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Detailed Description

See the Core J2EE Patterns

Detailed Example

The Java Pet Store sample application, v1.3.1 has two service locators: a Web-tier classServiceLocator, and an Enterprise JavaBeansTM (EJB) tier class, also calledServiceLocator

. Both classes manage lookup and caching of enterprise bean home interfaces, JMS and database

connection factories, and environment entries within their respective tiers. The only differencebetween them is that the Web-tier class is a singleton, and it caches the objects it looks up. The

EJB-tier class is not a singleton, and does not cache.

The following code discussion uses examples from the Web-tierServiceLocator:

Clients use ServiceLocator to access services.

The sample application class AdminRequestBDis a business delegate that uses the Web-

tierServiceLocator to access the order processing center enterprise bean

OPCAdminFacade. (See the Business Delegatedesign pattern for a more detailed

description ofAdminRequestBD.)

Figure 1 is a structure diagram that demonstrates how AdminRequestBD uses

ServiceLocator to find the remote home interface of the OPCAdminFacade enterprise

bean. The ServiceLocator returns a the remote enterprise bean interface

OPCAdminFacadeHome, by either retrieving it from the cache or looking it up using an

internal InitialContext instance. The client then uses the OPCAdminFacade to find or

create a remote component interface to an OPCAdminFacade

Figure 1. Structure diagram of ServiceLocator sample code

http://java.sun.com/blueprints/corej2eepatterns/Patterns/ServiceLocator.htmlhttp://java.sun.com/blueprints/code/index.html#petstorehttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/web/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/ejb/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/ejb/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/web/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/web/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/AdminRequestBD.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/opc/admin/ejb/OPCAdminFacade.java.htmlhttp://java.sun.com/blueprints/patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/code/index.html#petstorehttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/web/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/ejb/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/web/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/AdminRequestBD.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/opc/admin/ejb/OPCAdminFacade.java.htmlhttp://java.sun.com/blueprints/patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/corej2eepatterns/Patterns/ServiceLocator.html

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In the following code excerpt, AdminRequestBD calls the ServiceLocator static method

getInstance to get the singleton instance of the service locator, then calls

getRemoteHome to get the remote home interface of the OPCAdminFacade enterprise

bean. Notice that the caller must typecast the remote home interface toOPCAdminFacadeHome because getRemoteHome returns type EJBHome.

public class AdminRequestBD {...public AdminRequestBD() throws AdminBDException {try {OPCAdminFacadeHome home =(OPCAdminFacadeHome)

ServiceLocator.getInstance().getRemoteHome(OPC_ADMIN_NAME,OPCAdminFacadeHome.class);

opcAdminEJB = home.create();} catch (ServiceLocatorException sle) {...}

}

The service locator greatly simplifies the lookup of the enterprise bean home interface.

The singleton and caching strategies (discussed below) also improve performance,

because they avoid constructing unnecessary InitialContext and enterprise bean home

interfaces.

Public methods look up distributed resources.

The public methods of the service locator look up distributed resources by their JNDI

names. There are methods that find and return enterprise bean local home interfaces,

JDBCTM data sources, JMS queues and topics, and JMS queue and topic connectionfactories. There are also convenience methods that look up and perform type conversions

on environment entries.

As an example, method getLocalHome (for finding enterprise bean local home

interfaces) appears below. Each method that locates a particular type of resource returns

either a cached reference to the requested resource, or uses JNDI to find the resource,

placing a reference in the cache before returning it.

// Enterprise bean lookupspublic EJBLocalHome getLocalHome(String jndiHomeName)throws ServiceLocatorException {EJBLocalHome home = null;try {if (cache.containsKey(jndiHomeName)) {

home = (EJBLocalHome) cache.get(jndiHomeName);

http://java.sun.com/j2ee/sdk_1.3/techdocs/api/javax/ejb/EJBHome.htmlhttp://java.sun.com/j2ee/sdk_1.3/techdocs/api/javax/ejb/EJBHome.html

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} else {home = (EJBLocalHome) ic.lookup(jndiHomeName);cache.put(jndiHomeName, home);

}} catch (NamingException ne) {

throw new ServiceLocatorException(ne);} catch (Exception e) {

throw new ServiceLocatorException(e);}return home;

}

Methods that return enterprise bean home interface references are only type-safe to the

platform interface level; for example, getLocalHome returns a EJBLocalHome, but the

client must typecast the result.

