一、环境：
三个mysql数据库。一个master，两个slaver。master写数据，slaver读数据。
二、原理：
借助Spring的 AbstractRoutingDataSource 这个抽象实现。我们要实现 determineCurrentLookupKey()这个方法来动态的选择使用哪个数据源操着数据库
public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean { protected abstract Object determineCurrentLookupKey();}
三、实现步骤：
1、添加spring，mybatis，mysql相关的pom依赖。
2、写jdbc.properties，定义三个数据库。
jdbc.driver=com.mysql.jdbc.Driver#master库
master.jdbc.url=jdbc:mysql://127.0.0.1:3306/master?characterEncoding=utf8
master.jdbc.user=root
master.jdbc.password=tiger
#slave 一 库
slave.one.jdbc.url=jdbc:mysql://127.0.0.1:3306/slave-one?characterEncoding=utf8
slave.one.jdbc.user=root
slave.one.jdbc.password=tiger
#slave 二 库
slave.two.jdbc.url=jdbc:mysql://127.0.0.1:3306/slave-two?characterEncoding=utf8
slave.two.jdbc.user=root
slave.two.jdbc.password=tiger
3、配置三个数据源，分别写到三个配置文件中。
datasources-master.xml、datasource-slave-one.xml和datasource-slave-two.xml三个文件都一样，这里就写一个
<!--master数据源，支持读写-->
<bean id="dataSourceMaster" class="com.alibaba.druid.pool.DruidDataSource"
init-method="init" destroy-method="close">
<property name="driverClassName" value="${jdbc.driver}"/>
<property name="url" value="${master.jdbc.url}"/>
<property name="username" value="${master.jdbc.user}"/>
<property name="password" value="${master.jdbc.password}"/> <property name="filters" value="stat"/> <property name="maxActive" value="20"/>
<property name="initialSize" value="1"/>
<property name="maxWait" value="60000"/>
<property name="minIdle" value="1"/> <property name="timeBetweenEvictionRunsMillis" value="60000"/>
<property name="minEvictableIdleTimeMillis" value="300000"/> <property name="validationQuery" value="SELECT 'x'"/>
<property name="testWhileIdle" value="true"/>
<property name="testOnBorrow" value="false"/>
<property name="testOnReturn" value="false"/>
</bean>
4、写spring的配置文件applicationContext.xml。
<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<list>
<value>classpath*:jdbc.properties</value>
</list>
</property>
<property name="fileEncoding" value="UTF-8" />
<property name= "ignoreResourceNotFound" value="false"/>
</bean> <context:component-scan base-package="org.hope.lee"/> <aop:aspectj-autoproxy/> <!--spring的路由来管理数据源-->
<bean id="dynamicDataSource" class="org.hope.lee.utils.DynamicDataSource">
<property name="targetDataSources">
<map>
<entry value-ref="dataSourceMaster" key="db_master"/>
<entry value-ref="dataSourceSlaveOne" key="db_slave_one"/>
<entry value-ref="dataSourceSlaveTwo" key="db_slave_two"/>
</map>
</property>
</bean> <!--spring-mybatis整合-->
<bean id="dynamicsqlSessionFactory"
class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dynamicDataSource"/>
<property name="mapperLocations">
<array>
<value>classpath:mappers/*Mapper.xml</value>
</array>
</property>
<property name="configLocation" value="classpath:mybatis-config.xml"/>
<property name="typeAliasesPackage" value="org.hope.lee.model"/>
</bean> <!--自动扫描所有的Mapper接口与文件-->
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<property name="basePackage" value="org.hope.lee.dao"></property>
<property name="sqlSessionFactoryBeanName" value="dynamicsqlSessionFactory"></property>
</bean> <!--配置事务-->
<bean id="transactionManager"
class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="dynamicDataSource"/>
</bean>
<!--开启注解事务-->
<tx:annotation-driven transaction-manager="transactionManager"/> <import resource="classpath:datasource-master.xml"/>
<import resource="classpath:datasource-slave-one.xml"/>
<import resource="classpath:datasource-slave-two.xml"/>
5、新建DBContextHolder，DBType为动态设置数据库的util
package org.hope.lee.utils;public class DBType {
public final static String DB_TYPE_MASTER = "db_master"; public final static String DB_TYPE_SLAVE_ONE = "db_slave_one"; public final static String DB_TYPE_SLAVE_TWO = "db_slave_two";
}
package org.hope.lee.utils;public class DBContextHolder { private static ThreadLocal<String> contextHolder = new ThreadLocal<String>(); public static String getDBType() {
