HibernateTransactionManager 事务管理实现

HibernateTransactionManager对事务的实现，最终都是通过处理hibernate的Transaction的commit,rollback方法完成，

与单独的hibernate的Transaction事务没有太大的区别；但是，HibernateTransactionManager通过注入sessionfactory.

然后在得到session，把session包装成SessionHolder(),并通过threadlocal来对象的实现和线程的绑定(threadlocal实现重点)

最后到线程中的session取得的Transaction，

protected void doBegin(Object transaction, TransactionDefinition definition) {HibernateTransactionObject txObject = (HibernateTransactionObject) transaction;if (txObject.hasConnectionHolder() && !txObject.getConnectionHolder().isSynchronizedWithTransaction()) {throw new IllegalTransactionStateException(......Session session = null;                //如果SessionHolder没有被创建，那么这里openSession并放到SessionHolder中去try {if (txObject.getSessionHolder() == null || txObject.getSessionHolder().isSynchronizedWithTransaction()) {Interceptor entityInterceptor = getEntityInterceptor();Session newSession = (entityInterceptor != null ?getSessionFactory().openSession(entityInterceptor) : getSessionFactory().openSession());if (logger.isDebugEnabled()) {......}txObject.setSession(newSession);}//三种得到session(OpenSessionInView,getcurrentsession,opensession)，这里就从SessionHolder得到sessionsession = txObject.getSessionHolder().getSession();//设置<tx>isolation,read-only属性if (this.prepareConnection && isSameConnectionForEntireSession(session)) {// We're allowed to change the transaction settings of the JDBC Connection.......Connection con = session.connection();Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);txObject.setPreviousIsolationLevel(previousIsolationLevel);}else {// Not allowed to change the transaction settings of the JDBC Connection.......}if (definition.isReadOnly() && txObject.isNewSession()) {// Just set to NEVER in case of a new Session for this transaction.session.setFlushMode(FlushMode.MANUAL);}if (!definition.isReadOnly() && !txObject.isNewSession()) {// We need AUTO or COMMIT for a non-read-only transaction.FlushMode flushMode = session.getFlushMode();if (flushMode.lessThan(FlushMode.COMMIT)) {session.setFlushMode(FlushMode.AUTO);txObject.getSessionHolder().setPreviousFlushMode(flushMode);}}Transaction hibTx;// Register transaction timeout.int timeout = determineTimeout(definition);if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {// Use Hibernate's own transaction timeout mechanism on Hibernate 3.1+// Applies to all statements, also to inserts, updates and deletes!hibTx = session.getTransaction();hibTx.setTimeout(timeout);hibTx.begin();}else {// Open a plain Hibernate transaction without specified timeout.                                //创建并开始事务hibTx = session.beginTransaction();}// Add the Hibernate transaction to the session holder.                        //把hibtx事务放到txobject里面，原因是因为这个SessionHolder会和线程绑定txObject.getSessionHolder().setTransaction(hibTx);// Register the Hibernate Session's JDBC Connection for the DataSource, if set.if (getDataSource() != null) {Connection con = session.connection();ConnectionHolder conHolder = new ConnectionHolder(con);if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {conHolder.setTimeoutInSeconds(timeout);}if (logger.isDebugEnabled()) {logger.debug("Exposing Hibernate transaction as JDBC transaction [" + con + "]");}TransactionSynchronizationManager.bindResource(getDataSource(), conHolder);txObject.setConnectionHolder(conHolder);}// Bind the session holder to the thread.                        //如果是新的SessionHolder，把它和线程绑定if (txObject.isNewSessionHolder()) {TransactionSynchronizationManager.bindResource(getSessionFactory(), txObject.getSessionHolder());}                        //SessionHolder的状态标识，你的Transaction是一个线程中的session取得的txObject.getSessionHolder().setSynchronizedWithTransaction(true);}catch (Exception ex) {if (txObject.isNewSession()) {try {if (session.getTransaction().isActive()) {session.getTransaction().rollback();}}catch (Throwable ex2) {logger.debug("Could not rollback Session after failed transaction begin", ex);}finally {SessionFactoryUtils.closeSession(session);}}throw new CannotCreateTransactionException("Could not open Hibernate Session for transaction", ex);}}