多库数据源深入分析（Mybatis+ Spring + JTA）（二）

接上篇，为什么此种模式下，在spring托管CMT管理的JTA事务中，无法切换数据源，忙活了好久，对着日志流程和源代码，貌似问题出现在下面的代码中：

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org.mybatis.spring .SqlSessionUtilspublic static SqlSession getSqlSession方法：SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);      //7.当前在事务中，且session的holder存在，则取得当前事务的sessionif (holder != null && holder.isSynchronizedWithTransaction()) {                        if (logger.isDebugEnabled()) {                logger.debug("Fetched SqlSession [" + holder.getSqlSession() + "] from current transaction");            }            return holder.getSqlSession();        }if (logger.isDebugEnabled()) {            logger.debug("Creating SqlSession with JDBC Connection [" + conn + "]");}//1.创建SqlSessionSqlSession session = sessionFactory.openSession(executorType, conn);//2.判断当前有事务if (TransactionSynchronizationManager.isSynchronizationActive()) {            if (logger.isDebugEnabled()) {                logger.debug("Registering transaction synchronization for SqlSession [" + session + "]");            }//3.创建当前session的holder            holder = new SqlSessionHolder(session, executorType, exceptionTranslator);//4.将session的holder注册到事务中            TransactionSynchronizationManager.bindResource(sessionFactory, holder);            TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));            holder.setSynchronizedWithTransaction(true);            holder.requested();//5.（8.）执行sql。。。。public static void closeSqlSession方法：SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);//6.（9.）释放掉当前事务的sessionif ((holder != null) && (holder.getSqlSession() == session)) {     if (logger.isDebugEnabled()) {logger.debug("Releasing transactional SqlSession [" + session + "]");     }holder.released();public void beforeCommit(boolean readOnly) 方法：//10.session提交 if (TransactionSynchronizationManager.isActualTransactionActive()) {                try {                    if (logger.isDebugEnabled()) {                        logger.debug("Transaction synchronization committing SqlSession [" + this.holder.getSqlSession() + "]");                    }                    this.holder.getSqlSession().commit(); public void afterCompletion(int status) 方法：//11.解除事务绑定的session并关闭            if (!this.holder.isOpen()) {                TransactionSynchronizationManager.unbindResource(this.sessionFactory);                try {                    if (logger.isDebugEnabled()) {                        logger.debug("Transaction synchronization closing SqlSession [" + this.holder.getSqlSession() + "]");                    }                    this.holder.getSqlSession().close();  

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在事务中，mybatis操作两个数据库的步骤流程：

1.创建SqlSession--第一个DAO，操作第一个DB

2.判断当前有事务

3.创建当前session的holder

4.将当前session的sessionFacotry的holder注册到事务中

5.执行sql。。。。

6.holder释放掉当前事务的session

7.当前在事务中，且sessionFactory的holder存在，则取得当前事务的session--第二个DAO，操作第二个DB

8.执行sql。。。。

9.释放掉当前事务的session

10.session提交

11.解除事务绑定的sessionFactory并关闭

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可以知道在操作第二个DAO的时候取得的是，在事务中绑定的第一个SqlSession，整个Service用同一个SqlSession，所以无法切换数据源。

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问题解决思路：通过上面的源代码

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 SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);       //4.将session的holder注册到事务中            TransactionSynchronizationManager.bindResource(sessionFactory, holder);            TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));            holder.setSynchronizedWithTransaction(true);

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可以知道，事务绑定的是mybatis的当前SqlSessionFactory，如果SqlSessionFactory变了，则事务TransactionSynchronizationManager通过SqlSessionFactory（getResource(sessionFactory)）获取

的SqlSessionHolder必定不是上一个事务中的，即holder.isSynchronizedWithTransaction()为false。

由此，可以找出一个方法解决，动态切换SqlSessionFactory

OK，代码如下：

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/** * 上下文Holder * */@SuppressWarnings("unchecked") public class ContextHolder<T> {    private static final ThreadLocal contextHolder = new ThreadLocal();        public static <T> void setContext(T context)    {        contextHolder.set(context);    }        public static <T> T getContext()    {        return (T) contextHolder.get();    }        public static void clearContext()    {        contextHolder.remove();    }}

