespace级别的缓存
• 一级缓存在会话中，会话关闭就消失。存放在二级缓存中。
• <cache 标签，readonly=true则表示，cache存放的是引用，不要修改，false则表示存放的是序列化！

步骤

1. 开启全局缓存

在核心文件中添加
mybatis-config.xml

    <!--配置日志-->
    <settings>
        <setting name="cacheEnabled" value="true"/>
    </settings>

2. 在要使用二级缓存的Mapper中开启

如下例
UserMapper.xml

<cache
  eviction="FIFO"
  flushInterval="60000"
  size="512"
  readOnly="true"/>