Enterprise Java Beans (EJB) Interview Questions And Answers

The EJB specification is one of several Java APIs in the Java Platform, Enterprise Edition. EJB is a server-side model that encapsulates the business logic of an application. The EJB specification was originally developed in 1997 by IBM and later adopted by Sun Microsystems (EJB 1.0 and 1.1) and enhanced under the Java Community Process as JSR 19 (EJB 2.0), JSR 153 (EJB 2.1) and JSR 220 (EJB 3.0).

The EJB specification intends to provide a standard way to implement the back-end 'business' code typically found in enterprise applications (as opposed to 'front-end' user-interface code). Such code was frequently found to reproduce the same types of problems, and it was found that solutions to these problems are often repeatedly re-implemented by programmers. Enterprise JavaBeans were intended to handle such common concerns as persistence, transactional integrity, and security in a standard way, leaving programmers free to concentrate on the particular problem at hand.

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11. Does Stateful Session bean support instance pooling?

 Stateful Session Bean conceptually doesn't have instance pooling.

12. For Entity Beans, What happens to an instance field not mapped to any persistent storage, when the bean is passivated?

The specification infers that the container never serializes an instance of an Entity bean (unlike stateful session beans). Thus passivation simply involves moving the bean from the ?ready? to the ?pooled? bin. So what happens to the contents of an instance variable is controlled by the programmer. Remember that when an entity bean is passivated the instance gets logically disassociated from it?s remote object. Be careful here, as the functionality of passivation/activation for Stateless Session, Stateful Session and Entity beans is completely different. For entity beans the ejbPassivate method notifies the entity bean that it is being disassociated with a particular entity prior to reuse or for dereference.

13. How can I call one EJB from inside of another EJB?

EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the other bean, then acquire an instance reference, and so forth.

14. How EJB Invocation happens?

 
 Step 1: Retrieve Home Object reference from Naming Service via JNDI.
step 2: Return Home Object reference to the client.
step 3: Create me a new EJB Object through Home Object interface.
step 4: Create EJB Object from the Ejb Object
step 5: Return EJB Object reference to the client.
step 6: Invoke business method using EJB Object reference.
step 7: Delegate request to Bean (Enterprise Bean).

15. How is persistence implemented in enterprise beans?

Persistence in EJB is taken care of in two ways, depending on how you implement your beans: container managed persistence (CMP) or bean managed persistence (BMP) For CMP, the EJB container which your beans run under takes care of the persistence of the fields you have declared to be persisted with the databaseA: this declaration is in the deployment descriptor. So, anytime you modify a field in a CMP bean, as soon as the method you have executed is finished, the new data is persisted to the database by the container. For BMP, the EJB bean developer is responsible for defining the persistence routines in the proper places in the bean, for instance, the ejbCreate(), ejbStore(), ejbRemove() methods would be developed by the bean developer to make calls to the database. The container is responsible, in BMP, to call the appropriate method on the bean. So, if the bean is being looked up, when the create() method is called on the Home interface, then the container is responsible for calling the ejbCreate() method in the bean, which should have functionality inside for going to the database and looking up the data.

16. How to implement an entity bean which the PrimaryKey is an autonumeric field ?

The EJB 2 Spec (10.8.3A: Special case: Unknown primary key class) says that in cases where the PrimaryKeys are generated automatically by the underlying database, the bean provider must declare the findByPrimaryKey method to return java.lang.Object and specify the Primary Key Class as java.lang.Object in the Deployment Descriptor.

When defining the Primary Key for the Enterprise Bean, the Deployer using the Container Provider's tools will typically add additional container-managed fields to the concrete subclass of the entity bean class.

In this case, the Container must generate the Primary Key value when the entity bean instance is created (and before ejbPostCreate is invoked on the instance.)

What is clustering?
Clustering is grouping machines together to transparantly provide enterprise services. Clustering is an essential piece to solving the needs for today's large websites.

The client does not know the difference between approaching one server or approaching a cluster of servers.

17. 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.

18. If my session bean with single method insert record into 2 entity beans, how can know that the process is done in same transaction (the attributes for these beans are Required)?

If your session bean is using bean-managed transactions, you can ensure that the calls are handled in the same transaction by :

javax.transaction.UserTransaction tran= null;
try{
tran=ctx.getUserTransaction();
tran.begin();
myBeanHome1.create(....);
myBeanHome2.create(...);
tran.commit();
}catch(...){}
You may want to check if you're already running in a transaction by calling tran.getStatus().

19. Is is possible for an EJB client to marshal an object of class java.lang.Class to an EJB?

Technically yes, spec. compliant NO!A: The enterprise bean must not attempt to query a class to obtain information about the declared members that are not otherwise accessible to the enterprise bean because of the security rules of the Java language.

20. Is it legal to have static initializer blocks in EJB?

Although technically it is legal, static initializer blocks are used to execute some piece of code before executing any constructor or method while instantiating a class. Static initializer blocks are also typically used to initialize static fieldsA: which may be illegal in EJB if they are read/writeA: In EJB this can be achieved by including the code in either the ejbCreate(), setSessionContext() or setEntityContext() methods.

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