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Introduction To EJB (Enterprise Java Beans)

Introduction 

 
The Java Enterprise Edition architecture is based on Component-Container architecture. The container contains various components and supports the component in performing its functionality, by providing them an execution environment.
 
Java EE has 3 different types of components - EJB Components (Session bean, Message-driven bean), Web Components (Servlet, JSP, JSF) and Client Components (Application client component, Applet). The EJB components are contained by the EJB container which is a part of the Java EE Application Server.

 Let us understand the EJB components here.

What is EJB?

  • EJB stands for "Enterprise Java Beans". 
  • EJB is a development architecture that allows us to make component-based secure, scalable and robust applications. EJB enables the development of reusable business logic components.

Types of Enterprise Java Beans

  1.  Session beans
  2.  Message-driven beans
A. Session Beans
  • Session beans are Java beans which encapsulate the business logic in a centralized and reusable manner such that it can be used by a number of clients.
  • Session beans are plain old Java objects which function as server-side extension performing business logic.
  • As session beans are server-side extensions, they can be accessed by various types of clients, like another enterprise beans, a web component such as a Servlet, a plain Java class or a web service.
  • There are 2 types of client access models for session beans - Local client access and remote client access.
  • Local clients are components residing in the same enterprise application while remote clients are components executing in a remote JVM.
There are 3 types of Session beans -
  1. Stateless Session Beans (SLSB) - Provide user generic business processing and are not tied to a specific client.
  2. Stateful Session Beans (SFSB) - Provide user-specific business processing and are tied to a specific client.
  3. Singleton Session Beans (SSB) - Provide shared data access to clients and components within an application and are instantiated only once per application.
B. Message-driven beans
  • Message-driven beans are EJB beans which are driven with the help of messages.
  • There are 2 types of Message-driven beans -
    • JMS (Java Messaging System) Message-driven beans 
    • Non-JMS Message-driven beans