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Tuesday, June 27, 2006

EJB Interview Questions Part-1

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.

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.

What is the difference between Context, InitialContext and Session Context? How they are used?
javax.naming.Context is an interface that provides methods for binding a name to an object. It's much like the RMI Naming.bind() method.

javax.naming.InitialContext is a Context and provides implementation for methods available in the Context interface.

Where as SessionContext is an EJBContext object that is provided by the EJB container to a SessionBean in order for the SessionBean to access the information and/or services or the container.

There is EntityContext too which is also and EJBContext object that'll be provided to an EntityBean for the purpose of the EntityBean accessing the container details. In general, the EJBContext (SessionContext and EntityContext), AppletContext and ServletContext help the corresponding Java objects in knowing about its 'context' [environment in which they run], and to access particular information and/or service. Whereas, the javax.naming.Context is for the purpose of 'NAMING' [by the way of referring to] an object.

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