Enterprise Bean (EJB) programming tutorials

Enterprise Beans Tutorial

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Enterprise Beans Tutorial

21 Synchronizing a Session Bean's Instance Variables The SessionSynchonization interface, which is optional, allows you to synchronize the instance variables with their corresponding values in the database. The container invokes the SessionSynchonization methods-- afterBegin, beforeCompletion, and afterCompletion -- at each of the main stages of a transaction. The afterBegin method informs the instance that a new transaction has begun. The container invokes afterBegin immediately before it invokes the business method. The afterBegin method is a good place
to load the instance variables from the database. The container invokes the beforeCompletion method after the business method has finished, but just before the transaction commits. The beforeCompletion method is the last opportunity for the session bean to
roll back the transaction (by calling setRollbackOnly). If it hasn't already updated the database with the
values of the instance variables, the session bean may do so in the beforeCompletion method. The afterCompletion method indicates that the transaction has completed. It has a single boolean parameter, whose value is true if the transaction was committed and false if it was rolled back. If a rollback
occurred, the session bean can refresh its instance variables from the database in the afterCompletion
method Methods Not Allowed in Container-Managed Transactions You should not invoke any method that might interfere with the transaction boundaries set by the container. The
list of prohibited methods follows: commit, setAutoCommit, and rollback methods of java.sql.Connection
getUserTransaction method of javax.ejb.EJBContext
any method of javax.transaction.UserTransaction You may, however, use these methods to set boundaries in bean-managed transactions. Bean-Managed Transactions · In a bean-managed transaction, the session bean code invokes methods that mark the boundaries of the
transaction. · An entity bean may not have bean-managed transactions; it must use container-managed transactions
instead. · Although beans with container-managed transactions require less coding, they have one limitation:
When a method is executing, it can be associated with either a single transaction or no transaction at all.
If this limitation will make coding your session bean difficult, you should consider using bean-managed
transactions. The following pseudo-code illustrates the kind of fine-grained control you can obtain with bean-managed
transactions. By checking various conditions, the pseudo-code decides whether to start and stop different
transactions within the business method. begin transaction
...
update table-a
...
if (condition-x)
commit transaction
else if (condition-y)
update table-b
commit transaction
else
rollback transaction
begin transaction
update table-c