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EJB Interview Questions & Answers - Learning Mode
COOLINTERVIEW.COM J2EE INTERVIEW QUESTIONS EJB INTERVIEW QUESTIONS QUESTIONS & ANSWERS - LEARNING MODE

EJB Interview Questions & Answers - Learning Mode

Enterprise Java Beans (EJB) is a development architecture for building highly scalable and robust enterprise level applications to be deployed on J2EE compliant Application Server such as JBOSS, Web Logic etc. Enterprise JavaBeans (EJB) is an architecture for setting up program components, written in the Java programming language, that run in the server parts of a computer network that uses the client/server model.

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EJB Interview Questions & Answers - Learning Mode
Try EJB Interview Questions & Answers - Exam Mode
Question: What is EJB architecture(components)?

Answer: Enterprise beans

An enterprise bean is a non-visual component of a distributed, transaction-oriented enterprise application. Enterprise beans are typically deployed in EJB containers and run on EJB servers.

There are three types of enterprise beans: session beans, entity beans, and message-driven beans.

Session beans: Session beans are non-persistent enterprise beans. They can be stateful or stateless. A stateful session bean acts on behalf of a single client and maintains c Source: CoolInterview.com
Question: What is the difference between ejbCreate() and ejbPostCreate

Answer: The purpose of ejbPostCreate() is to perform clean-up database operations after SQL INSERTs (which occur when ejbCreate() is called) when working with CMP entity beans. ejbCreate() is called before database INSERT operations. You need to use ejbPostCreate() to define operations, like set a flag, after INSERT completes successfully.

When working with BMP entity beans, this is not necessary. You have full control over the entire process, so you can place all the necessary logic surrounding Source: CoolInterview.com
Question: Can I invoke Runtime.gc() in an EJB?

Answer: You shouldn't. What will happen depends on the implementation, but the call will most likely be ignored. Source: CoolInterview.com
Question: Why an onMessage call in Message-driven bean is always a seperate transaction?

Answer: EJB 2.0 specification: "An onMessage call is always a separate transaction, because there is never a transaction in progress when the method is called."

When a message arrives, it is passed to the Message Driven Bean through the onMessage() method, that is where the business logic goes.

Since there is no guarantee when the method is called and when the message will be processed, is the container that is responsible of managing the environment, including transactions ?

Hence Source: CoolInterview.com
Question: Is there a way to get the original exception object from inside a nested or wrapped Exception (for example an EJBException or RemoteException)

Answer: Yes.
The way to get, depends on the Exception, since there are no standards. Some examples:

* When we have an javax.ejb.EJBException, we can use the getCausedByException() that returns a java.lang.Exception.

* A java.rmi.RemoteException there is a public field called detail of type java.lang.Throwable

* With a java.sql.SQLException we need to use the method getNextException() to get the chained java.sql.SQLException.

* When we have an java.lang.refl Source: CoolInterview.com
Question: How do you check whether the session is active in Stateful session bean


Answer: 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 Source: CoolInterview.com
Question: Which are features in EJB 2.0 ? and which are features in EJB 3.0?

Answer: POJO - Plain old Java Objects..

Look for article on EJB3.0 on javaworld

1) EJB2.0 has Deployment descriptors but in EJB3.0 has no Deployment Descriptors.

2) EJB2.0 we have to write Home and Remote Interfaces But in EJB3.0 No need to write Home and remote interfaces.

3) In EJB3.0 We r identifying all entities with '@' symbol.

Deven Mehta USA accentue Source: CoolInterview.com
Question: The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes?

Answer: The EJB container maintains an instance pool. The container uses these instances for the EJB Home reference irrespective of the client request. while referring the EJB Object classes the container creates a separate instance for each client request. The instance pool maintenance is up to the implementation of the container. If the container provides one, it is available otherwise it is not mandatory for the provider to implement it. Having said that, yes most of the container providers implement Source: CoolInterview.com
Question: Does WebLogic support auto generating primary keys for entity beans?

Answer: Yes, this feature was added in WLS 6.1. For more information, see the DTD comments for the <automatic-key-generation> element.http://www.bea.com/servers/wls810/dtd/weblogic-ejb-jar.dtd
Source: CoolInterview.com
Question: What is the life cycle of Stateless session bean?

Answer: The stateless session bean's life cycle has two states: the Does Not Exist state and the Method-Ready Pool. The Method-Ready Pool is similar to the instance pool used for entity beans. This is one of the significant life-cycle differences between stateless and stateful session beans; stateless beans define instance pooling in their life cycle and stateful beans do not.


Visit http://www.unix.org.ua/orelly/java-ent/ebeans/ch07_02.htm for more details on this topic. Source: CoolInterview.com
Question: What is the difference between EJB and RMI

Answer: Both of them are java solution for distributed computing.



RMI offers remote access to an object running in another JVM and no other services.



But EJB offers far more services than RMI apart from remote method calling. EJB leverages this remote-object feature of RMI and ORB (RMI-IIOP) which can be called by any COBRA client, but also provides other services such as persistence, transaction management, security, resource management, object pooling and messaging. Source: CoolInterview.com
Question: What is the use of activate, passivate methods in EJB?






Answer: Ejb activate() and passivate() methods are playing good role in ejbs.

ejb activate() is the method that donotes the objects which are at active state and at the same time passivate() tells us that objects which are at passive state(at sleeping condition).That means if we take an example like bottle.

If there are 10 bottels are in the box which currently using and means they are active and if u want to fill ane more bottle into that box then we should kept that bottle at another Source: CoolInterview.com
Question: How will you propagate exception thrown inside session bean to JSP or Servlet client

Answer: 2 Types of exceptions can occur

1.System Exception - We can define some error codes the in the property file and show it to the JSP
2.Application Exception.. - We can customise the exception and show the appropriate error
Source: CoolInterview.com
Question: What is meant by Serialization and Externalization? Serialization is a Marker interface,so what is the use of WriteObject() anf ReadObject(), Where it is actually used? Give me some real time examples?

Answer: Marker Interface is used by java runtime engine (JVM) to identify the class for special processing.



Sterilization is a process of converting state of an object to stream of bytes that can be sent over network or can be stored in a file or in database to be reconstructed into Object Later when required. Remember EJB spec, all EJB (Session or Entity) implements Serializable interface indirectly, in case of Statefull session bean it can be Passivated or Activated (Persisted by writi Source: CoolInterview.com
Question: If i throw a custom ApplicationException from a business method in Entity bean which is participating

Answer: For declarative transactions, container will rollback on systemException. Container has no way to know whether a speicific application exception is serious enough to roll back the participated transaction. Use setRollbackOnly() to doom the transaction. Source: CoolInterview.com
Question: What is the difference between CMP 1.1 and CMP 2.0


Answer: CMR
and sub classing of the CMP bean by the container Source: CoolInterview.com
Question: 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?

Answer: You can pass the HttpSession as parameter to an EJB method, only if all objects in session are serializable.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.The “pass-by-reference” can be used between EJBs Remote Interfaces, as they are remote references. While it IS possible to pass an HttpSession as a parameter to an EJB object, Source: CoolInterview.com
Question: What are the key benefits of the EJB technology?

Answer: ► Rapid application development
► Broad industry adoption
► Application portability
► Protection of IT investment Source: CoolInterview.com
Question: Why CMP beans are abstract classes?


Answer: 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. Source: CoolInterview.com
Question: Can undefined primary keys are possible with Entity beans?If so, what type is defined?

Answer: Yes,undefined primary keys are possible with Entity Beans.The type is defined as java.lang.Object. Source: CoolInterview.com

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