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/** * 动态切换SqlSessionFactory的SqlSessionDaoSupport * * @see org.mybatis.spring.support.SqlSessionDaoSupport */public class DynamicSqlSessionDaoSupport extends DaoSupport {    private Map<Object, SqlSessionFactory> targetSqlSessionFactorys;    private SqlSessionFactory              defaultTargetSqlSessionFactory;    private SqlSession                     sqlSession;    private boolean                        externalSqlSession;    @Autowired(required = false)    public final void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {        if (!this.externalSqlSession) {            this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);        }    }    @Autowired(required = false)    public final void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {        this.sqlSession = sqlSessionTemplate;        this.externalSqlSession = true;    }    /**     * Users should use this method to get a SqlSession to call its statement     * methods This is SqlSession is managed by spring. Users should not     * commit/rollback/close it because it will be automatically done.     *      * @return Spring managed thread safe SqlSession     */    public final SqlSession getSqlSession() {        SqlSessionFactory targetSqlSessionFactory = targetSqlSessionFactorys.get(ContextHolder                .getContext());        if (targetSqlSessionFactory != null) {            setSqlSessionFactory(targetSqlSessionFactory);        } else if (defaultTargetSqlSessionFactory != null) {            setSqlSessionFactory(defaultTargetSqlSessionFactory);        }        return this.sqlSession;    }    /**     * {@inheritDoc}     */    protected void checkDaoConfig() {        Assert.notNull(this.sqlSession,                "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");    }    public Map<Object, SqlSessionFactory> getTargetSqlSessionFactorys() {        return targetSqlSessionFactorys;    }    /**     * Specify the map of target SqlSessionFactory, with the lookup key as key.     * @param targetSqlSessionFactorys     */    public void setTargetSqlSessionFactorys(Map<Object, SqlSessionFactory> targetSqlSessionFactorys) {        this.targetSqlSessionFactorys = targetSqlSessionFactorys;    }    public SqlSessionFactory getDefaultTargetSqlSessionFactory() {        return defaultTargetSqlSessionFactory;    }    /**     * Specify the default target SqlSessionFactory, if any.     * @param defaultTargetSqlSessionFactory     */    public void setDefaultTargetSqlSessionFactory(SqlSessionFactory defaultTargetSqlSessionFactory) {        this.defaultTargetSqlSessionFactory = defaultTargetSqlSessionFactory;    }}

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//每一个DAO由继承SqlSessionDaoSupport全部改为DynamicSqlSessionDaoSupportpublic class xxxDaoImpl extends DynamicSqlSessionDaoSupport implements xxxDao {    public int insertUser(User user) {                return this.getSqlSession().insert("xxx.xxxDao.insertUser", user);    }}

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spring配置如下：

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<!-- 创建数据源。 --><bean id="ds1" ref="ds1" /><property name="configLocation" value="classpath:config/mybatis-config.xml" /></bean><bean id="sqlSessionFactory1" ref="ds2" /><property name="configLocation" value="classpath:config/mybatis-config.xml" /></bean><!-- 动态切换SqlSessionFactory  --><bean id="dynamicSqlSessionDaoSupport" value-ref="sqlSessionFactory" /><entry key="ds2" value-ref="sqlSessionFactory1" /></map></property><property name="defaultTargetSqlSessionFactory" ref="sqlSessionFactory" /></bean><bean id="xxxDao" parent="dynamicSqlSessionDaoSupport"></bean>

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Service测试代码如下：

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@Transactional    public void testXA() {        ContextHolder.setContext("ds1");        xxxDao.insertUser(user);                ContextHolder.setContext("ds2");        xxxDao.insertUser(user);                    }

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通过Service代码，每个DAO访问都会调用getSqlSession()方法，此时就会调用DynamicSqlSessionDaoSupport的如下代码：

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public final SqlSession getSqlSession() {        SqlSessionFactory targetSqlSessionFactory = targetSqlSessionFactorys.get(ContextHolder                .getContext());        if (targetSqlSessionFactory != null) {            setSqlSessionFactory(targetSqlSessionFactory);        } else if (defaultTargetSqlSessionFactory != null) {            setSqlSessionFactory(defaultTargetSqlSessionFactory);        }        return this.sqlSession;    }

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起到动态切换SqlSessionFactory（每一个SqlSessionFactory对应一个DB）。OK，到此圆满解决，动态切换和事务这两个问题。

在此，我补充下为什么到用到动态切换，其实每一个SqlSessionFactory对应一个DB，而关于此DB操作的所有DAO对应此SqlSessionFactory，在Service中不去切换，直接用对应不同SqlSessionFactory

的DAO也可以，此种方式可以参考附件：《Spring下mybatis多数据源配置》

问题就在于，项目中不同DB存在相同的Table，动态可以做到只配置一个DAO，且操作哪个DB是通过路由Routing或者通过什么获取才能知道（延迟到Service时才知道对应哪个DB），此种情况用到动态切换，就显得方便很多。。。

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