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EJB Components And Programming Restrictions

 Introduction

An entity bean represents a business object in a persistent storage mechanism. Some examples of business objects are customers, orders, and products. In the Application Server, the persistent storage mechanism is a relational database. Typically, each entity bean has an underlying table in a relational database, and each instance of the bean corresponds to a row in that table.
Types of Enterprise Beans are
  • Session Beans
    1. Stateless Session Beans
    2. Stateful Session Beans
  • Entity Beans
    1. Bean Managed Persistent (BMP) Entity Beans
    2. Container Managed Persistent (CMP) Entity Beans
  • Message-Driven Bean

 The Remote Interface

  • Components of an enterprise bean are:
    • Enterprise Bean Class
    • EJB Object
    • Remote Interface
    • Home Object
    • Home Interface
    • Local Interfaces
  • The Enterprise Bean Class
    • The enterprise bean class that contains the methods to implement the business logic of an EJB application.
    • To create an enterprise bean class for a session bean, you need to implement the javax.ejb.SessionBean interface.
    • You need to implement the javax.ejb.EntityBean and javax.ejb.MessageDrivenBean interfaces for creating the enterprise bean class for entity and message-driven beans, respectively.
  • The Remote Interface
    • The remote interface declares the business methods of an enterprise bean, which can be invoked by a client that is stored at a remote JVM.
    • The rules to create a remote interface file are:
      • It should extend the javax.EJBObject interface.
      • It should contain a declaration of all the methods that are accessible to the clients.
      • All the methods that is remotely accessible to client must throw an exception of type java.rmi.RemoteException. This type of exception is raised when a network connection failure occurs.
    • The remote interface inherits following methods from the javax.ejb.EJBObject interface:
      • getEJBHome()
      • getPrimaryKey()
      • remove()
      • getHandle()
      • isIdentical()
  • EJB Object:
    • Provides services, such as transaction support, security, resource management, persistence, and remote accessibility.
  • The Home Interface:
    • Should import the following packages:
      • java.io.Serializable
      • java.rmi.RemoteException
      • javax.ejb.CreateException
      • javax.ejb.EJBHome
    • Should extend the EJBHome interface and its name should not be a keyword in Java.
    • Should contain a create() method whose return type is the same as the type of the remote interface of the enterprise bean. The create() method is responsible for creating a new instance of enterprise bean.
    • Inherits methods from the javax.ejb.EJBHome interface
      • getEJBMetaData()
      • getHomeHandle()
      • remove()
    • EJB Home interface is used to create an EJB object:
  • The Home Object:
    • Enables clients to instantiate an EJB object that is stored in a remote JVM.
    • Performs the following functions:
      • Creating EJB objects
      • Searching for EJB objects
      • Removing EJB objects
  • The Local Interfaces:
    • Are created when the client and the enterprise bean are stored on the same JVM.
    • Are of two types:
      • local interface
      • local home interface
  • Creating the Local Interface
    • Local interface:
      • Contains the declaration of the methods, which are accessible to the clients.
      • Is created by extending the javax.ejb.EJBLocalObject interface.
  • Creating the Local Home Interface
    • Local home interface:
      • Contains the same set of functions as the home interface.
      • Is created by extending the javax.ejb.EJBLocalHome interface.

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