三种事务的创建由对应的JDBCTransactionFactory,JTATransactionFactroy,CMTTransactionFactory完成
这三者区别在
  connectionReleaseMode
     JTATransactionFactroy和CMTTransactionFactory连接释放模式执行每条语句后释放连接，JDBCTransactionFactory事务提交后释放，因此jta事务连接的重用高
  TransactionManager
    JDBCTransactionFactory和JTATransactionFactroy不需要TransactionManager,CMTTransactionFactory必须
  isTransactionInProgress
    JDBCTransactionFactory确定执行JDBCTransaction begin方法，但没提交或回滚过视为正在进行
    JTATransactionFactroy
       通过缓存的JTATransaction.getUserTransaction状态确定
       如果找不到，通过transactionManager状态确定
       如果找不到，通过JNDI查找userTransaction状态确定
   CMTTransactionFactory
       通过transactionManager.getTransaction状态确定
factory通过org.hibernate.transaction.TransactionFactoryFactory.buildTransactionFactory确定，简单表述下
    通过hibernate.transaction.factory_class确定，如果为空，返回JDBCTransactionFactory
    否则实例化hibernate.transaction.factory_class指定类，并调用configure方法

package org.hibernate.transaction;

import java.util.Properties;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.hibernate.HibernateException;
import org.hibernate.cfg.Environment;
import org.hibernate.util.ReflectHelper;

/**
 * @author Gavin King
 */
public final class TransactionFactoryFactory {

	private static final Log log = LogFactory.getLog(TransactionFactoryFactory.class);

	/**
	 * Obtain a TransactionFactory with the transaction handling strategy
	 * specified by the given Properties.
	 *
	 * @param transactionProps transaction properties
	 * @return TransactionFactory
	 * @throws HibernateException
	 */
	public static TransactionFactory buildTransactionFactory(Properties transactionProps) throws HibernateException {

		String strategyClassName = transactionProps.getProperty(Environment.TRANSACTION_STRATEGY);
		if (strategyClassName==null) {
			log.info("Using default transaction strategy (direct JDBC transactions)");
			return new JDBCTransactionFactory();
		}
		log.info("Transaction strategy: " + strategyClassName);
		TransactionFactory factory;
		try {
			factory = (TransactionFactory) ReflectHelper.classForName(strategyClassName).newInstance();
		}
		catch (ClassNotFoundException e) {
			log.error("TransactionFactory class not found", e);
			throw new HibernateException("TransactionFactory class not found: " + strategyClassName);
		}
		catch (IllegalAccessException e) {
			log.error("Failed to instantiate TransactionFactory", e);
			throw new HibernateException("Failed to instantiate TransactionFactory: " + e);
		}
		catch (java.lang.InstantiationException e) {
			log.error("Failed to instantiate TransactionFactory", e);
			throw new HibernateException("Failed to instantiate TransactionFactory: " + e);
		}
		factory.configure(transactionProps);
		return factory;
	}

	private TransactionFactoryFactory() {}
}