1、整体流程

• 从Datasource获取conn
• 设置为非自动提交（这就相当于开启事务了）
• 缓存当前链接
• 开始执行业务SQL：先从当前线程缓存获取conn
• 判断是否自动提交
• 执行sql
• 模版commit
• 模版清除线程的conn
• 模版reset conn为自动提交

  //获取链接，自动事务提交设为false，预示接下来的操作是一个事务，缓存当前connectionHolder到当前线程
            TransactionStatus status = this.transactionManager.getTransaction(this);
            T result;
            try {
//执行sql，尝试从当前线程获取conn，并检查事务提交方式
                result = action.doInTransaction(status);
            }
            catch (RuntimeException ex) {
                // Transactional code threw application exception -> rollback
                rollbackOnException(status, ex);
                throw ex;
            }
            catch (Error err) {
                // Transactional code threw error -> rollback
                rollbackOnException(status, err);
                throw err;
            }
            catch (Throwable ex) {
                // Transactional code threw unexpected exception -> rollback
                rollbackOnException(status, ex);
                throw new UndeclaredThrowableException(ex, "TransactionCallback threw undeclared checked exception");
            }
//提交事务，恢复链接的为自动提交，清空当前线程的conn
            this.transactionManager.commit(status);

2、开启事务操作

if (txObject.getConnectionHolder() == null ||
                    txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
                Connection newCon = this.dataSource.getConnection();
                if (logger.isDebugEnabled()) {
                    logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
                }
//从链接池拿链接过程
                txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
            }

    txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
            con = txObject.getConnectionHolder().getConnection();

            Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
            txObject.setPreviousIsolationLevel(previousIsolationLevel);

            // Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
            // so we don't want to do it unnecessarily (for example if we've explicitly
            // configured the connection pool to set it already).
            if (con.getAutoCommit()) {
                txObject.setMustRestoreAutoCommit(true);
                if (logger.isDebugEnabled()) {
                    logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
                }
//此处设置为非自动提交
                con.setAutoCommit(false);
            }

3、业务SQL执行时通过当前线程缓存获取conn

ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
        if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
            conHolder.requested();
            if (!conHolder.hasConnection()) {
                logger.debug("Fetching resumed JDBC Connection from DataSource");
                conHolder.setConnection(dataSource.getConnection());
            }
            return conHolder.getConnection();
        }

4结束事务操作的清理工作

DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;

        // Remove the connection holder from the thread, if exposed.
        if (txObject.isNewConnectionHolder()) {
            TransactionSynchronizationManager.unbindResource(this.dataSource);
        }

        // Reset connection. 重新设置为自动提交
        Connection con = txObject.getConnectionHolder().getConnection();
        try {
            if (txObject.isMustRestoreAutoCommit()) {
                con.setAutoCommit(true);
            }
            DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());
        }