首先我们需要添加多个数据源,找到mybatis的配置类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>
    <settings>
        <!-- 设置日志输出为LOG4J -->
        <setting name="logImpl" value="LOG4J" />
        <!--将以下画线方式命名的数据库列映射到 Java 对象的驼峰式命名属性中 -->
        <setting name="mapUnderscoreToCamelCase" value="true" />
        <setting name="cacheEnabled" value="false" />
    </settings>
    <!--简化类命名空间 -->
    <typeAliases>
        <package name="tk.mybatis.simple.model" />
    </typeAliases>

    <environments default="reader">
        <environment id="reader">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver" />
                <property name="url"
                    value="jdbc:mysql://localhost:3306/mybatis?useSSL=false" />
                <property name="username" value="root" />
                <property name="password" value="root" />
                <property name="poolPingEnabled" value="true" />
                <property name="poolPingQuery" value="select 1" />
                <property name="poolPingConnectionsNotUsedFor"
                    value="3600000" />
            </dataSource>
        </environment>
        <environment id="write">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver" />
                <property name="url"
                    value="jdbc:mysql://localhost:3306/mybatis?useSSL=false" />
                <property name="username" value="root" />
                <property name="password" value="root" />
                <property name="poolPingEnabled" value="true" />
                <property name="poolPingQuery" value="select 1" />
                <property name="poolPingConnectionsNotUsedFor"
                    value="3600000" />
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--常规做法 -->
        <!--<mapper resource="tk/mybatis/simple/mapper/CountryMapper.xml" /> -->
        <!--<mapper resource="tk/mybatis/simple/mapper/PrivilegeMapper.xml" /> -->
        <!--<mapper resource="tk/mybatis/simple/mapper/RoleMapper.xml" /> -->
        <!--<mapper resource="tk/mybatis/simple/mapper/RolePrivilegeMapper.xml"/> -->
        <!--<mapper resource="tk/mybatis/simple/mapper/UserMapper.xml" /> -->
        <!--<mapper resource="tk/mybatis/simple/mapper/UserRoleMapper.xml" /> -->
        <!--第二种做法 -->
        <package name="tk.mybatis.simple.mapper" />
    </mappers>
</configuration>

一个是 <environment id="reader">, 一个是 <environment id="write"> 就是一个是读库一个是写库

然后简单的改造下BaseMapperTest.java

package simple;

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 org.junit.BeforeClass;

import java.io.IOException;
import java.io.Reader;

public class BaseMapperTest {
    private static SqlSessionFactory sqlSessionFactory;  
    private static SqlSessionFactory wirteSqlSessionFactory;  
    private static String mybatisfilename="mybatis-config.xml";

    @BeforeClass
    public static void init() { 
        try {
            Reader readerinputStream  = Resources.getResourceAsReader(mybatisfilename);
            Reader wireteinputStream  = Resources.getResourceAsReader(mybatisfilename);
            SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
            sqlSessionFactory = builder.build(readerinputStream, "reader");
            wirteSqlSessionFactory = builder.build(wireteinputStream, "write");
            readerinputStream.close();
            wireteinputStream.close(); 
        } catch (IOException ignore) {
            ignore.printStackTrace();
        }
    }

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

    public SqlSession getWriteSqlSession() {
        return wirteSqlSessionFactory.openSession();
    }
}

为什么要两个Reader , 因为在SqlSessionFactoryBuilder 的build方法之后直接吧Reader 给close了, 所以我们这里需要new两个, 当然有别的好方法麻烦提供,谢谢
UserMapperTest.java测试类, 里面找到testInsert3() 代码稍微改动下

public class UserMapperTest extends BaseMapperTest{
.
.
.
    @Test
    public void testInsert3() {
        SqlSession sqlSession = getWriteSqlSession();
        try {
            SysUser sysUser = new SysUser();
            sysUser.setUserName("obj");
            sysUser.setUserInfo("ifno");
            sysUser.setUserEmail("obj@sina.com");
            sysUser.setUserPassword("123456");
            sysUser.setCreateTime(new Date());
            UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
            int result = userMapper.insert3(sysUser);
            System.out.println(result);
            System.out.println("返回数据库生成的id" + sysUser.getId());
        } finally {
            sqlSession.commit();
            sqlSession.close();
        }
    }
}

getWriteSqlSession();就是我们的写库, 然后执行发现成功

至于读库我们默认的就是读库,所以代码完全一样, 另外我个人来说一定会把mybatis的缓存去掉 在所有查询节点上加上flushCache="true"

<select id="selectByid" resultMap="userMap" flushCache="true">
            select * from sys_user where id =#{id}
    </select>

下一个章节为事务, 既然涉及到读写分离就应该多事务严格起来