DAO pattern - where do transactions fit in?

Question

So I\'ve got this generic DAO thing going on and at face value it appears to be ok. It\'s basically modeled after the CaveatEmptor sample application from the Hibernate guys.

Answer 1

In the past I've put the transaction logic in the root DAO for a hierarchy of DAOs that match to a hierarchy of Objects in your model that represent a single solid entity in the system.

I.e., if you have and X that has many Ys, and you want to store and retrieve Xs and their Ys at the same time as a single compound object, then your DAO for X should also call the DAO for Y. Then you can put a transaction around everything in your add() and update() methods in the DAO for X - and even make the Y DAO package private to hide it from your main business logic. I.e., instead of business logic:

XDAO xDAO = new XDAO(conn);
xDAO.startTransaction();
boolean success = xDAO.add(x);
if (success)
    for (Y y : x.getYs()) {
        success = YDAO.add(y);
        if (!success) break;
    }
if (success)
    xDAO.commit();
else
    xDAO.rollback();

You would just have:

XDAO xDAO = new XDAO(conn);
xDAO.add(x);

(with the success / commit / rollback logic internal to that DAO)

However this doesn't cover every situation, and yours may be different (for example mine works with JDBC, I don't know how Hibernate works or if it is possible there).