当项目中使用到读写分离的时候，我们就会遇到多数据源的问题。多数据源让人最头痛的，不是配置多个数据源，而是如何能灵活动态的切换数据源。例如在一个spring和Mybatis的框架的项目中，我们在spring配置中往往是配置一个dataSource来连接数据库，然后绑定给sessionFactory，在dao层代码中再指定sessionFactory来进行数据库操作。

正如上图所示，每一块都是指定绑死的，如果是多个数据源，也只能是下图中那种方式。

可看出在Dao层代码中写死了两个SessionFactory，这样日后如果再多一个数据源，还要改代码添加一个SessionFactory，显然这并不符合开闭原则。

那么正确的做法应该是：

具体代码与配置如下:

1、applicationContext-mgr.xml

<?xml version="1.0" encoding="utf-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xmlns:aop="http://www.springframework.org/schema/aop"
 xmlns:context="http://www.springframework.org/schema/context" 
 xmlns:tx="http://www.springframework.org/schema/tx"
 xmlns:p="http://www.springframework.org/schema/p"
 xsi:schemaLocation="http://www.springframework.org/schema/beans 
 http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
 http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd
 http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
    <!-- use annotation -->
    <context:annotation-config />
        <context:component-scan base-package="com.carl.o2o.**.mgr">
    </context:component-scan>

    <!-- master -->
    <bean id="master" class="com.mchange.v2.c3p0.ComboPooledDataSource">
        <property name="driverClass" value="${driverClassName_master}"/>
        <property name="user" value="${username_master}"/>
        <property name="password" value="${password_master}"/>
        <property name="jdbcUrl" value="${url_master}?Unicode=true&amp;characterEncoding=UTF-8&amp;allowMultiQueries=true"/>

        <property name="maxPoolSize" value="150"/>  
        <property name="minPoolSize" value="10"/>  
        <property name="initialPoolSize" value="20"/>  
        <property name="maxIdleTime" value="3600"/>  
        <property name="acquireIncrement" value="10"/>  
        <property name="idleConnectionTestPeriod" value="1800"/>    
    </bean>

    <!-- slave -->
    <bean id="slave" class="com.mchange.v2.c3p0.ComboPooledDataSource">
        <property name="driverClass" value="${driverClassName_slave}"/>
        <property name="user" value="${username_slave}"/>
        <property name="password" value="${password_slave}"/>
        <property name="jdbcUrl" value="${url_slave}?Unicode=true&amp;characterEncoding=UTF-8"/>

        <property name="maxPoolSize" value="150"/>  
        <property name="minPoolSize" value="10"/>  
        <property name="initialPoolSize" value="20"/>  
        <property name="maxIdleTime" value="3600"/>  
        <property name="acquireIncrement" value="10"/>  
        <property name="idleConnectionTestPeriod" value="1800"/>    
    </bean>

    <!-- spring 动态数据源 -->
    <bean id="dynamicDataSource" class="com.carl.dbUtil.DynamicDataSource">
        <property name="targetDataSources">  
           <map key-type="java.lang.String">  
                <entry key="slave" value-ref="slave" />  
            </map>  
       </property>  
       <property name="defaultTargetDataSource" ref="master" />      
   </bean>  

    <!-- mybatis mapper config -->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dynamicDataSource"/>
        <property name="configLocation" value="classpath:o2o_mybatis_config.xml"/>
        <property name="mapperLocations" >
            <list>
                <value>classpath:sqlMap/*.xml</value>
                <value>classpath*:/com/carl/o2o/**/*.xml</value>
            </list>
        </property>
    </bean>

    <bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate">
        <constructor-arg index="0" ref="sqlSessionFactory"></constructor-arg>
    </bean>

    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="com.carl.o2o.**.mgr.dao" />
    </bean>

    <!-- 多数据源 aop -->
    <bean id="DataSourceAspect" class="com.carl.dbUtil.DataSourceAspect" />
    <aop:config>  
        <aop:advisor pointcut="execution(* com.carl.o2o.mgr.*.*(..))" advice-ref="DataSourceAspect" />
    </aop:config>  

    <!-- 事务 -->
    <bean name="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">   
        <property name="dataSource" ref="dynamicDataSource"></property>
    </bean>
 </beans>

2、DynamicDataSource

DynamicDataSource使用Spring中的代码结合AOP实现多数据源切换.

public class DynamicDataSource extends AbstractRoutingDataSource {
    public DynamicDataSource() {
    }

    protected Object determineCurrentLookupKey() {
        return DBContextHolder.getDbType();
    }

    public Logger getParentLogger() {
        return null;
    }
}

3、DBContextHolder

DynamicDataSource的辅助类，用于实际的切换多数据源。

public class DBContextHolder {
    private static ThreadLocal<String> contextHolder = new ThreadLocal();
    public static String MASTER = "master";
    public static String SLAVE = "slave";

    public DBContextHolder() {
    }

    public static String getDbType() {
        String db = (String)contextHolder.get();
        if(db == null) {
            db = MASTER;
        }

        return db;
    }

    public static void setDbType(String str) {
        contextHolder.set(str);
    }

    public static void setMaster() {
        contextHolder.set(MASTER);
    }

    public static void setSlave() {
        contextHolder.set(SLAVE);
    }

    public static void clearDBType() {
        contextHolder.remove();
    }
}

4、DataSourceAspect

多数据源AOP切面编程实现。

public class DataSourceAspect implements MethodBeforeAdvice, AfterReturningAdvice, ThrowsAdvice {
    private static final Logger log = LogManager.getLogger(DataSourceAspect.class);

    public DataSourceAspect() {
    }

    public void before(Method m, Object[] args, Object target) throws Throwable {
        try {
            if(m != null) {
                if((m.getName().startsWith("list") || m.getName().startsWith("select") || m.getName().startsWith("get") 
                || m.getName().startsWith("count")) && !m.getName().contains("FromMaster")) {
                    DBContextHolder.setDbType("slave");
                } else {
                    DBContextHolder.setDbType("master");
                }
            }
        } catch (Exception var5) {
            log.error("data source aspect error.", var5);
        }

    }

    public void after(JoinPoint point) {
        log.info("clear db type after method.current id {}", new Object[]{Long.valueOf(Thread.currentThread().getId())});
        DBContextHolder.clearDBType();
    }

    public void afterReturning(Object returnValue, Method method, Object[] args, Object target) throws Throwable {
    }

    public void afterThrowing(Method method, Object[] args, Object target, Exception ex) throws Throwable {
        log.info("current db type {} when exception", new Object[]{DBContextHolder.getDbType()});
        DBContextHolder.setDbType("master");
    }
}