Need to understand transaction.factory_class of Hibernate configuration

Question

In my hibernate.cfg.xml file, one of the properties is -

<property name = "transaction.factory_class"> <!--1-->
org.hibernate.transaction.JDBCTransactionFactory <!--2-->
</property> <!--3-->

Other properties are easy to understand. But, many questions came to my mind when I saw the above property.

line 1 - this specifies the class implementing the Transaction*Factory* interface.

Q1 - I saw the java docs for TransactionFactory but did not understand what it really is. What does this "Factory" mean ? Why don't they call it TransactionGenerator as per the line -

Contract for generating Hibernate Transaction instances.

Q2 - TransactionFactory lead me to Transaction. Is this exactly the same as a Database transaction ?

Q3 -

A single session might span multiple transactions since the notion of a session (a conversation between the application and the datastore) is of coarser granularity than the notion of a transaction. However, it is intended that there be at most one uncommitted Transaction associated with a particular Session at any time.

...the notion of a session is of coarser granularity than the notion of a transaction.

What does that mean in simple words ?

--

However, it is intended that there be at most one uncommitted Transaction associated with a particular Session at any time.

Why do you intend this ?

I don't think that the API docs are clear. Makes a n00b life miserable.

Answer 1

Q1 - The reasoning is clear. The transaction creation follows a factory pattern. For example, in an environment where application servers (like JBOSS, WebSphere etc) manage the transaction creation via JTA APIs, you will use a JTA Transaction factory to create transactions and that transactions will be further used by hibernate. In a purely Spring managed environment, Spring can be configured to use a HibernateTransactionFactory. In short, it would follow a similar fashion as:

IFactory f= FactoryCreatorForYourEnvironment.create();
Transaction t = f.create();

Q2 and Q3 - Yes, a hibernate transaction factory creates database transactions on underlying DB. Note that a session may be spanning over multiple transactions - A hibernate session abstracts a database connection. Over the same connection, multiple begin transaction, commit transaction cycles are possible. Example: REQUIRE_NEW properties if the participating beans (EJBs, or SPRING beans). So session is a broader (coarser) term as mentioned in the documentation.

Transactions are required for any writes, deletes, protected reads. So the session holds an implicit transaction. You can read a related stack overflow article here.

HTH.