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EJB Questions et reponses d'entretien

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Differences associees

Differences associees

EJB 2.0 vs EJB 3.0	EJB 3.0 vs Spring	EJB 1.0 vs EJB 2.0

JavaBeans vs EJB

Question 61. Can I develop an Entity Bean without implementing the create() method in the home interface?

As per the specifications, there can be �ZERO� or �MORE� create() methods defined in an Entity Bean. In cases where create() method is not provided, the only way to access the bean is by knowing its primary key, and by acquiring a handle to it by using its corresponding finder method. In those cases, you can create an instance of a bean based on the data present in the table. All one needs to know is the primary key of that table. i.e. a set a columns that uniquely identify a single row in that table. Once this is known, one can use the �getPrimaryKey()� to get a remote reference to that bean, which can further be used to invoke business methods.

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Question 62. 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 responsibilities between the two interfaces. The home interface is your way to communicate with the container, that is who is responsible 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.

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Question 63. What is bean managed transaction?

If a developer doesn�t want a Container to manage transactions, it�s possible to implement all database operations manually by writing the appropriate JDBC code. This often leads to productivity increase, but it makes an Entity Bean incompatible with some databases and it enlarges the amount of code to be written. All transaction management is explicitly performed by a developer.

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Question 64. How can I call one EJB from inside of another EJB?

EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the other bean, then acquire an instance reference, and so forth.

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Question 65. What happens if remove( ) is never invoked on a session bean?

In case of a stateless session bean it may not matter if we call or not as in both cases nothing is done. The number of beans in cache is managed by the container.

In case of stateful session bean, the bean may be kept in cache till either the session times out, in which case the bean is removed or when there is a requirement for memory in which case the data is cached and the bean is sent to free pool.

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