项目背景：项目开发中数据库使用了读写分离，所有查询语句走从库，除此之外走主库。

最简单的办法其实就是建两个包，把之前数据源那一套配置copy一份，指向另外的包，但是这样扩展很有限，所有采用下面的办法。

参考了两篇文章如下：

http://blog.csdn.net/zl3450341/article/details/20150687

http://www.blogjava.net/hoojo/archive/2013/10/22/405488.html

这两篇文章都对原理进行了分析，下面只写自己的实现过程其他不再叙述。

实现思路是：

第一步，实现动态切换数据源：配置两个DataSource，配置两个SqlSessionFactory指向两个不同的DataSource，两个SqlSessionFactory都用一个SqlSessionTemplate，同时重写Mybatis提供的SqlSessionTemplate类，最后配置Mybatis自动扫描。

第二步，利用aop切面，拦截dao层所有方法，因为dao层方法命名的特点，比如所有查询sql都是select开头，或者get开头等等，拦截这些方法，并把当前数据源切换至从库。

spring中配置如下：

主库数据源配置：

<bean id="masterDataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
    <property name="driverClass" value="${master_mysql_jdbc_driver}" />
    <property name="jdbcUrl" value="${master_mysql_jdbc_url}" />
    <property name="user" value="${master_mysql_jdbc_user}" />
    <property name="password" value="${master_mysql_jdbc_password}" />
</bean>

从库数据源配置：

<bean id="masterDataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
    <property name="driverClass" value="${slave_mysql_jdbc_driver}" />
    <property name="jdbcUrl" value="${slave_mysql_jdbc_url}" />
    <property name="user" value="${slave_mysql_jdbc_user}" />
    <property name="password" value="${slave_mysql_jdbc_password}" />
</bean>

主库SqlSessionFactory配置：

<bean id="masterSqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="dataSource" ref="masterDataSource" />
    <property name="mapperLocations"  value="classpath:com/sincetimes/slg/dao/*.xml"/>
</bean>

从库SqlSessionFactory配置：

<bean id="slaveSqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="dataSource" ref="slaveDataSource" />
    <property name="mapperLocations"  value="classpath:com/sincetimes/slg/dao/*.xml"/>
</bean>

两个SqlSessionFactory使用同一个SqlSessionTemplate配置：

<bean id="MasterAndSlaveSqlSessionTemplate" class="com.sincetimes.slg.framework.core.DynamicSqlSessionTemplate">
    <constructor-arg index="0" ref="masterSqlSessionFactory" />
    <property name="targetSqlSessionFactorys">
        <map>  
            <entry value-ref="masterSqlSessionFactory" key="master"/>  
            <entry value-ref="slaveSqlSessionFactory" key="slave"/>  
        </map>  
    </property>
</bean>

配置Mybatis自动扫描dao

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
    <property name="basePackage" value="com.sincetimes.slg.dao" />
    <property name="sqlSessionTemplateBeanName" value="MasterAndSlaveSqlSessionTemplate" />
</bean>

自己重写了SqlSessionTemplate代码如下：

package com.sincetimes.slg.framework.core;

import static java.lang.reflect.Proxy.newProxyInstance;
import static org.apache.ibatis.reflection.ExceptionUtil.unwrapThrowable;
import static org.mybatis.spring.SqlSessionUtils.closeSqlSession;
import static org.mybatis.spring.SqlSessionUtils.getSqlSession;
import static org.mybatis.spring.SqlSessionUtils.isSqlSessionTransactional;
 
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.sql.Connection;
import java.util.List;
import java.util.Map;

import org.apache.ibatis.exceptions.PersistenceException;
import org.apache.ibatis.executor.BatchResult;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ExecutorType;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.MyBatisExceptionTranslator;
import org.mybatis.spring.SqlSessionTemplate;
import org.springframework.dao.support.PersistenceExceptionTranslator;
import org.springframework.util.Assert;

import com.sincetimes.slg.framework.util.SqlSessionContentHolder;


/**
 * 
 * TODO         重写SqlSessionTemplate
 * @author      ccg
 * @version        1.0
 * Created        2017年4月21日 下午3:15:15
 */
public class DynamicSqlSessionTemplate extends SqlSessionTemplate {
 
    private final SqlSessionFactory sqlSessionFactory;
    private final ExecutorType executorType;
    private final SqlSession sqlSessionProxy;
    private final PersistenceExceptionTranslator exceptionTranslator;
 
    private Map<Object, SqlSessionFactory> targetSqlSessionFactorys;
    private SqlSessionFactory defaultTargetSqlSessionFactory;
 
    public void setTargetSqlSessionFactorys(Map<Object, SqlSessionFactory> targetSqlSessionFactorys) {
        this.targetSqlSessionFactorys = targetSqlSessionFactorys;
    }
    
    public Map<Object, SqlSessionFactory> getTargetSqlSessionFactorys(){
        return targetSqlSessionFactorys;
    }
 
