EJB 3.0 Architechtural Overview

To use Enterprise JavaBeans and Java Persistence effectively, you need to understand their architectures and how enterprise beans are distributed as business objects and how they interact with Java Persistence.

Remote interface - The remote interface defines a session bean's business methods, which can be accessed from applications outside the EJB container: the business methods a bean presents to the outside world to do its work. The remote interface is a plain Java interface. It is tagged with the @javax.ejb.Remote annotation to identify that it is a remote interface.

Remote interfaces are denoted by the @javax.ejb.Remote annotation:

import javax.ejb.Remote;

@Remote
public interface CalculatorRemote {
public int add(int x, int y);
public int subtract(int x, int y);
}

Local interface - The local interface defines a session bean's business methods that can be used by other beans in the same EJB container: the business methods a bean presents to other beans running in the same JVM. It allows beans to interact without the overhead of a distributed object protocol, which improves their performance. The local interface is a plain Java interface. It is tagged with the @javax.ejb.Local annotation to identify that it is a local interface.

Endpoint interface - The endpoint interface defines business methods that can be accessed from applications outside the EJB container via SOAP. The endpoint interface is based on Java API for XML-RPC (JAX-RPC) and is designed to adhere to the SOAP and WSDL standards. The endpoint interface is a plain Java interface that is annotated with the @javax.jws.WebService annotation.

Message interface - Message-driven beans implement the message interface, which defines the methods by which messaging systems, such as the JMS, can deliver messages to the bean.

Bean class - The session bean class contains business logic and must have at least one remote, local, or endpoint interface. It usually implements these interfaces, but it is not required to. A bean class may also have more than one interface of a given type. The EJB container usually determines whether a session bean is remote and/or local by the interfaces it implements. The session bean class must also be tagged with the @javax.ejb.Stateful or @javax.ejb.Stateless annotation so that the EJB container knows what session bean type it is.

import javax.ejb.*;

@Stateless
public class CalculatorBean implements CalculatorRemote {
public int add(int x, int y) {
return x + y;
}
public int subtract(int x, int y) {
return x - y;
}
}

EJB Container - In EJB 3.0, clients outside the container system always use the enterprise bean's remote component interfaces. Clients outside the container system have the option of accessing stateless session beans as web services as well.

The three pieces you need in order to understand this architecture are the proxy stub, the container, and the bean instance itself. You will probably never deal with the internal architecture of your EJB container because the purpose of middleware is to alleviate these concerns so that you can focus on writing business logic.

When your business logic interacts with a session bean, it is not working directly with instances of the bean class; it is working through the bean's remote or local interface. When you invoke methods on the remote or local interface, the object instance you are using is something called a proxy stub. This proxy stub implements the remote or local interface of the session bean and is responsible for sending your session bean method invocation over the network to your remote EJB container or routing the request to an EJB container that is local in the JVM. The proxy stub can be generated by a precompiler, such as RMI's rmic. Or, as in the case of JBoss, it is dynamically generated at deployment time using the java.lang.reflect.Proxy facilities that come with the JDK.

The proxy stub routes the method invocation to the EJB container on the server (or in the server if it is a local interface). It is the EJB container's job to manage bean class instances as well as security and transaction demarcation.