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Saturday, March 04, 2006

EJB Interview Questions Part-2

Why an onMessage call in Message-driven bean is always a seperate transaction?

EJB 2.0 specification: "An onMessage call is always a separate transaction, because there is never a transaction in progress when the method is called."

When a message arrives, it is passed to the Message Driven Bean through the onMessage() method, that is where the business logic goes.

Since there is no guarantee when the method is called and when the message will be processed, is the container that is responsible of managing the environment, including transactions.

Why are ejbActivate() and ejbPassivate() included for stateless session bean even though they are never required as it is a nonconversational bean?

To have a consistent interface, so that there is no different interface that you need to implement for Stateful Session Bean and Stateless Session Bean.

Both Stateless and Stateful Session Bean implement javax.ejb.SessionBean and this would not be possible if stateless session bean is to remove ejbActivate and ejbPassivate from the interface.

Static variables in EJB should not be relied upon as they may break in clusters.Why?
Static variables are only ok if they are final. If they are not final, they will break the cluster. What that means is that if you cluster your application server (spread it across several machines) each part of the cluster will run in its own JVM.

Say a method on the EJB is invoked on cluster 1 (we will have two clusters - 1 and 2) that causes value of the static variable to be increased to 101. On the subsequent call to the same EJB from the same client, a cluster 2 may be invoked to handle the request. A value of the static variable in cluster 2 is still 100 because it was not increased yet and therefore your application ceases to be consistent. Therefore, static non-final variables are strongly discouraged in EJBs.

If I throw a custom ApplicationException from a business method in Entity bean which is participating in a transaction, would the transaction be rolled back by container?
EJB Transaction is automatically rolled back only when a SystemException (or a subtype of it) is thrown.

Your ApplicationExceptions can extend from javax.ejb.EJBException, which is a sub class of RuntimeException. When a EJBException is encountered the container rolls back the transaction. EJB Specification does not mention anything about Application exceptions being sub-classes of EJBException.

You can tell container to rollback the transaction, by using setRollBackOnly on SessionContext/EJBContext object as per type of bean you are using.

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