String db = contextHolder.get();
if(db == null) {
db = DBType.DB_TYPE_MASTER; //默认是master库
} return db;
} public static void setDBType(String dbType) {
contextHolder.set(dbType);
} public static void clearDBType() {
contextHolder.remove();
}}
6、继承Spring的 AbstractRoutingDataSource 来动态的进行数据库路由
package org.hope.lee.utils;import org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource;public class DynamicDataSource extends AbstractRoutingDataSource {
@Override
protected Object determineCurrentLookupKey() {
return DBContextHolder.getDBType();
}
}
7、创建三个数据库master、slave-one、slave-two。三个库建同一张user表进行测试。
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
8、写mybatis的dao层,service层,model层
package org.hope.lee.model;public class User {
private int id;
private String name; setters()&getters()
}
package org.hope.lee.dao;import org.hope.lee.model.User;
import org.springframework.stereotype.Repository;@Repository
public interface UserMapper {
void insert(User user);
User selectOne(int id);
}
<?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="org.hope.lee.dao.UserMapper">
<resultMap id="usersResultMap" type="User">
<id column="id" property="id" javaType="Integer" />
<result column="name" property="name" />
</resultMap> <insert id="insert" useGeneratedKeys="true" parameterType="User">
INSERT INTO `user`(name) VALUES(#{name, jdbcType=VARCHAR});
</insert> <select id="selectOne" resultMap="usersResultMap" parameterType="int" >
SELECT id, name FROM `user` WHERE id=#{id, jdbcType=INTEGER}
</select>
</mapper>
package org.hope.lee.service;import org.hope.lee.dao.UserMapper;
import org.hope.lee.model.User;
import org.hope.lee.utils.DBContextHolder;
import org.hope.lee.utils.DBType;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;@Service("userService")
public class UserServiceImpl implements UserService { @Autowired
private UserMapper userMapper; public void addUser(User user) {
//设置数据库
DBContextHolder.setDBType(DBType.DB_TYPE_MASTER);
userMapper.insert(user);
} public User getUserById(int id) {
//设置数据库，单元测试的时候自己手动修改一下，看看效果
DBContextHolder.setDBType(DBType.DB_TYPE_SLAVE_ONE);
return userMapper.selectOne(id);
}
}
9、单元测试。
import org.hope.lee.model.User;
import org.hope.lee.service.UserService;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"classpath*:applicationContext.xml"})
public class UserServiceTest { @Autowired
private UserService userService; @Test
public void addUserTest() {
User user = new User();
user.setName("马云"); userService.addUser(user);
} @Test
public void getUserOneTest() {
int id = 1;
User u = userService.getUserById(id);
System.out.println(u.getName());
}
}
10、在service层修改 DBContextHolder.setDBType()来看看效果。
四、遇到的问题：
1、遇到Spring的PropertyPlaceholderConfigurer不起效果，在数据源配置的${jdbc.driver}中获取不到jdbc.properties中的值。
<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<list>
<value>classpath*:jdbc.properties</value>
</list>
</property>
<property name="fileEncoding" value="UTF-8" />
<property name= "ignoreResourceNotFound" value="false"/>
</bean>
解决：
<!--
原来id的名称是sqlSessionFactory,但是在spring里使用org.mybatis.spring.mapper.MapperScannerConfigurer
进行自动扫描的时候，设置了sqlSessionFactory 的话，他会优先于PropertyPlaceholderConfigurer执行。
从而导致PropertyPlaceholderConfigurer失效，
这时在xml中用${url}、${username}、${password}等这样之类的表达式，
将无法获取到properties文件里的内容。
-->
<bean id="dynamicsqlSessionFactory"
class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dynamicDataSource"/>
<property name="mapperLocations">
<array>
<value>classpath:mappers/*Mapper.xml</value>
</array>
</property>
<property name="configLocation" value="classpath:mybatis-config.xml"/>
<property name="typeAliasesPackage" value="org.hope.lee.model"/>
</bean>
六、还有一种方式是使用mysql自带的replicationDriver来实现读写分离。大家自己也可以试试
http://blog.csdn.net/lixiucheng005/article/details/17391857
https://gitee.com/huayicompany/mybatis-learn/tree/master/separated-read-write
参考：
[1] 博客，http://blog.csdn.net/xtj332/article/details/43953699
[2] 博客，http://blog.csdn.net/keda8997110/article/details/16827215