Method getRemoteHome is similar to getLocalHome, except that it returns an enterprise

bean remote, instead of local, home interface. It also requires a reference to a class object

for the specific remote home interface, because remote home lookups use methodPortableRemoteObject.narrow to perform the type conversion from the object

returned from the JNDI lookup to the actual home interface type. The client that callsgetRemoteHome must still typecast the result to the remote home interface type, as shown

in the first example above.

public EJBHome getRemoteHome(String jndiHomeName, Class className)throws ServiceLocatorException {EJBHome home = null;try {if (cache.containsKey(jndiHomeName)) {

home = (EJBHome) cache.get(jndiHomeName);} else {

Object objref = ic.lookup(jndiHomeName);Object obj = PortableRemoteObject.narrow(objref, className);home = (EJBHome)obj;cache.put(jndiHomeName, home);

}} catch (NamingException ne) {

throw new ServiceLocatorException(ne);} catch (Exception e) {

throw new ServiceLocatorException(e);}

return home;}

As mentioned above, the service locator returns JMS resources, JDBC data sources, andperforms type conversion on values in environment entries. The table below summarizes

the names and return types of these methods.

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Table 1. Additional ServiceLocator methods

Method

Name

Return

Type

Resource

Type

getQueueConnectionFactory QueueConnectionFactory JMS

getQueue Queue JMS

getTopicConnectionFactory TopicConnectionFactory JMS

getTopic Topic JMS

getDataSource DataSource JDBC

getUrl URL env-entry

getBoolean boolean env-entry

getString String env-entry

Improving performance with the Singleton pattern and caching.

The Singleton pattern [ GHJV95] ensures that only a single instance of a class exists in an

application. The meaning of the term "singleton" is not always clear in a distributed

environment; in ServiceLocator it means that only one instance of the class exists per

class loader.

The Singleton pattern improves performance because it eliminates unnecessary

construction ofServiceLocator objects, JNDI InitialContext objects, and enables

caching (see below).

The Web-tier service locator also improves performance by caching the objects it finds.The cache lookup ensures that a JNDI lookup only occurs once for each name.

Subsequent lookups come from the cache, which is typically much faster than a JNDIlookup.

The code excerpt below demonstrates how the ServiceLocator improves performance

with the Singleton pattern and an object cache.

public class ServiceLocator {

private InitialContext ic;

private Map cache;

private static ServiceLocator me;

static {try {me = new ServiceLocator();

} catch(ServiceLocatorException se) {System.err.println(se);se.printStackTrace(System.err);

http://java.sun.com/blueprints/patterns/references.html#GHJV95http://java.sun.com/blueprints/patterns/references.html#GHJV95

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}}private ServiceLocator() throws ServiceLocatorException {

try {ic = new InitialContext();cache = Collections.synchronizedMap(new HashMap());

} catch (NamingException ne) {throw new ServiceLocatorException(ne);

}}

static public ServiceLocator getInstance() {return me;

}

A private class variable me contains a reference to the only instance of the

ServiceLocator class. It is constructed when the class is initialized in the static

initialization block shown. The constructor initializes the instance by creating the JNDI

InitialContext and the HashMap that is used a cache. Note that the no-argumentconstructor is private: only class ServiceLocator can construct a ServiceLocator.

Because only the static initialization block creates the instance, there can be only oneinstance per class loader.

Classes that use service locator access the singleton ServiceLocator instance by calling

public method getInstance.

Each object looked up has a JNDI name which, being unique, can be used as a cacheHashMap key for the object. Note also that the HashMap used as a cache is synchronized

so that it may be safely accessed from multiple threads that share the singleton instance.

EX 2---Design Pattern Sample Application - Service Locator

Date: 05-Nov-2004

Table of Contents

Introduction

Application Overview

Software RequirementsTerminology

Configuring the ApplicationDeploying and Running the Application

Sample Application FilesAdditional References

Introduction

Prerequisite

http://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#introhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#overhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#reqnhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#termshttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#configurehttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#deployhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#fileshttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#addrefhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#prehttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#introhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#overhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#reqnhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#termshttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#configurehttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#deployhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#fileshttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#addrefhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#pre

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Technical Overview

Prerequisite

To understand this sample application the user is expected to have knowledge in the following

area,

JSP

EJB

Technical Overview

In an enterprise application, clients are always required to interact with various service

components, such as Enterprise Javabeans (EJB) and Java Message Service (JMS) as they

provide the business methods to run the service. This is possible only after locating a businessservice (referred as a lookup operation). J2EE uses the JNDI tree to lookup, access and invoke

business services on passing a unique registered JNDI name. If the service is used by variousclients, then the code for looking up the object gets duplicated in various forms which makes itdifficult to maintain the application.

