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caller is transactional, the EJB method will join the existing transaction and won t cause the exiting transaction to be suspended. This approach avoids any needless overhead in suspending or resuming the client transaction. The SUPPORTS attribute is typically useful for methods that perform read-only operations such as retrieving a record from a database table. In our Snag-It example, the session bean method for checking whether a bid exists on the item about to be ordered can probably have a SUPPORTS attribute since it modifies no data. MANDATORY MANDATORY really means requires existing that is, the caller must have a transaction before calling an EJB method and the container should never create a transaction on behalf of the client. If the EJB method using the MANDATORY attribute is invoked from a nontransactional client, the container throws an EJBTransactionRequiredException. This value is also very rarely used. You should use this value if you want to make sure the client fails if you request a rollback. We can make a reasonable case to require a MANDATORY transaction on a session bean method that charges the customer. After all, we want to make sure nothing is accidentally given away for free if the client neglects to detect a failure in the method charging the customer, and the invoker s transaction can be forcibly rolled back by us when necessary. NOT_SUPPORTED If we assign NOT_SUPPORTED as the transaction attribute, the EJB method cannot be invoked in a transactional context. If a caller with an associated transaction invokes the method, the container will suspend the transaction, invoke the method, and then resume the transaction when the method returns. This attribute is typically useful only for an MDB supporting a JMS provider in nontransactional, autoacknowledge mode. To recap from chapter 5, in such cases the message is acknowledged as soon as it is successfully delivered and the MDB has no capability or apparent need to support rolling back message delivery. NEVER In a CMT, NEVER really means never. In other words, this attribute means that the EJB method can never be invoked from a transactional client. If such an attempt is made, a javax.ejb.EJBException is thrown. This is probably the least-used transaction attribute value. It could be used if your method is changing a nontransactional resource (such as a text file) and you want to make sure the client knows about the nontransactional nature of the method.
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Container-managed transactions
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Transaction attributes and MDBs As we mentioned in chapter 4, MDBs don t support all of the six transaction attributes we just discussed. Although you can apply any of the attributes to a stateful or stateless session bean, MDBs only support the REQUIRED and NOT_SUPPORTED attributes. This relates to the fact that no client ever invokes MDB methods directly; it is the container that invokes MDB methods when it receives an incoming message. Since there is no existing client transaction to suspend or join, REQUIRES_NEW, SUPPORTS, and MANDATORY make no sense (refer to table 6.2). NEVER makes no sense either, since we don t need that strong a guard against the container. In effect, depending on message acknowledgment on method return, we need only tell the container of two conditions: we need a transaction (REQUIRED) that encapsulates the message listener method, or we do not need transaction support (NOT_SUPPORTED). So far, we ve taken a detailed look at how transactions are created and managed by the container. We know that the successful return of a CMT method causes the container to commit a method or at least not roll it back if it is a joined transaction. We ve explained how a CMT method can mark an available transaction as rolled back, but we ve not yet discussed the actual mechanics. Let s dig into the underpinnings next.
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6.2.4 Marking a CMT for rollback If the appropriate business conditions arise, a CMT method can ask the container to roll back a transaction as soon as possible. The important thing to note here is that the transaction is not rolled back immediately, but a flag is set for the container to do the actual rollback when it is time to end the transaction. Let s go back to a snippet of our scenario in listing 6.1 to see exactly how this is done:
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@Resource private SessionContext context; ... public void placeSnagItOrder(Item item, Customer customer){ try { ... validateCredit(customer); ... } catch (CreditValidationException cve) { context.setRollbackOnly(); ...
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As this snippet shows, the setRollbackOnly method of the injected javax.ejb. EJBContext marks the transaction to be rolled back when we are unable to validate the user s credit card, a CreditValidationException is thrown, and we cannot
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