• All bean classes must implement javax.ejb.EnterpriseBean, or one of its subinterfaces, at some level.

  • javax.ejb.EnterpriseBean extends java.io.Serializable.
• Session beans implement javax.ejb.SessionBean.
  • javax.ejb.SessionBean extends javax.ejb.EnterpriseBean.

  • It defines methods: ejbActivate, ejbPassivate, ejbRemove, and setSessionContext.

• Entity Beans implement javax.ejb.EntityBean.
  • javax.ejb.EntityBean extends javax.ejb.EnterpriseBean.

  • It defines methods: ejbActivate, ejbPassivate, ejbRemove, setEntityContext, ejbLoad, ejbStore, and unsetEntityContext.

• Message Driven Beans implement javax.ejb.MessageDrivenBean.

  • javax.ejb.MessageDrivenBean extends javax.ejb.EnterpriseBean.

  • It defines methods: ejbRemove and setMessageDrivenContext.

Client never invokes methods directly on an actual bean instance. All invocations go through the EJB object, which is a tool-generated class. Message requests are intercepted by the EJB object and then delegated to the actual bean instance. That is why it is called a request interceptor.

A bean class exposes business methods that are cloned by the EJB object. But how do the tools that generate the EJB object know which methods to clone? The answer is the remote interface. This interface duplicates all the methods that the corresponding bean class exposes (remember that they must comply with the EJB specification). Remote clients then access the bean via this remote interface. All remote interfaces must extend javax.ejb.EJBObject.

How do clients acquire references to EJB objects? The client asks for the EJB object from the EJB object factory. This factory is responsible for instantiating and destroying the EJB objects. This factory is called the Home object.

How does a home object know how you would like to initialize your EJB object? This information is provided to the container through the home interface. The home interface contains the methods for creating, destroying, and finding EJB objects. All home interfaces are derived from the javax.ejb.EJBHome interface and all home objects implement the home interface. This interface extends java.rmi.Remote.

Creating beans through the home interface and then calling the beans though the remote interface is very slow. Local interfaces speed up beans' calls. Use local objects (implemented from a local interface rather than from a remote interface) instead of EJB objects whenever possible. Also, use local home objects (implemented from a local home interface rather than from a home interface) instead of home objects. Local interfaces extend javax.ejb.EJBLocalObject and local home interfaces extend javax.ejb.EJBLocalHome.

You state your deployment configuration in the deployment descriptor file named ejb-jar.xml. Different application servers may have their own additional proprietary deployment descriptor files. These XML files can either be edited by hand or with a tool provided by the container vendor. You can specify:

• Bean management and lifecycle requirements.

• Persistence requirements for entity beans.

• Transaction requirements.

• Security requirements, including access control entries.

Once you have finished compiling your classes and creating all of the necessary configuration files, you can place all of these files into an EJB-JAR file for deployment. EJB-JAR files are zip files that contain Java classes and all the other files needed for deployment. You can generate these files by hand or with tools, such as Apache Ant.