EJB stands for "Enterprise JavaBeans" which are distributed network aware components for developing secure, scalable, transactional and multi-user components in a J2EE environment.

Above definition actually describes EJBs from functional point of view i.e. what they do. A more structural definition would be : "EJBs are collection of Java classes, interfaces and XML files adhering to given rules".
Written in the Java programming language, an enterprise bean is a server-side component that encapsulates the business logic of an application.

For several reasons, enterprise beans simplify the development of large, distributed applications. First, because the EJB container provides system-level services to enterprise beans, the bean developer can concentrate on solving business problems. The EJB container--not the bean developer--is responsible for system-level services such as transaction management and security authorization.

Second, because the beans--and not the clients--contain the application's business logic, the client developer can focus on the presentation of the client. The client developer does not have to code the routines that implement business rules or access databases. As a result, the clients are thinner, a benefit that is particularly important for clients that run on small devices.

Third, because enterprise beans are portable components, the application assembler can build new applications from existing beans. These applications can run on any compliant J2EE server.

You should consider using enterprise beans if your application has any of the following requirements:

• The application must be scalable. To accommodate a growing number of users, you may need to distribute an application's components across multiple machines. Not only can the enterprise beans of an application run on different machines, but their location will remain transparent to the clients.





• Transactions are required to ensure data integrity. Enterprise beans support transactions, the mechanisms that manage the concurrent access of shared objects.





• The application will have a variety of clients. With just a few lines of code, remote clients can easily locate enterprise beans. These clients can be thin, various, and numerous.

To have an EJB deployment up and running, one needs more than an application server and components. There are six more parties that are involved:

1. The Bean provider: The bean provider supplies the business components to the enterprise applications. These business components are not complete applications but can be combined to form complete enterprise applications. These bean providers could be an ISV selling components or an internal component provider.





2. The Application Assembler: The application assembler is responsible for integrating the components. This party writes applications to combine components so as to develop the target application that can be deployed under various environments.





3. The EJB Deployer: After the application developer builds the application, the application must be deployed on the server. This involves configuring the security parameter settings, performance tuning, etc. An application assembler is not familiar with these issues. This is where the EJB deployer comes into play.





4. The System Administrator: The system administrator is responsible for the upkeep and monitoring of the deployed system and may make use of monitoring and management tools to closely observe the deployed system.





5. The Container and Server providers: The container provider supplies the EJB container (an application server). This is the runtime environment where the beans live. The container supplies the middleware services to the beans and manages them. Some of the various containers are: BEA's WebLogic, iPlanet's iPlanet Application Server, IBM's WebSphere, Oracle's Oracle 9i Application Server and Oracle 10g Application Server, and the JBoss open source Application Server. The server provider is the same as the container provider.





6. The Tool Vendors: There are various IDEs available to assist the developer in rapidly building and debugging components, for example Eclipse, NetBeans, and JBuilder. For the modeling of components one can use Rational Rose. There are many other tools, some used for testing (JUnit) and others used for building (Ant, XDoclet).