Enterprise JavaBeans, Fourth Edition

Transactional Stateful Session Beans

Session beans can interact directly with the database as easily as they can manage the taskflow of other enterprise beans. The ProcessPayment EJB, for example, makes inserts into the PAYMENT table when the byCredit( ) method is invoked, and the TravelAgent EJB queries the database directly when the listAvailableCabins( ) method is invoked. Stateless session beans such as the ProcessPayment EJB have no conversational state, so each method invocation must make changes to the database immediately. With stateful session beans, however, we may not want to make changes to the database until the transaction is complete. Remember, a stateful session bean can be one of many participants in a transaction, so it may be advisable to postpone database updates until the entire transaction is committed or to avoid updates if it is rolled back.

There are several different scenarios in which a stateful session bean might cache changes before applying them to the database. For example, think of a shopping cart implemented by a stateful session bean that accumulates several items for purchase. If the stateful bean implements SessionSynchronization, it can cache the items and write them to the database only when the transaction is complete.

The javax.ejb.SessionSynchronization interface allows a session bean to receive additional notification of the session’s involvement in transactions. The addition of these transaction callback methods by the SessionSynchronization ...