Lazy JCA Enlistment

There is a little known feature of JavaEE where you can do things in the "wrong order" for UserTransactions and it still does the transaction enlistment.

CMT enlistment

Normally in JavaEE you use CMT where the transaction is already started before your method begins you can then do

DataSource ds = ...
Connection c = ds.getConnection(); // <======= Connection is enlisted in transaction here

this is called automatic enlistment.

Similar BMT enlistment

With a UserTransaction you are in control of the transaction so this works as well

UserTransaction ut = ...
ut.begin();
DataSource ds = ...
Connection c = ds.getConnection(); // <======= Connection is enlisted in transaction here

Lazy BMT enlistment

What is not often appreciated is that you can do it the "wrong way around"

DataSource ds = ...
Connection c = ds.getConnection();
UserTransaction ut = ...
ut.begin();  // <======= Connection is enlisted in transaction here

This is so you can do

DataSource ds = ...
Connection c = ds.getConnection();
UserTransaction ut = ...
ut.begin();  // <======= Connection is enlisted in transaction here
// Do work
ut.commit(); // <======= Connection is de-enlisted here but not returned to the pool
ut.begin();  // <======= Connection is enlisted in the new transaction here

Implementation

To facilitate this feature, the application server keeps track of all the connections opened for

BMT beans and webapps and whenever you start a user transaction it enlists them automatically,

it does this for every user transaction you create.