# EJB architecture

This topic provides a high-level overview of the distributed component architecture defined in the Sun Microsystems Enterprise JavaBeans™ (EJB) architecture specification.

The complete Enterprise JavaBeans specifications and descriptions of the technology are available from the java.sun.com Web site.

Enterprise beans provide several benefits for application developers:

- They allow you to build distributed applications by combining components developed using tools from different vendors.
- They make it easy to write applications. You do not have to deal with low-level details of transaction and state management, multithreading, resource pooling, and other complex low-level APIs. However, if necessary, expert programmers can still gain direct access to the low-level APIs.
- They can be developed once and then deployed on multiple platforms without recompilation or source code modification.
- The EJB specification that governs the use of enterprise beans is compatible with other Java™ APIs and CORBA. It also provides for interoperability between enterprise beans and non-Java applications.

Enterprise beans
  An enterprise bean is a non-visual component of a distributed, transaction-oriented enterprise application. Enterprise beans are typically deployed in EJB containers and run on EJB servers. You can customize them by changing their deployment descriptors and you can assemble them with other beans to create new applications. There are three types of enterprise beans: session beans, entity beans, and message-driven beans.

  > - Session beans: Session beans are non-persistent enterprise beans. They can be stateful or stateless. A stateful session bean acts on behalf of a single client and maintains client-specific session information (called conversational state) across multiple method calls and transactions. It exists for the duration of a single client/server session. A stateless session bean, by comparison, does not maintain any conversational state. Stateless session beans are pooled by their container to handle multiple requests from multiple clients.
  > - Entity beans: Entity beans are enterprise beans that contain persistent data and that can be saved in various persistent data stores. Each entity bean carries its own identity. Entity beans that manage their own persistence are called bean-managed persistence (BMP) entity beans. Entity beans that delegate their persistence to their EJB container are called container-managed persistence (CMP) entity beans.
  > - Message-driven beans: Message-driven beans are enterprise beans that receive and process JMS messages. Unlike session or entity beans, message-driven beans have no interfaces. They can be accessed only through messaging and they do not maintain any conversational state. Message-driven beans allow asynchronous communication between the queue and the listener, and provide separation between message processing and business logic.

Remote client view
  The remote client view specification became available beginning with EJB 1.1. The remote client view of an enterprise bean is location independent. A client running in the same JVM as a bean instance uses the same API to access the bean as a client running in a different JVM on the same or different machine.
  - Remote interface: The remote interface specifies the remote business methods that a client can call on an enterprise bean.
  - Remote home interface: The remote home interface specifies the methods used by remote clients for locating, creating, and removing instances of enterprise bean classes.

Local client view
  The local client view specification is available in EJB 2.0 or later. Unlike the remote client view, the local client view of a bean is location dependent. Local client view access to an enterprise bean requires both the local client and the enterprise bean that provides the local client view to be in the same JVM. The local client view therefore does not provide the location transparency provided by the remote client view. Local interfaces and local home interfaces provide support for lightweight access from enterprise bean that are local clients. Session and entity beans can be tightly couple with their clients, allowing access without the overhead typically associated with remote method calls.
  - Local interface: The local interface is a lightweight version of the remote interface, but for local clients. It includes business logic methods that can be called by a local client.
  - Local home interface: The local home interface specifies the methods used by local clients for locating, creating, and removing instances of enterprise bean classes.
Web service client view
  In the EJB 2.1 specification, the EJB architecture introduced the support for Web services. A client for a session bean can be a Web service client. A Web service client can make use of the Web service client view of a stateless session bean, which has a corresponding service endpoint interface.
  Service endpoint interface
    The service endpoint interface for a stateless session bean exposes the functionality of the session bean as a Web service endpoint. The Web Service Description Language (WSDL) document for a Web service describes the Web service as a set of endpoints operating on messages. A WSDL document can include the service endpoint interface of a stateless session bean as one of its endpoints. An existing stateless session bean can be modified to include a Web service client view, or a service endpoint interface can be mapped from an existing WSDL to provide the correct interface.

    A Web service client view is independent of location and can be accessed through remote calls.

EJB client JAR file
  An EJB client JAR file is an optional JAR file that can contain the client interfaces that a client program needs to use the client views of the enterprise beans that are contained in the EJB JAR file. If you decide not to create an EJB client JAR file for an EJB module, all of the client interface classes will be in the EJB JAR file. By default, the workbench creates EJB client JAR projects for each corresponding EJB project.

EJB container
  An EJB container is a runtime environment that manages one or more enterprise beans. The EJB container manages the life cycles of enterprise bean objects, coordinates distributed transactions, and implements object security. Generally, each EJB container is provided by an EJB server and contains a set of enterprise beans that run on the server.
Deployment descriptor
  A deployment descriptor is an XML file packaged with the enterprise beans in an EJB JAR file or an EAR file. It contains metadata describing the contents and structure of the enterprise beans, and runtime transaction and security information for the EJB container.
EJB server
  An EJB server is a high-level process or application that provides a runtime environment to support the execution of server applications that use enterprise beans. An EJB server provides a JNDI-accessible naming service, manages and coordinates the allocation of resources to client applications, provides access to system resources, and provides a transaction service. An EJB server could be provided by, for example, a database or application server.

(C) Copyright IBM Corporation 2000, 2005. All Rights Reserved.