The Service Locator Design Pattern solves this issue by abstracting the complexities and network

dependencies into a single Service Locator class. This service layer pattern can be used forstoring lookup values for all services and provide it on request. A caching mechanism can also

be incorporated within the service to enhance performance as the JNDI lookup is made once

only.

Application Overview

This application demonstrates the implementation of Service Locator design pattern. In anenterprise application, generally there is a module for getting the feedback from the customer.

The sample application targets this scenario to explain how to implement the Service Locator

pattern.In this application, the customer provides the feedback with his/her name and e-mail ID. Using

an Entity bean (Feedback), the application first enters the feedback in the database table. After

this is successfully executed, the information is entered in a queue (MailQueue). The lookup for

the EJB, QueueConnectionFactory and the Queue is done through a Service Locator class. An

MDB (MailMDB) is registered to listen to the queue. The MDB is asynchronously invoked and

sends an e-mail to the customer thanking for the feedback.The objects are also cached within the Service Locator class which improves performance as the

lookup is done only once during the first request and not for subsequent requests.

The following class diagram shows the important classes in the Service Locator pattern.

http://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#techhttp://www.oracle.com/technology/sample_code/tech/java/jsps/index.htmlhttp://www.oracle.com/technology/sample_code/tech/java/ejb_corba/index.htmlhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/readme.html#techhttp://www.oracle.com/technology/sample_code/tech/java/jsps/index.htmlhttp://www.oracle.com/technology/sample_code/tech/java/ejb_corba/index.html

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The following sequence diagram depicts the interactions in this sample application.

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Software Requirements

Following is the list of software's required for configuring and running this sample application.

Oracle Containers for J2EE (OC4J) - version 9.0.4

andANT - version 1.5 or above for building the EAR fileandJ2ESDK1.4.x

Oracle Database 10g

Terminology

Term Definition

http://www.oracle.com/technology/software/content.htmlhttp://ant.apache.org/http://ant.apache.org/http://java.sun.com/productshttp://java.sun.com/productshttp://www.oracle.com/technology/software/content.htmlhttp://www.oracle.com/technology/software/content.htmlhttp://www.oracle.com/technology/software/content.htmlhttp://www.oracle.com/technology/software/content.htmlhttp://ant.apache.org/http://java.sun.com/productshttp://www.oracle.com/technology/software/content.html

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Deploying and Running the Application

This section describes the steps required in deploying this application to the Standalone OC4J

using ANT Tool and running using the browser.

Note: Make sure that the environment variables[, /bin in the

PATH;, /bin in the PATH] have been set before proceeding further. For moreinformation on how to setup these environment variables, please referenvironment set up

readme document.

Edit /j2ee/home/config/jms.xml in your favorite XML

editor. Add the following lines under the tag.

Ensure that OC4J is up and running. To start the OC4J server, navigate to/j2ee/home and execute the following command,

> java -jar oc4j.jar

Open the /config/server.properties in a text editor.

Change the following parameters to with your OC4J details.

OC4J_HOME = Folder where OC4J is installed.Ex: D:\oc4j10g

OC4J_HOST = Machine name or IP address on

which OC4J is running

OC4J_PORT = ORMI port on which your OC4J

listens for ORMI requests

OC4J_USERNAME = User name of OC4J server,

default is admin.

OC4J_PASSWORD = Password of OC4J server.

Build and deploy the EAR file using ANT. Fromdirectory, execute the command,

> ant

Open your favorite browser and access the sample application, using thefollowing URL,

http://hostName:port/locator/Feedback.jsp

http://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/envsetup.htmlhttp://www.oracle.com/technology/sample_code/tech/java/j2ee/designpattern/businesstier/servicelocator/envsetup.html

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where,

is the machine on which OC4J is running and is Port in

which the OC4J server listens to HTTP requests. By default OC4J listens forHTTP requests in port # 8888

Example: http://localhost:8888/locator/Feedback.jsp

Sample Application Files

This section provides a tabular listing of the sample application files, along with their respectivedirectory locations and a description of what they do in the overall scheme of the application.

