EJB Notes

• Enterprise JavaBeans -- remote specification


• Enterprise Java Bean -- remote component

EJB Container	runtime environment for managing enterprise beans
	instantiates and controls the enterprise beans providing system-level services

System-level services:

• transaction management for the bean


• security for the bean


• persistence of the bean


• remote access to the bean


• database connection pooling


• instance pooling for the bean

Enterprise Beans

Session Beans stateful stateless	invoked synchronously by an enterprise bean client
Entity Beans	invoked synchronously by an enterprise bean client
Message-driven Beans (MDBs)	invoked by a message container e.g. publish/subscribe topic

★ Session Beans

• 
  performs business logic for the client
  


• 
  represents a single client and not shared across clients
  


• 
  not persisted across multiple sessions
  

■ Stateful Session Bean
⇒ maintains a conversational state with one client for the duration of a single session
⇒ maintain instance variables across multiple invocations from one client during a single session
⇒ life ends when EJB container removes the enterprise bean

■ Stateless Session Bean
⇒ all data is transient between invocations for each client
⇒ each invocation is a request to a brand new object instance

★ Entity Beans

• share the same qualities found in relational databases


• intended to represent the business logic for an entity existing in persistent storage


• responsible for inserting, updating, selecting, removing data within the data source

⇒ persistent
⇒ allow shared access (container handles concurrency)
⇒ have primary keys
⇒ may participate in relationships
⇒ can participate in transactions

Persistence

• process of writing information to an external data source

▲ Bean Managed Persistence (BMP)
⇒ all access to data source is determined by programmer

▲ Container Managed Persistence (CMP)
⇒ programmer is freed from writing data-access code
⇒ entity bean can be deployed in different containers and/or against different data sources



★ Message Driven Beans

• allows enterprise applications to handle messages asynchronously

EJB can help solve

• Scalability (deployable across many different hardware platforms transparently)


• Transaction Support


• Client-location Transparency (architecture is designed to hide the location of the EJB from the client program)


• Data Source Portability (data source access can be handled by container)


• Reusability (business logic can be written once and reused easily as needed)


• Asynchronous Messaging (for applications requiring asynchronous communication)

Steps in EJB Deployment

• 
  Write classes and interfaces for enterprise bean
  
  
  
  • an interface that extends javax.ejb.EJBObject (component interface)
  
  
  • an interface that extends javax.ejb.EJBHome (home interface)
  
  
  • a class that implements either javax.ejb.SessionBean or javax.ejb.EntityBean (enterprise bean class)
  
  
  
  


• 
  Write deployment descriptor (ejb-jar.xml)
  

  
             <? xml version="1.0" encoding="UTF-8" ?>
             <ejb-jar>
                  <description>xxx</description>
                  <display-name>XXX</display-name>
                  <enterprise-beans>
                     <session>
                         <ejb-name/>
                         <home/>
                         <remote/>
                         <ejb-class/>
                         <session-type/>
                         <transaction-type/>
                     </session>
                  </enterprise-beans>
             </ejb-jar>
        

  
  


• 
  Package enterprise bean and associated files into a jar file
  


• 
  Deploy the bean