In the Caveatemptor example, it creates a DAO object (e.g. UserDAO, ItemDAO) every time a command is executed (e.g. BidForItemCommand)

see below:


Why is this necessary? Can we have a static object for each DAO? so we don' t need to create every time?

And in the constructor of UserDAO(), it begins a transaction.
when does it end?

public UserDAO() {
HibernateUtil.beginTransaction();
}

Thank you.

Why not create a new one every time? It's very lightweight. Read chapter 8 again and you will understand the design, incl. the transaction demarcation.

i implemented a singleton for the dao. (ok, it is not neccessary)
or you can provide a method/constructor with an attribute (e.g.
public ItemDAO(boolean beginTransaction) {..}
to determine if a transaction should be started or if it has been already created previously

Thats the job of the threadlocal binding... read again :)

OK,
my post was only half of the story. When using only one transaction for the whole thread, the DAO do not need to be a singleton or something like that. Session, transaction etc. is stored in a ThreadLocal attribute.

I implemented the usage of two or more datasources and a feature to start a new user transaction. This requires XA-Transactions (Where i still having trouble when using this within a webcontainer).