Directory File Description

ServiceLocator\doc Readme.html This file

ServiceLocator\src\oracle\otnsamples\designpatterns\servicelocator *.java

Java source

files used inthis sample

application

ServiceLocator\public_html *.jsp

JSP files

used in thissample

application

ServiceLocator\public_html\WEB-INF\web.xml web.xml

Web

deploymentdescriptor

ServiceLocator\config mail.properties

Theproperties

file that holds

the mailserver details

ServiceLocator\configserver.propertie

s

The

properties

file that holds

the OC4Jserver details

ServiceLocator\src\META-INF ejb-jar.xml

EJB

deployment

descriptor

ServiceLocator\src\META-INF orion-ejb-

jar.xml

OC4J

specific EJBdeployment

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descriptor

ServiceLocator\src\META-INFdata-

sources.xml

Datasourceconfiguration

file

ServiceLocator\src\META-INF application.xml

Application

deploymentdescriptor.

ServiceLocator\src\META-INForion-

application.xml

OC4Jspecific

deployment

descriptor.

ServiceLocator build.xmlThe ANTbuild file

Business Delegate

Brief Description

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In distributed applications, lookup and exception handling for remote business components can

be complex. When applications use business components directly, application code must change

to reflect changes in business component APIs.

These problems can be solved by introducing an intermediate class called a business delegate,

which decouples business components from the code that uses them. The Business Delegatepattern manages the complexity of distributed component lookup and exception handling, and

may adapt the business component interface to a simpler interface for use by views.

Detailed Description

See the Core J2EETM Patterns

Detailed Example

Sample application business delegate class AdminRequestBDhandles distributed

lookup and catches and adapts exceptions in the sample application orderprocessing center (OPC).

The AdminRequestBD business delegate manages distributed componentlookup and handles exceptions.

The structure diagram in Figure 1 shows the ApplRequestProcessor servlet using

AdminRequestBD to find and use distributed business components.

Figure 1. AdminRequestBD locates and adapts other business

components

The code sample below shows the constructor forAdminRequestBD. It uses class

ServiceLocatorto acquire the remote home interface of session facade

OPCAdminFacade. (See the Service Locatorand Session Facadedesign patterns.) It uses

the remote home interface to create a OPCAdminFacade remote component interface,

which it maintains in a private field. The block that locates and creates the enterprise

http://java.sun.com/blueprints/corej2eepatterns/Patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/corej2eepatterns/Patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/corej2eepatterns/Patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/corej2eepatterns/Patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/corej2eepatterns/Patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/AdminRequestBD.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/web/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/opc/admin/ejb/OPCAdminFacade.java.htmlhttp://java.sun.com/blueprints/patterns/ServiceLocator.htmlhttp://java.sun.com/blueprints/patterns/ServiceLocator.htmlhttp://java.sun.com/blueprints/patterns/SessionFacade.htmlhttp://java.sun.com/blueprints/patterns/SessionFacade.htmlhttp://java.sun.com/blueprints/corej2eepatterns/Patterns/BusinessDelegate.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/AdminRequestBD.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/servicelocator/web/ServiceLocator.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/opc/admin/ejb/OPCAdminFacade.java.htmlhttp://java.sun.com/blueprints/patterns/ServiceLocator.htmlhttp://java.sun.com/blueprints/patterns/SessionFacade.html

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bean reference catches exceptions related to finding the home interface and to creating

the component interface. Any exception that occurs is then wrapped in anAdminBDException, which effectively hides the implementation details of the business

delegate from its clients.

public AdminRequestBD() throws AdminBDException {try {

OPCAdminFacadeHome home = (OPCAdminFacadeHome)ServiceLocator.getInstance().getRemoteHome(OPC_ADMIN_NAME,OPCAdminFacadeHome.class);

opcAdminEJB = home.create();} catch (ServiceLocatorException sle) {

throw new AdminBDException(sle.getMessage());} catch (CreateException ce) {

throw new AdminBDException(ce.getMessage());} catch (RemoteException re) {

throw new AdminBDException(re.getMessage());}

}

The OPC request processor uses AdminRequestBD for simple access to businesscomponents components.

