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EJB Interview Questions and Answers


What is EJB QL?
EJB QL is a Query Language provided for navigation across a network of enterprise beans and dependent objects defined by means of container managed persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query language defines finder methods for entity beans with container managed persistenceand is portable across containers and persistence managers. EJB QL is used for queries of two types of finder methods: Finder methods that are defined in the home interface of an entity bean and which return entity objects. Select methods, which are not exposed to the client, but which are used by the Bean Provider to select persistent values that are maintained by the Persistence Manager or to select entity objects that are related to the entity bean on which the query is defined.

Can the primary key in the entity bean be a Java primitive type such as int?
The primary key canít be a primitive typeĖuse the primitive wrapper classes, instead. For example, you can use java.lang.Integer as the primary key class, but not int (it has to be a class, not a primitive)

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).

What are transaction attributes?
The transaction attribute specifies how the Container must manage transactions for a method when a client invokes the method via the enterprise beanís home or component interface or when the method is invoked as the result of the arrival of a JMS message. (Sunís EJB Specification) Below is a list of transactional attributes:

1. NotSupported - transaction context is unspecified.

2. Required - beanís method invocation is made within a transactional context. If a client is not associated with a transaction, a new transaction is invoked automatically.

3. Supports - if a transactional context exists, a Container acts like the transaction attribute is Required, else - like NotSupported.

4. RequiresNew - a method is invoked in a new transaction context.

5. Mandatory - if a transactional context exists, a Container acts like the transaction attribute is Required, else it throws a javax.ejb.TransactionRequiredException.

6. Never - a method executes only if no transaction context is specified.

What is Session Bean?
A session bean is a non-persistent object that implements some business logic running on the server. One way to think of a session object is as a logical extension of the client program that runs on the server.

Session beans are used to manage the interactions of entity and other session beans,access resources, and generally perform tasks on behalf of the client.

There are two basic kinds of session bean: stateless and stateful.

Stateless session beans are made up of business methods that behave like procedures; they operate only on the arguments passed to them when they are invoked. Stateless beans are called stateless because they are transient; they do not maintain business state between method invocations.Each invocation of a stateless business method is independent from previous invocations. Because stateless session beans are stateless, they are easier for the EJB container to manage, so they tend to process requests faster and use less resources.

Stateful session beans encapsulate business logic and state specific to a client. Stateful beans are called ďstatefulĒ because they do maintain business state between method invocations, held in memory and not persistent. Unlike stateless session beans, clients do not share stateful beans. When a client creates a stateful bean, that bean instance is dedicated to service only that client. This makes it possible to maintain conversational state, which is business state that can be shared by methods in the same stateful bean.

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.

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