是从springmvc的思路上来做的，主要就是配置主、从DataSource，
再继承AbstractRoutingDataSource，重写determineCurrentLookupKey
方法，通过Context结合 aop 进行数据主、从库的切换。

/*

*    重写的函数决定了最后选择的DataSource

*    因为AbstractRoutingDataSource中获取连接方法为：

@Override

    public Connection getConnection(String username, String password) throws SQLException {

        return determineTargetDataSource().getConnection(username, password);

    }

*/

public class MultiRouteDataSource extends AbstractRoutingDataSource {

    @Override

    protected Object determineCurrentLookupKey() {

        return DataSourceContextHolder.getDataSource();

    }

}

@Target({ ElementType.METHOD, ElementType.TYPE })

@Retention(RetentionPolicy.RUNTIME)

@Documented

public @interface TargetDataSource {

    String value();

}

public class BaseDataSourceConfig {

    private String url;

    private String username;

    private String password;

    private String driverClassName;

    // 添加上getter、setter方法

}

@Configuration

public class DataSourceComponent {

    @Resource

    MasterDataSourceConfig masterDataSourceConfig;

    @Resource

    FirstDataSourceConfig firstDataSourceConfig;

    @Resource

    SecondDataSourceConfig secondDataSourceConfig;

    /*

     * 一开始以为springboot的自动配置还是会生效，直接加了@Resource DataSource dataSource；

     * 显示是不work的，会报create bean 错误

     */

    public DataSource masterDataSource() {

        DataSource dataSource = new DataSource();

        dataSource.setUrl(masterDataSourceConfig.getUrl());

        dataSource.setUsername(masterDataSourceConfig.getUsername());

        dataSource.setPassword(masterDataSourceConfig.getPassword());

        dataSource.setDriverClassName(masterDataSourceConfig.getDriverClassName());

        return dataSource;

    }

    /*

     * 一开始在这里加了@Bean的注解，当然secondDataSource()也加了

     * 会导致springboot识别的时候，发现有多个

     * 所以，其实都不要加@Bean，最终有效的的DataSource就只需要一个multiDataSource即可

     */

    public DataSource firstDataSource() {

        DataSource dataSource = new DataSource();

        dataSource.setUrl(firstDataSourceConfig.getUrl());

        dataSource.setUsername(firstDataSourceConfig.getUsername());

        dataSource.setPassword(firstDataSourceConfig.getPassword());

        dataSource.setDriverClassName(firstDataSourceConfig.getDriverClassName());

        return dataSource;

    }

    public DataSource secondDataSource() {

        DataSource dataSource = new DataSource();

        dataSource.setUrl(secondDataSourceConfig.getUrl());

        dataSource.setUsername(secondDataSourceConfig.getUsername());

        dataSource.setPassword(secondDataSourceConfig.getPassword());

        dataSource.setDriverClassName(secondDataSourceConfig.getDriverClassName());

        return dataSource;

    }

    @Bean(name = "multiDataSource")

    public MultiRouteDataSource exampleRouteDataSource() {

        MultiRouteDataSource multiDataSource = new MultiRouteDataSource();

        Map<Object, Object> targetDataSources = new HashMap<>();

        targetDataSources.put("master", masterDataSource());

        targetDataSources.put("first", firstDataSource());

        targetDataSources.put("second", secondDataSource());

        multiDataSource.setTargetDataSources(targetDataSources);

        multiDataSource.setDefaultTargetDataSource(masterDataSource());

        return multiDataSource;

    }

    @Bean(name = "transactionManager")

    public DataSourceTransactionManager dataSourceTransactionManager() {

        DataSourceTransactionManager manager = new DataSourceTransactionManager();

        manager.setDataSource(exampleRouteDataSource());

        return manager;

    }

    @Bean(name = "sqlSessionFactory")

    public SqlSessionFactoryBean sqlSessionFactory() {

        SqlSessionFactoryBean sessionFactoryBean = new SqlSessionFactoryBean();

        sessionFactoryBean.setDataSource(exampleRouteDataSource());

        return sessionFactoryBean;

    }

}

public class DataSourceContextHolder {

    private static final ThreadLocal<String> contextHolder = new ThreadLocal<>();

    public static void setDataSource(String value) {

        contextHolder.set(value);

    }

    public static String getDataSource() {

        return contextHolder.get();

    }

    public static void clearDataSource() {

        contextHolder.remove();

    }

}

@Aspect

@Component

public class DynamicDataSourceAspect {

    @Around("execution(public * com.wdm.example.service..*.*(..))")

    public Object around(ProceedingJoinPoint pjp) throws Throwable {

        MethodSignature methodSignature = (MethodSignature) pjp.getSignature();

        Method targetMethod = methodSignature.getMethod();

        if(targetMethod.isAnnotationPresent(TargetDataSource.class)){

            String targetDataSource = targetMethod.getAnnotation(TargetDataSource.class).value() ;

            DataSourceContextHolder.setDataSource(targetDataSource);

        }

        Object result = pjp.proceed();

        DataSourceContextHolder.clearDataSource();

        return result;

    }

}

package com.wdm.example.service;

import java.util.Date;

import javax.annotation.Resource;

import org.springframework.stereotype.Service;

import com.wdm.example.dao.UserDao;

import com.wdm.example.datasource.TargetDataSource;

import com.wdm.example.model.User;

import com.wdm.example.service.UserService;

/*

 * @author wdmyong

 * 20170416

 */

@Service

public class UserService {

    @Resource

    UserDao userDao;

    public User getById(Integer id) {

        return userDao.getById(id);

    }

    @TargetDataSource("master")

    public User getById0(Integer id) {

        return userDao.getById(id);

    }

    @TargetDataSource("first")

    public User getById1(Integer id) {

        return userDao.getById(id);

    }

    @TargetDataSource("second")

    public User getById2(Integer id) {

        return userDao.getById(id);

    }

    public void insert(User user) {

        Date now = new Date();

        user.setCreateTime(now);

        user.setModifyTime(now);

        userDao.insert(user);

    }

    public void update(User user) {

        user.setModifyTime(new Date());

        userDao.update(user);

    }

}

PS：其实之前一直以为DataSource听起来挺悬乎，没去细想，当然主要由于自己是半路出家的Java、web开发，本身也没那么熟悉，所以没理解哈，

现在想想DataSource其实就是保存了些配置，说白了是url和账号密码，就是连接数据库的，相当于你用命令行连接了数据库进行了操作一样，各种

数据库DataSource的实现高功能多半应该是做了些连接池的管理，以及连接的打开关闭之类，其实实质上我觉得应该就是说最后用的就是那个url加

上账号密码就能连接并操作了。这样的话，多数据源的切换就好理解了，结合 aop 在函数入口之前设置好当前线程数据源，以及根据路由数据库类

AbstractRoutingDataSource将选择数据源留给子类实现的方法

determineCurrentLookupKey，从而在service方法入口设置数据源，在使用时取到数据源。