Ejb Interview Questions

Q)What are the different kinds of enterprise beans?

Stateless session bean- An instance of these non-persistent EJBs provides a service without storing an interaction or conversation state between methods. Any instance can be used for any client.

Stateful session bean- An instance of these non-persistent EJBs maintains state across methods and transactions. Each instance is associated with a particular client.

Entity bean- An instance of these persistent EJBs represents an object view of the data, usually rows in a database. They have a primary key as a unique identifier. Entity bean persistence can be either container-managed or bean-managed.

Message-driven bean- An instance of these EJBs is integrated with the Java Message Service (JMS) to provide the ability for message-driven beans to act as a standard JMS message consumer and perform asynchronous processing between the server and the JMS message producer

Q)What is session bean ?

There are two types of session beans: stateful and stateless.

Stateful Session Beans

The state of an object consists of the values of its instance variables. In a stateful session bean, the instance variables represent the state of a unique client-bean session. Because the client interacts ("talks") with its bean, this state is often called the conversational state.

The state is retained for the duration of the client-bean session. If the client removes the bean or terminates, the session ends and the state disappears. This transient nature of the state is not a problem, however, because when the conversation between the client and the bean ends there is no need to retain the state.

Stateless Session Beans

A stateless session bean does not maintain a conversational state for a particular client. When a client invokes the method of a stateless bean, the bean's instance variables may contain a state, but only for the duration of the invocation. When the method is finished, the state is no longer retained. Except during method invocation, all instances of a stateless bean are equivalent, allowing the EJB container to assign an instance to any client.

Because stateless session beans can support multiple clients, they can offer better scalability for applications that require large numbers of clients. Typically, an application requires fewer stateless session beans than stateful session beans to support the same number of clients.

At times, the EJB container may write a stateful session bean to secondary storage. However, stateless session beans are never written to secondary storage. Therefore, stateless beans may offer better performance than stateful beans.

Q)What is Entity Bean?

The entity bean is used to represent data in the database. It provides an object-oriented interface to data that would normally be accessed by the JDBC or some other back-end API. More than that, entity beans provide a component model that allows bean developers to focus their attention on the business logic of the bean, while the container takes care of managing persistence,transactions, and access control.

There are two basic kinds of entity beans: container-managed ersistence (CMP) andbean-managed persistence (BMP).

Container-managed persistence beans are the simplest for the bean developer to create and the most difficult for the EJB server to support. This is because all the logic for synchronizing the bean's state with the database is handled automatically by the container. This means that the bean developer doesn't need to write any data access logic, while the EJB server is
supposed to take care of all the persistence needs automatically. With CMP, the container manages the persistence of the entity bean. Vendor tools are used to map the entity fields to the database and absolutely no database access code is written in the bean class.

The bean-managed persistence (BMP) enterprise bean manages synchronizing its state with the database as directed by the container. The bean uses a database API to read and write its fields to the database, but the container tells it when to do each synchronization operation and manages the transactions for the bean automatically. Bean-managed persistence gives the bean developer the flexibility to perform persistence operations that are too complicated for the container or to use a data source that is not supported by the container.


Q)what are Container-Managed Transactional attributes ?

NotSupported

The bean is not involved in a transaction. If the bean invoker calls the bean while involved in a transaction, the invoker's transaction is suspended, the bean executes, and when the bean returns, the invoker's transaction is resumed.

Required

The bean must be involved in a transaction. If the invoker is involved in a transaction, the bean uses the invoker's transaction. If the invoker is not involved in a transaction, the container starts a new transaction for the bean.

Supports

Whatever transactional state that the invoker is involved in is used for the bean. If the invoker has begun a transaction, the invoker's transaction context is used by the bean. If the invoker is not involved in a transaction, neither is the bean.

RequiresNew

Whether or not the invoker is involved in a transaction, this bean starts a new transaction that exists only for itself. If the invoker calls while involved in a transaction, the invoker's transaction is suspended until the bean completes.

Mandatory

The invoker must be involved in a transaction before invoking this bean. The bean uses the invoker's transaction context.

Never

The bean is not involved in a transaction. Furthermore, the invoker cannot be involved in a transaction when calling the bean. If the invoker is involved in a transaction, a RemoteException is thrown.

Q)What's difference between httpsession and EJB session bean ?

A session in a Servlet, is maintained by the Servlet Container through the HttpSession object, that is acquired through the request object. You cannot really instantiate a new HttpSession object, and it doesn't contains any business logic, but is more of a place where to store objects.

A session in EJB is maintained using the SessionBeans. You design beans that can contain business logic, and that can be used by the clients. You have two different session beans: Stateful and Stateless. The first one is somehow connected with a single client. It maintains the state for that client, can be used only by that client and when the client "dies" then the session bean is "lost".

A Stateless Session Bean doesn't maintain any state and there is no guarantee that the same client will use the same stateless bean, even for two calls one after the other. The lifecycle of a Stateless Session EJB is slightly different from the one of a Stateful Session EJB. Is EJB Container's responsability to take care of knowing exactly how to track each session and redirect the request from a client to the correct instance of a Session Bean. The way this is done is vendor dependant, and is part of the contract.


Q)What is the default transaction attribute for an EJB?

There is no default transaction attribute for an EJB.
Deployer must specify a value for the transaction attribute for those methods having container managed transaction.
In WebLogic, the default transaction attribute for EJB is SUPPORTS.
WAS6.0 its "Required".

Q) What is the need of Remote and Home interface. Why cant it be in one?

The main reason is because there is a clear division of roles and responsabilities between the two interfaces. The home interface is your way to communicate with the container, that is who is responsable of creating, locating even removing one or more beans. The remote interface is your link to the bean, that will allow you to remotely access to all its methods and members. As you can see there are two distinct elements (the container and the beans) and you need two different interfaces for accessing to both of them.

Q) What is the diffrence between Bean Managed Persistance and Container Managed Persistance?

CMP means Container Managed Persistence. When we write CMP bean , we dont need to write any JDBC code to connect to Database. The container will take care of connection our enitty beans fields with database. The Container manages the persistence of the bean. Absolutely no database access code is written inside the bean class.

BMP means Bean Managed Persistence. When we write BMP bean, it is programmer responsiblity to write JDBC code to connect to Database.

Q)Why is ejbFindByPrimaryKey mandatory?

An Entity Bean represents persistent data that is stored outside of the EJB Container/Server. The ejbFindByPrimaryKey is a method used to locate and load an Entity Bean into the container, similar to a SELECT statement in SQL. By making this method mandatory, the client programmer can be assured that if they have the primary key of the Entity Bean, then they can retrieve the bean without having to create a new bean each time


Q)What are the Differences between EJB 3.0 and EJB 2.1?

1)EJB 3.0 allows developers to program EJB components as ordinary Java objects with ordinary Java business interfaces rather than as heavy weight components like EJB 2 (home,remote).

2)In EJB 3.0 you can use annotaion or deployment descriptors but in
EJB 2 you have to use deployment descriptors.

3) EJB 3 introduced persistence API for database access. In EJB 2 you can use Entity bean.

4) EJB 3.0 is much faster the EJB 2.