Inside EJBs

Inside EJBs
Session and message-driven beans require a EJB container to run and entities require a persistence provider. A Java EE Container is an application server solution that supports EJB 3, a web container and other Java EE APIs and services. There are a number of Java EE containers that you can use

• JBoss Application Server
• BEA WebLogic Server
• IBM WebSphere
• Oracle Application Server
• Sun GlassFish

The container is an extension of the basic idea of a JVM, but manages the memory on your behalf and provides services such as transactions, security management, remoting, web services. The EJB 3 part will provide services applicable to session beans and MDBs only, and putting a EJB 3 component inside a container is called deployment, once deployed it can be accessed via a client. To provide persistence you will require a JPA which can be plugged into the container, this means that you are freely to choice what JPA you want to use, JBoss Hibernate and Oracle TopLink are popular and you can mix and match containers with persistence providers.
There are a number of EJB 3 component services

Service	Applies To	What's offered
Integration	Session beans, MDBs	Helps glue components together, in EJB 3 you use dependency injection (DI) as well as lookup
Pooling	Stateless session beans and MDBs	The EJB platform creates a pool of component instances that are shared by clients. All instances are returned to the pool when the client has finished with them.
Thread-Safety	Session beans, MDBs	EJB makes all components thread-safe and highly performant in ways that are completely invisible, however complex the application.
State Management	Stateful session beans	The EJB container manages state transparently.
Messaging	MDBs	You can write messaging aware components with out to much detail of the JMS service
Transactions	Session beans, MDBs	EJB supports declarative transaction management using simple configuration instead of code, the container can handle the commit and rollback of transactions.
Security	Session beans	Integration of JAAS API, means that it is easy to implement security with simple configuration instead of code.
Interceptors	Session beans, MDBs	EJB 3 introduces AOP in a very lightweight, accessible manner using interceptors. This allows you to separate out crossing cutting concerns such as logging, auditing and so on in a configurable way.
Remote Access	Session beans	EJB 3 can make components remotely accessible without writing code. It allows clients to access components as if they were local using DI
Web Services	Stateless session beans	EJB 3 can transparently turn business components into robust web services with minimal code change.
Persistence	Entities	Automated persistence as an alternative to verbose and error-prone JDBC/SQL code.
Caching and Performance	Entities	JPA provides a number of services geared toward data caching, performance optimization and application tuning, especially in large environments.