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 <h1 class="topictitle1">EJB 2.x architecture</h1>


 <div><p>This topic provides a high-level overview of the distributed component
 architecture defined in the Sun Microsystems Enterprise JavaBeans™ (EJB)
 version 2.x architecture specification.</p>

 <p>The complete Enterprise JavaBeans specifications and descriptions
 of the technology are available from the <a href="http://java.sun.com/products/ejb/docs.html#specs" target="_blank" title="">java.sun.com</a> Web site.</p>

 <p>Enterprise beans provide several benefits for application developers. They
 do the following: </p>

 <ul>
 <li>Allow you to build distributed applications by combining components developed
 using tools from different vendors. </li>

 <li>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.</li>

 <li>Are developed once and then deployed on multiple platforms without recompilation
 or source code modification. </li>

 <li>Offer compatibility between the EJB specification that governs the use
 of enterprise beans and other Java™ APIs and CORBA. This also provides
 for interoperability between enterprise beans and non-Java applications.</li>

 </ul>

 <dl>
 <dt class="dlterm"><strong>Enterprise beans</strong></dt>

 <dd>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 and message-driven beans are coarse-grained
 components designed to model business process while entity beans are used
 to model fine-grained data objects.<div class="p"> <ul>
 <li><strong>Session beans</strong>: Session beans are non-persistent enterprise beans.
 They can be stateful or stateless.<ul>
 <li><strong>Stateful session beans</strong>:  Act on behalf of a single client and maintain
 client-specific session information (called conversational state) across multiple
 method calls and transactions. They exist for the duration of a single client/server
 session.</li>

 <li><strong>Stateless session beans</strong>: Do not maintain any conversational state
 and are pooled by their container to handle multiple requests from multiple
 clients.</li>

 </ul>
 </li>

 <li><strong>Entity beans</strong>: 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.</li>

 <li><strong>Message-driven beans</strong>: 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.</li>

 </ul>
 </div>
 </dd>

 </dl>

 <dl>
 <dt class="dlterm"><strong>Remote client view</strong></dt>

 <dd>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.
 The remote client view consists of two interfaces<ul>
 <li><strong>Remote interface</strong>: The remote interface specifies the remote business
 methods that a client can call on an enterprise bean.</li>

 <li><strong>Remote home interface</strong>: The remote home interface specifies the
 methods used by remote clients for locating, creating, and removing instances
 of enterprise bean classes.</li>

 </ul>
 </dd>

 </dl>

 <dl>
 <dt class="dlterm"><strong>Local client view</strong></dt>

 <dd>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 beans
 that are local clients. Session and entity beans can be tightly coupled with
 their clients, allowing access without the overhead typically associated with
 remote method calls. The local client view consists of two interfaces:<ul>
 <li><strong>Local interface</strong>: 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.</li>

 <li><strong>Local home interface</strong>: The local home interface specifies the methods
 used by local clients for locating, creating, and removing instances of enterprise
 bean classes.</li>

 </ul>
 </dd>


 <dt class="dlterm"><strong>Web service client view</strong></dt>

 <dd>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.<dl>
 <dt class="dlterm"><strong>Service endpoint interface</strong></dt>

 <dd>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.<p>A Web service client view is independent
 of location and can be accessed through remote calls.</p>
 </dd>

 </dl>
 </dd>

 </dl>

 <dl>
 <dt class="dlterm"><strong>EJB client JAR file</strong></dt>

 <dd>An EJB client JAR file is an optional JAR file that can contain the client
 interfaces that a client program needs to use and 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.</dd>

 </dl>

 <dl>
 <dt class="dlterm"><strong>EJB container</strong></dt>

 <dd>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.</dd>


 <dt class="dlterm"><strong>Deployment descriptor</strong></dt>

 <dd>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.</dd>


 <dt class="dlterm"><strong>EJB server</strong></dt>

 <dd>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.</dd>

 </dl>

 </div>

 <div>
 <div class="familylinks">
 <div class="parentlink"><strong>Parent topic:</strong> <a href="../topics/ceover.html" title="The workbench provides a specialized environment that you can use to develop and test enterprise beans that conform to the distributed component architecture defined in the Sun Microsystems Enterprise JavaBeans (EJB) version specification. This product supports the Enterprise JavaBeans 1.1, 2.0, 2.1, and 3.0 specification levels.">EJB 2.x application development</a></div>
 </div>
 </div>