    public void setDefaultTargetSqlSessionFactory(SqlSessionFactory defaultTargetSqlSessionFactory) {
        this.defaultTargetSqlSessionFactory = defaultTargetSqlSessionFactory;
    }
 
    public DynamicSqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
        this(sqlSessionFactory, sqlSessionFactory.getConfiguration().getDefaultExecutorType());
    }
 
    public DynamicSqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType) {
        this(sqlSessionFactory, executorType, new MyBatisExceptionTranslator(sqlSessionFactory.getConfiguration()
                .getEnvironment().getDataSource(), true));
    }
 
    public DynamicSqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
            PersistenceExceptionTranslator exceptionTranslator) {
 
        super(sqlSessionFactory, executorType, exceptionTranslator);
 
        this.sqlSessionFactory = sqlSessionFactory;
        this.executorType = executorType;
        this.exceptionTranslator = exceptionTranslator;
        
        this.sqlSessionProxy = (SqlSession) newProxyInstance(
                SqlSessionFactory.class.getClassLoader(),
                new Class[] { SqlSession.class }, 
                new SqlSessionInterceptor());
 
        this.defaultTargetSqlSessionFactory = sqlSessionFactory;
    }
 
    @Override
    public SqlSessionFactory getSqlSessionFactory() {
 
        SqlSessionFactory targetSqlSessionFactory = targetSqlSessionFactorys.get(SqlSessionContentHolder.getContextType());
        if (targetSqlSessionFactory != null) {
            return targetSqlSessionFactory;
        } else if (defaultTargetSqlSessionFactory != null) {
            return defaultTargetSqlSessionFactory;
        } else {
            Assert.notNull(targetSqlSessionFactorys, "Property 'targetSqlSessionFactorys' or 'defaultTargetSqlSessionFactory' are required");
            Assert.notNull(defaultTargetSqlSessionFactory, "Property 'defaultTargetSqlSessionFactory' or 'targetSqlSessionFactorys' are required");
        }
        return this.sqlSessionFactory;
    }
 
    @Override
    public Configuration getConfiguration() {
        return this.getSqlSessionFactory().getConfiguration();
    }
 
    public ExecutorType getExecutorType() {
        return this.executorType;
    }
 
    public PersistenceExceptionTranslator getPersistenceExceptionTranslator() {
        return this.exceptionTranslator;
    }
 
    /**
     * {@inheritDoc}
     */
    public <T> T selectOne(String statement) {
        return this.sqlSessionProxy.<T> selectOne(statement);
    }
 
    /**
     * {@inheritDoc}
     */
    public <T> T selectOne(String statement, Object parameter) {
        return this.sqlSessionProxy.<T> selectOne(statement, parameter);
    }
 
    /**
     * {@inheritDoc}
     */
    public <K, V> Map<K, V> selectMap(String statement, String mapKey) {
        return this.sqlSessionProxy.<K, V> selectMap(statement, mapKey);
    }
 
    /**
     * {@inheritDoc}
     */
    public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey) {
        return this.sqlSessionProxy.<K, V> selectMap(statement, parameter, mapKey);
    }
 
    /**
     * {@inheritDoc}
     */
    public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds) {
        return this.sqlSessionProxy.<K, V> selectMap(statement, parameter, mapKey, rowBounds);
    }
 
    /**
     * {@inheritDoc}
     */
    public <E> List<E> selectList(String statement) {
        return this.sqlSessionProxy.<E> selectList(statement);
    }
 
    /**
     * {@inheritDoc}
     */
    public <E> List<E> selectList(String statement, Object parameter) {
        return this.sqlSessionProxy.<E> selectList(statement, parameter);
    }
 
    /**
     * {@inheritDoc}
     */
    public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
        return this.sqlSessionProxy.<E> selectList(statement, parameter, rowBounds);
    }
 
    /**
     * {@inheritDoc}
     */
    public void select(String statement, ResultHandler handler) {
        this.sqlSessionProxy.select(statement, handler);
    }
 
    /**
     * {@inheritDoc}
     */
    public void select(String statement, Object parameter, ResultHandler handler) {
        this.sqlSessionProxy.select(statement, parameter, handler);
    }
 
    /**
     * {@inheritDoc}
     */
    public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
        this.sqlSessionProxy.select(statement, parameter, rowBounds, handler);
    }
 
    /**
     * {@inheritDoc}
     */
    public int insert(String statement) {
        return this.sqlSessionProxy.insert(statement);
    }
 
    /**
     * {@inheritDoc}
     */
    public int insert(String statement, Object parameter) {
        return this.sqlSessionProxy.insert(statement, parameter);
    }
 
    /**
     * {@inheritDoc}
     */
    public int update(String statement) {
        return this.sqlSessionProxy.update(statement);
    }
 
    /**
     * {@inheritDoc}
     */
    public int update(String statement, Object parameter) {
        return this.sqlSessionProxy.update(statement, parameter);
    }
 
