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51). Can I use session beans and hibernate (instead of entity beans) for persitance?

Yes, we can. It's same as BMP.

52). If session has thrown ApplicaitonException would you use EJBContext. setRollBackOnly method?

According to the EJB specification, when the ApplicationException is thrown, the EJBContext.setRollBackOnly method is not called.

Typically, an enterprise bean marks a transaction for rollback to protect data integrity before throwing an application exception, because application exceptions do not automatically cause the Container to rollback thetransaction.

For example, an AccountTransfer bean which debits one account and credits another account could mark a transaction for rollback if it successfully performs the debit operation, but encounters a failure during the credit operation.

53). What is the difference between activation and passivation?

This would be the difference between Activation and Passivation:

While the bean is in the ready stage, the EJB container may decide to deactivate, or passivate, the bean by moving it from memory to secondary storage. (Typically, the EJB container uses a least-recently-used algorithm to select a bean for passivation.) The EJB container invokes the bean's ejbPassivate method immediately before passivating it. If a client invokes a business method on the bean while it is in the passive stage, the EJB container activates the bean, moving it back to the ready stage, and then calls the bean's ejbActivate method.

54). How do you check whether the session is active in Stateful session bean ?

In Stateful session bean session is not itself a separate entity. it is contained in the bean it self. So in order to check tht we need the check whether the Stateful session bean is present or not which is done by just invoking the home interface with the jndi

55). What is the difference between find and select methods in EJB?
# A select method can return a persistent field (or a collection thereof) of a related entity bean. A finder method can return only a local or remote interface (or a collection of interfaces).

# Because it is not exposed in any of the local or remote interfaces, a select method cannot be invoked by a client. It can be invoked only by the methods implemented within the entity bean class. A select method is usually invoked by either a business or a home method.

# A select method is defined in the entity bean class. For bean-managed persistence, a finder method is defined in the entity bean class, but for container-managed persistence it is not.

56). What is the difference between local interface and remote interface?

We can describe the following common rules for choosing whether to use remote client view or local client view:

When you will potentially use a distributed environment (if your enterprise bean should be independent of its deployment place), you should obviously choose remote client view.

Use remote client view when you need to be sure that parameters passed between your EJB and the client (and/or other enterprise beans) should be passed "by value" instead of "by reference." With pass-by-value, the bean will have its own copy of the data, completely separated from the copy of the data at the client. With local client view, you can do pass-by-reference, which means your bean, as well as the client, will work directly with one copy of the data. Any changes made by the bean will be seen by the client and vice versa. Pass-by-reference eliminates time/system expenses for copying data variables, which provides a performance advantage.

If you create an entity bean, you need to remember that it is usually used with a local client view. If your entity bean needs to provide access to a client outside of the existing JVM (i.e., a remote client), you typically use a session bean with a remote client view. This is the so-called Session Facade pattern, the goal of which is that the session bean provides the remote client access to the entity bean.

If you want to use container-managed relationship (CMR) in your enterprise bean, you must expose local interfaces, and thus use local client view. This is mentioned in the EJB specification.

Enterprise beans that are tightly coupled logically are good candidates for using local client view. In other words, if one enterprise bean is always associated with another, it is perfectly appropriate to co-locate them (i.e., deploy them both in one JVM) and organize them through a local interface.

57). Why CMP beans are abstract classes?
We have to provide abstract data to object mapping that maps the fields in our bean to a batabase, and abstract methods methods that corelate these fields.

58). What is the difference between normal Java object and EJB?
Java Object: is a reusable component.
EJB : is a distributed component used to develop business applications. Container provides runtime environment for EJBs.

59). What is abstract schema?
Abstract schema is part of an entity bean's deployment descriptor which defines the bean's persistent fields and their relationship. Abstract schema is specifed for entity beans with container managed persistence. We specify the name of the Abstract schema name in the deployment descriptor. The queries written in EJB QL for the finder methods references this name. The information provided in this Abstract Schema is used by the container for persistence management and relationship management.

60). What is clustering. What are the different algorithms used for clustering?

Clustering is the use of multiple computers and storage devices to create what seems to be a single system. Clustering is often used to increase a system's availability and for load balancing on highly-trafficked Web sites.

Clustering algorithms find groups of items that are similar. For example, clustering could be used by an insurance company to group customers according to income, age, types of policies purchased and prior claims experience. It divides a data set so that records with similar content are in the same group, and groups are as different as possible from each other. Since the categories are unspecified, this is sometimes referred to as unsupervised learning.

Main strategies of clustering:
1. Hierarchical clustering
2. K-clustering (partitioning)
3. Self Organizing Maps (SOM)
4. Hybrids (incremental)