EJB Questions Part - 4

46. Why are ejbActivate() and ejbPassivate() included for stateless session bean even though they are never required as it is a no conversational 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.

47. 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.

48. 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 ApplicationException can extend from javax.ejb.EJBException, which is a sub class of RuntimeException. When an 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.

49. Does Stateful Session bean support instance pooling?

Stateful Session Bean conceptually doesn't have instance pooling.

50. Can I map more than one table in a CMP?

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.

51. Can I invoke Runtime.gc() in an EJB?

You shouldn't. What will happen depends on the implementation, but the call will most likely be ignored.

52. Can a Session Bean be defined without ejbCreate() method?

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)

 

53. How to implement an entity bean which the PrimaryKey is an auto numeric field?

 The EJB 2 Spec (10.8.3 - Special case: Unknown primary key class) says that in cases where the Primary Keys 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.

54. 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.)

55. What is clustering?

Clustering is grouping machines together to transparently 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 and approaching a cluster of servers.

56. Is it possible to share an HttpSession between a JSP and EJB?

What happens when I change a value in the HttpSession from inside an EJB? You can pass the HttpSession as parameter to an EJB method, only if all objects in session are serializable.This. This has to be consider as "passed-by-value", that means that it's read-only in the EJB. If anything is altered from inside the EJB, it won't be reflected back to the HttpSession of the Servlet Container.

57. 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().

58. When should I adopt BMP and when I should use CMP?

You can use CMP and BMP beans in the same application... obviously, a bean can be BMP or CMP, not both at the same time (they are mutually exclusive).

There is a common approach that is normally used and considered a good one. You should start developing CMP beans, unless you require some kind of special bean, like multi-tables, that cannot be completely realized with a single bean. Then, when you realize that you need something more or that you would prefer handling the persistence (performance issue are the most common reason), you can change the bean from a CMP to a BMP.