    /**
     * {@inheritDoc}
     */
    public int delete(String statement) {
        return this.sqlSessionProxy.delete(statement);
    }
 
    /**
     * {@inheritDoc}
     */
    public int delete(String statement, Object parameter) {
        return this.sqlSessionProxy.delete(statement, parameter);
    }
 
    /**
     * {@inheritDoc}
     */
    public <T> T getMapper(Class<T> type) {
        return getConfiguration().getMapper(type, this);
    }
 
    /**
     * {@inheritDoc}
     */
    public void commit() {
        throw new UnsupportedOperationException("Manual commit is not allowed over a Spring managed SqlSession");
    }
 
    /**
     * {@inheritDoc}
     */
    public void commit(boolean force) {
        throw new UnsupportedOperationException("Manual commit is not allowed over a Spring managed SqlSession");
    }
 
    /**
     * {@inheritDoc}
     */
    public void rollback() {
        throw new UnsupportedOperationException("Manual rollback is not allowed over a Spring managed SqlSession");
    }
 
    /**
     * {@inheritDoc}
     */
    public void rollback(boolean force) {
        throw new UnsupportedOperationException("Manual rollback is not allowed over a Spring managed SqlSession");
    }
 
    /**
     * {@inheritDoc}
     */
    public void close() {
        throw new UnsupportedOperationException("Manual close is not allowed over a Spring managed SqlSession");
    }
 
    /**
     * {@inheritDoc}
     */
    public void clearCache() {
        this.sqlSessionProxy.clearCache();
    }
 
    /**
     * {@inheritDoc}
     */
    public Connection getConnection() {
        return this.sqlSessionProxy.getConnection();
    }
 
    /**
     * {@inheritDoc}
     * @since 1.0.2
     */
    public List<BatchResult> flushStatements() {
        return this.sqlSessionProxy.flushStatements();
    }
 
    /**
     * Proxy needed to route MyBatis method calls to the proper SqlSession got from Spring's Transaction Manager It also
     * unwraps exceptions thrown by {@code Method#invoke(Object, Object...)} to pass a {@code PersistenceException} to
     * the {@code PersistenceExceptionTranslator}.
     */
    private class SqlSessionInterceptor implements InvocationHandler {
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            final SqlSession sqlSession = getSqlSession(
                    DynamicSqlSessionTemplate.this.getSqlSessionFactory(),
                    DynamicSqlSessionTemplate.this.executorType, 
                    DynamicSqlSessionTemplate.this.exceptionTranslator);
            try {
                Object result = method.invoke(sqlSession, args);
                if (!isSqlSessionTransactional(sqlSession, DynamicSqlSessionTemplate.this.getSqlSessionFactory())) {
                    // force commit even on non-dirty sessions because some databases require
                    // a commit/rollback before calling close()
                    sqlSession.commit(true);
                }
                return result;
            } catch (Throwable t) {
                Throwable unwrapped = unwrapThrowable(t);
                if (DynamicSqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
                    Throwable translated = DynamicSqlSessionTemplate.this.exceptionTranslator
                        .translateExceptionIfPossible((PersistenceException) unwrapped);
                    if (translated != null) {
                        unwrapped = translated;
                    }
                }
                throw unwrapped;
            } finally {
                closeSqlSession(sqlSession, DynamicSqlSessionTemplate.this.getSqlSessionFactory());
            }
        }
    }
 
}

SqlSessionContentHolder类代码如下：

package com.sincetimes.slg.framework.util;

public abstract class SqlSessionContentHolder {

    public final static String SESSION_FACTORY_MASTER = "master";
    public final static String SESSION_FACTORY_SLAVE = "slave";
    
    private static final ThreadLocal<String> contextHolder = new ThreadLocal<String>();  
    
    public static void setContextType(String contextType) {  
        contextHolder.set(contextType);  
    }  
      
    public static String getContextType() {  
        return contextHolder.get();  
    }  
      
    public static void clearContextType() {  
        contextHolder.remove();  
    } 
}

最后就是写切面去对dao所有方法进行处理了，代码很简单如下：

package com.sincetimes.slg.framework.core;

import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
import org.aspectj.lang.annotation.Pointcut;

import com.sincetimes.slg.framework.util.SqlSessionContentHolder;

@Aspect
public class DynamicDataSourceAspect {

    @Pointcut("execution( * com.sincetimes.slg.dao.*.*(..))")
    public void pointCut(){
        
    }
    @Before("pointCut()")
    public void before(JoinPoint jp){
        String methodName = jp.getSignature().getName();  
        //dao方法查询走从库
        if(methodName.startsWith("query") || methodName.startsWith("get") || methodName.startsWith("count") || methodName.startsWith("list")){
            SqlSessionContentHolder.setContextType(SqlSessionContentHolder.SESSION_FACTORY_SLAVE);
        }else{
            SqlSessionContentHolder.setContextType(SqlSessionContentHolder.SESSION_FACTORY_MASTER);
        }
    }
    
}