Imagine this code:
foo() {
Connection conn = ...;
}
foo() has been called from a method that has the annotation @Transactional. How do I get the current JDBC connection? Note that foo() is in a bean (so it can have @Autowired fields) but foo() can't have parameters (so I can't pass the connection in from somewhere).
[EDIT] I'm using jOOQ which needs either a data source or a connection. My problem: I don't know which transaction manager is configured. It could be anything; Java EE, DataSource based, something which gets the data source via JNDI. My code isn't an application, it's a library. I need to swallow what others put on my plate. Along the same lines, I can't request a Hibernate session factory because the application using me might not use Hibernate.
But I know that other code, like the Spring Hibernate integration, somehow can get the current connection from the transaction manager. I mean, Hibernate doesn't support Spring's transaction manager, so the glue code must adapt the Spring API to what Hibernate expects. I need to do the same thing but I couldn't figure out how it works.
[EDIT2] I know that there is an active transaction (i.e. Spring has a Connection instance somewhere or at least a transaction manager which can create one) but my method isn't @Transactional. I need to call a constructor which takes java.sql.Connection as parameter. What should I do?
The transaction manager is completely orthogonal to data sources. Some transaction managers interact directly with data sources, some interact through an intermediate layer (eg, Hibernate), and some interact through services provided by the container (eg, JTA).
When you mark a method as @Transactional, all that means is that Spring will generate a proxy when it loads your bean, and that proxy will be handed to any other class that wants to use your bean. When the proxy's method is invoked, it (the proxy) asks the transaction manager to either give it an outstanding transaction or create a new one. Then it calls your actual bean method. When your bean method returns, the proxy interacts with the transaction manager again, to either say "I can commit" or "I must rollback". There are twists to this process; for example, a transactional method can call another transactional method and share the same transaction.
While the transaction manager interacts with the DataSource, it does not own the DataSource. You cannot ask the transaction manager to give you a connection. Instead, you must inject a frame-specific object that will return connections (such as the Hibernate SessionFactory). Alternatively, you can use the static transaction-aware utility classes, but these again are tied to a specific framework.
Edit: I completely rewrote my answer based on comment thread; not sure why my original answer was focused on Hibernate, other than that's what I'm working with right now.