For Entity Beans, What happens to an instance field not mapped to any persistent storage, when the bean is passivated?

The specification infers that the container never serializes an instance of an Entity bean (unlike stateful session beans). Thus passivation simply involves moving the bean from the “ready” to the “pooled” bin. So what happens to the contents of an instance variable is controlled by the programmer. Remember that when an entity bean is passivated the instance gets logically disassociated from it’s remote object. Be careful here, as the functionality of passivation/activation for Stateless Session, Stateful Session and Entity beans is completely different. For entity beans the ejbPassivate method notifies the entity bean that it is being disassociated with a particular entity prior to reuse or for dereference.

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Are we allowed to change the transaction isolation property in middle of a transaction?

No. You cannot change the transaction isolation level in the middle of transaction.

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Should synchronization primitives be used on bean methods?

No. The EJB specification specifically states that the enterprise bean is not allowed to use thread primitives. The container is responsible for managing concurrent access to beans at runtime.

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Is method overloading allowed in EJB?

Yes you can overload methods

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How is persistence implemented in enterprise beans?

Persistence in EJB is taken care of in two ways, depending on how you implement your beans: container managed persistence (CMP) or bean managed persistence (BMP) For CMP, the EJB container which your beans run under takes care of the persistence of the fields you have declared to be persisted with the database - this declaration is in the deployment descriptor. So, anytime you modify a field in a CMP bean, as soon as the method you have executed is finished, the new data is persisted to the database by the container. For BMP, the EJB bean developer is responsible for defining the persistence routines in the proper places in the bean, for instance, the ejbCreate(), ejbStore(), ejbRemove() methods would be developed by the bean developer to make calls to the database. The container is responsible, in BMP, to call the appropriate method on the bean. So, if the bean is being looked up, when the create() method is called on the Home interface, then the container is responsible for calling the ejbCreate() method in the bean, which should have functionality inside for going to the database and looking up the data.

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What is the difference between a Server, a Container, and a Connector?

An EJB server is an application, usually a product such as BEA WebLogic, that provides (or should provide) for concurrent client connections and manages system resources such as threads, processes, memory, database connections, network connections, etc. An EJB container runs inside (or within) an EJB server, and provides deployed EJB beans with transaction and security management, etc. The EJB container insulates an EJB bean from the specifics of an underlying EJB server by providing a simple, standard API between the EJB bean and its container. A Connector provides the ability for any Enterprise Information System (EIS) to plug into any EJB server which supports the Connector architecture. See Sun’s J2EE Connectors for more in-depth information on Connectors.

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Why do we have a remove method in both EJBHome and EJBObject?

With the EJBHome version of the remove, you are able to delete an entity bean without first instantiating it (you can provide a PrimaryKey object as a parameter to the remove method). The home version only works for entity beans. On the other hand, the Remote interface version works on an entity bean that you have already instantiated. In addition, the remote version also works on session beans (stateless and stateful) to inform the container of your loss of interest in this bean.

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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 - which would mean creating duplications of persistent data and break the integrity of EJB.

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Is there any default cache management system with Entity beans ? In other words whether a cache of the data in database will be maintained in EJB ?

Caching data from a database inside the Application Server are what Entity EJB’s are used for.The ejbLoad() and ejbStore() methods are used to synchronize the Entity Bean state with the persistent storage(database). Transactions also play an important role in this scenario. If data is removed from the database, via an external application - your Entity Bean can still be “alive” the EJB container. When the transaction commits, ejbStore() is called and the row will not be found, and the transaction rolled back.


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What is the difference between session and entity beans? When should I use one or the other?

An entity bean represents persistent global data from the database; a session bean represents transient user-specific data that will die when the user disconnects (ends his session). Generally, the session beans implement business methods (e.g. Bank.transferFunds) that call entity beans (e.g. Account.deposit, Account.withdraw)

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What is the default transaction attribute for an EJB?

There is no default transaction attribute for an EJB. Section 11.5 of EJB v1.1 spec says that the 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.

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Is it possible to stop the execution of a method before completion in a SessionBean?

Stopping the execution of a method inside a Session Bean is not possible without writing code inside the Session Bean. This is because you are not allowed to access Threads inside an EJB.

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Is it legal to have static initializer blocks in EJB?

Although technically it is legal, static initializer blocks are used to execute some piece of code before executing any constructor or method while instantiating a class. Static initializer blocks are also typically used to initialize static fields - which may be illegal in EJB if they are read/write - In EJB this can be achieved by including the code in either the ejbCreate(), setSessionContext() or setEntityContext() methods.

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Is is possible for an EJB client to marshal an object of class java.lang.Class to an EJB?

Technically yes, spec. compliant NO! - The enterprise bean must not attempt to query a class to obtain information about the declared members that are not otherwise accessible to the enterprise bean because of the security rules of the Java language.

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What are the key benefits of the EJB technology?

*Rapid application development
*Broad industry adoption
*Application portability
*Protection of IT investment

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What are the key features of the EJB technology?

1.EJB components are server-side components written entirely in the Java programming language
2.EJB components contain business logic only - no system-level programming & services, such as transactions, security, life-cycle, threading, persistence, etc. are automatically managed for the EJB component by the EJB server.
3.EJB architecture is inherently transactional, distributed, portable multi-tier, scalable and secure.
4.EJB components are fully portable across any EJB server and any OS.
5.EJB architecture is wire-protocol neutral--any protocol can be utilized like IIOP,JRMP, HTTP, DCOM,etc.

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What is the difference between EJB and Java beans?

EJB is a specification for J2EE server, not a product; Java beans may be a graphical component in IDE.

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What is EJB role in J2EE?

EJB technology is the core of J2EE. It enables developers to write reusable and portable server-side business logic for the J2EE platform.

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