OPC servlet class ApplRequestProcessorreceives service requests from the admin

client in the form of XML messages transmitted using HTTP. One of these request is for

statistics about orders that have a given status.

Method ApplRequestProcessor.getOrders receives part of an XML DOM tree

representing a Web service request. It extracts the status code from the document anduses AdminRequestBD.getOrdersByStatus to retrieve a list of order information. The

interface to that list is transfer object interface OrdersTO. (See the Transfer Object

pattern.) The code in the request processor that retrieves the order information appears inthe following code sample.

public class ApplRequestProcessor extends HttpServlet {...

String getOrders(Element root) {try {AdminRequestBD bd = new AdminRequestBD();NodeList nl = root.getElementsByTagName("Status");String status = getValue(nl.item(0));OrdersTO orders = bd.getOrdersByStatus(status);

...}

http://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/AdminBDException.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/ApplRequestProcessor.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/opc/admin/ejb/OrdersTO.java.htmlhttp://java.sun.com/blueprints/patterns/TransferObject.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/AdminBDException.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/admin/web/ApplRequestProcessor.java.htmlhttp://java.sun.com/blueprints/code/jps131/src/com/sun/j2ee/blueprints/opc/admin/ejb/OrdersTO.java.htmlhttp://java.sun.com/blueprints/patterns/TransferObject.html

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Because it has already created the reference to an OPCAdminFacadeEJB, the

AdminRequestBD object can simply forward the call to the enterprise bean's method

getOrdersByStatus, as follows:

public class AdminRequestBD {...

public OrdersTO getOrdersByStatus(String status)throws AdminBDException {

try {return opcAdminEJB.getOrdersByStatus(status);

} catch (RemoteException re) {throw new AdminBDException(re.getMessage());

} catch (OPCAdminFacadeException oafee) {throw new AdminBDException(oafee.getMessage());

}}

...}

Notice again that the method catches any exceptions that the enterprise bean may throw

and re-throws an exception type that is specific to the business delegate's interface. Thishides the business delegate's implementation details from the client.

J2EE DESIGN PATTERNS - Business Delegate

When the presentation-tier components access the business services implemented by thebusiness-directly through the exposed API of the services. The clients are exposed to the

complexity of dealing with distributed components. The client is tightly coupled to the EJBlayer, creating dependencies between client and server that affect both development, run-

time and project management concerns. Due to this

1. The presentation-tier components are vulnerable to changes in the implementation of thebusiness services, when ever the implementation of the business services change, the

exposed implementation code in the presentation tier must change too.

2. All the client side components have to be aware of the location details of the business

services say that each components has to use the JNDI lookup service to locate the requiredremote interfaces.

3. There may be a detrimental impact on network performance because presentation-tiercomponents that use the business service API make too many invocations over the network.

This happens when presentation-tier components use the underlying API directly, with noclient-side caching mechanism or aggregating service.

The solution is to create a layer of Business Delegate to reduce coupling between

presentation-tier clients and business services. The Business Delegate hides the underlyingimplementation details of the business service, such as lookup and access details of the EJB

architecture.

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Business delegate handle all the code that access the business services in the selected

vendor software. When the vendor or the vendor software changes only changes that needto be made is to the companys allocation software are changes to Business delegate, to

access the business services. The presentation layer need not to be modified.

The business delegate object abstracts the business services API and provides standardinterface to al the client components. It hides the underlying implementation detail such aslookup mechanism and the API of the business services. This reduces the coupling between

the clients and the business services.

Another benefit is that the delegate may cache results and references to remote business

services. Caching can significantly improve performance, because it limits unnecessary andpotentially costly round trips over the network.

The responsibilities of the components participating in this pattern are:

Client components : The client components which are jsp pages and servlets in the

presentation tier, delegate the work of locating the business service providers and the work

of invoking the business service API methods to the Business Delegate objects.

Business Delegate : The business Delegate acts as a representative of the clinet

components. It knows how to look up and access the business services. It invokes theappropriate business services method in the required order . If the API of the business

service component changes, only Business Delegate needs to be modified without affecting

the client components.

Business Service : A business service component implements the actual business logic.

Some examples of the business service components are stateless session EJB and entity EJBa CORBA object or an RPC server.