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	<title>EJB 2.x architecture</title>
	</head>
	<body id="cearch"><a name="cearch"><!-- --></a>


	<h1 class="topictitle1">EJB 2.x architecture</h1>



	<div><p>This topic provides a high-level overview of the distributed component
	architecture defined in the Sun Microsystems Enterprise JavaBeans™ (EJB)
	version 2.x architecture specification.</p>

	<p>The complete Enterprise JavaBeans specifications and descriptions
	of the technology are available from the <a href="http://java.sun.com/products/ejb/docs.html#specs" target="_blank" title="">java.sun.com</a> Web site.</p>

	<p>Enterprise beans provide several benefits for application developers. They
	do the following: </p>

	<ul>
	<li>Allow you to build distributed applications by combining components developed
	using tools from different vendors. </li>

	<li>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.</li>

	<li>Are developed once and then deployed on multiple platforms without recompilation
	or source code modification. </li>

	<li>Offer compatibility between the EJB specification that governs the use
	of enterprise beans and other Java™ APIs and CORBA. This also provides
	for interoperability between enterprise beans and non-Java applications.</li>

	</ul>

	<dl>
	<dt class="dlterm"><strong>Enterprise beans</strong></dt>

	<dd>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 and message-driven beans are coarse-grained
	components designed to model business process while entity beans are used
	to model fine-grained data objects.<div class="p"> <ul>
	<li><strong>Session beans</strong>: Session beans are non-persistent enterprise beans.
	They can be stateful or stateless.<ul>
	<li><strong>Stateful session beans</strong>: Act on behalf of a single client and maintain
	client-specific session information (called conversational state) across multiple
	method calls and transactions. They exist for the duration of a single client/server
	session.</li>

	<li><strong>Stateless session beans</strong>: Do not maintain any conversational state
	and are pooled by their container to handle multiple requests from multiple
	clients.</li>

	</ul>
	</li>

	<li><strong>Entity beans</strong>: 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.</li>

	<li><strong>Message-driven beans</strong>: 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.</li>

	</ul>
	</div>
	</dd>

	</dl>

	<dl>
	<dt class="dlterm"><strong>Remote client view</strong></dt>

	<dd>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.
	The remote client view consists of two interfaces<ul>
	<li><strong>Remote interface</strong>: The remote interface specifies the remote business
	methods that a client can call on an enterprise bean.</li>

	<li><strong>Remote home interface</strong>: The remote home interface specifies the
	methods used by remote clients for locating, creating, and removing instances
	of enterprise bean classes.</li>

	</ul>
	</dd>

	</dl>

	<dl>
	<dt class="dlterm"><strong>Local client view</strong></dt>

	<dd>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 beans
	that are local clients. Session and entity beans can be tightly coupled with
	their clients, allowing access without the overhead typically associated with
	remote method calls. The local client view consists of two interfaces:<ul>
	<li><strong>Local interface</strong>: 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.</li>

	<li><strong>Local home interface</strong>: The local home interface specifies the methods
	used by local clients for locating, creating, and removing instances of enterprise
	bean classes.</li>

	</ul>
	</dd>


	<dt class="dlterm"><strong>Web service client view</strong></dt>

	<dd>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.<dl>
	<dt class="dlterm"><strong>Service endpoint interface</strong></dt>

	<dd>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.<p>A Web service client view is independent
	of location and can be accessed through remote calls.</p>
	</dd>

	</dl>
	</dd>

	</dl>

	<dl>
	<dt class="dlterm"><strong>EJB client JAR file</strong></dt>

	<dd>An EJB client JAR file is an optional JAR file that can contain the client
	interfaces that a client program needs to use and 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.</dd>

	</dl>

	<dl>
	<dt class="dlterm"><strong>EJB container</strong></dt>

	<dd>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.</dd>


	<dt class="dlterm"><strong>Deployment descriptor</strong></dt>

	<dd>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.</dd>


	<dt class="dlterm"><strong>EJB server</strong></dt>

	<dd>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.</dd>

	</dl>

	</div>

	<div>
	<div class="familylinks">
	<div class="parentlink"><strong>Parent topic:</strong> <a href="../topics/ceover.html" title="The workbench provides a specialized environment that you can use to develop and test enterprise beans that conform to the distributed component architecture defined in the Sun Microsystems Enterprise JavaBeans (EJB) version specification. This product supports the Enterprise JavaBeans 1.1, 2.0, 2.1, and 3.0 specification levels.">EJB 2.x application development</a></div>
	</div>
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