Continuing our Java/J2EE/JEE interview questions and answers series, today's questions are:

Q. Why an onMessage call in Message-driven bean is always a seperate transaction?
A. EJB 2.0 specification says: "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.

Q. Why are ejbActivate() and ejbPassivate() included for stateless session bean even though they are never required as it is a non conversational bean?
A. 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.

Q. 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?
A. 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 the container to rollback the transaction, by using setRollBackOnly on SessionContext/EJBContext object as per type of bean you are using.

Q. Does Stateful Session bean support instance pooling?
A. Stateful Session Bean conceptually doesn't have instance pooling.

Q. Can I map more than one table in a CMP?
A. No, you cannot map more than one table to a single CMP Entity Bean. CMP has been, in fact, designed to map a single table.

Q. Can a Session Bean be defined without ejbCreate() method?
A. The ejbCreate() methods is part of the bean's lifecycle, so, the compiler will not return an error because there is no ejbCreate() method. However, the J2EE spec is explicit, the home interface of a Stateless Session Bean must have a single create() method with no arguments, while the session bean class must contain exactly one ejbCreate() method, also without arguments. Stateful Session Beans can have arguments (more than one create method)

Previous Java, J2EE, JEE Interview Questions can be read here.

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