Source:

1
EJB Patterns

• Source
• http//java.sun.com/j2ee/blueprints/patterns/index
  .html

2
Bimodal Data Access

• http//java.sun.com/blueprints/patterns/j2ee_patte
  rns/fast_lane_reader/index.html
• Provide efficient read-only access to server
  object state while accounting for transactional
  updates
• Motivation
• When more important to read objects efficiently
  than to read their most current state
• Key Aspects
• Implement direct read-only access to database in
  the client
• Implement transactional updates in the EJB

3
Data Access Objects

• http//java.sun.com/blueprints/patterns/j2ee_patte
  rns/data_access_object/index.html
• Description
• encapsulate access to object storage away from
  usage/representation
• Motivation
• Allows one to implement the same business logic
  over multiple forms of data storage forms.
• Key Aspects
• Use in EJB and non-EJB clients
• Use of interface/dynamic implementation best

4
Front Component

• http//java.sun.com/j2ee/blueprints/patterns/front
  _component/index.html
• Description
• Simplify interaction with a series of objects
• Motivation
• Provide ease-of-use interface to well factored
  application
• Key Aspects
• Session Bean or Servlet managing workflow through
  business methods

5
Model-View-Controller

• http//java.sun.com/blueprints/patterns/j2ee_patte
  rns/model_view_controller/index.html
• Description
• Decouples data presentation, representation, and
  operations
• Motivation
• Increase reuse
• Key Aspects
• Model - manages state, notifies Views of change
• View - render state
• Control - updates state, selects Views

6
Page List

• http//java.sun.com/blueprints/patterns/j2ee_patte
  rns/page_by_page_iterator/index.html
• Description
• Traverse large collections more efficiently by
  accessing elements in page increments
• Motivation
• Only chunks of collection are retrieved when not
  all of collection desired
• Key Aspects
• Page List Access providing Client Access Pages
• Client Access providing clients single elements

7
Session Entity Façade

• Description
• Provide a single API for collection of EJBs
• Motivation
• Simplifies or focuses client interaction with
  application
• Key Aspects
• Layer of Session Bean(s) hide application
  implementation from Clients view of system

8
Value Object

• http//java.sun.com/blueprints/patterns/j2ee_patte
  rns/value_object/index.htmlDescription
• Provides bulk access to related information
• Motivation
• Cut down on unnecessary overhead accessing
  properties individually that are normally
  accessed together
• Key Aspects
• Serializable structure passed between bean and
  client

9
Service Locator

• Factors
• EJB clients repeatedly look up home interfaces
  using JNDI
• Requires too much knowledge
• Can lead to inefficient design
• Solution
• Develop a ServiceLocator object
• Abstract JNDI usage
• Simplify client interaction with the system
• Cache resources when possible

10
Service Locator (Cont)

• Implementation Notes
• InitialContext lookup is slow
• InitialContext object may not be thread-safe
• EJBHome lookup is slow
• Beans Home object can be shared among threads

11
Implementation
package corej2ee.project.ejb import
corej2ee.project.ejb.ServiceLocatorException impo
rt java.util. import javax.naming. import
java.rmi.RemoteException import
javax.ejb. import javax.rmi.PortableRemoteObject
import java.io. import java.util.Map import
java.util.HashMap / Basic service locator
implementation that caches home references for
clients /
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/ Basic service locator implementation that
caches home references for clients / public
class ServiceLocator / Singleton
constructor _at_throws
ServiceLocatorException / private
ServiceLocator() throws ServiceLocatorException
try context new
InitialContext() catch
(NamingException ne) throw new
ServiceLocatorException(ne)
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/ Returns singleton instance
_at_throws ServiceLocatorException /
public static ServiceLocator getInstance() throws
ServiceLocatorException if
(myServiceLocator null)
myServiceLocator new corej2ee.project.ejb.Servic
eLocator() return
myServiceLocator
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/ Retrieves the EJBObject instance from
a previosly serialized handle
_at_param id serialized EJBObject handle
_at_throws ServiceLocatorException _at_return
EJBObject reference for handle / public
static EJBObject getService(String id)throws
ServiceLocatorException if (id null)
return null try
byte bytes new
String(id).getBytes() InputStream io
new ByteArrayInputStream(bytes)
ObjectInputStream os new ObjectInputStream(io)
javax.ejb.Handle handle
(javax.ejb.Handle) os.readObject()
return handle.getEJBObject() catch
(Exception ex) throw new
ServiceLocatorException(ex)
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/ Creates a serialized representation
for an EJBObject so it can be re-created
at a later time _at_param session EJBObject
to create a handle for _at_throws
ServiceLocatorException _at_return serialized
handle for the EJBObject / public
static String getId(EJBObject session)throws
ServiceLocatorException try
javax.ejb.Handle handle session.getHandle()
ByteArrayOutputStream fo new
ByteArrayOutputStream()
ObjectOutputStream so new ObjectOutputStream(fo)
so.writeObject(handle)
so.flush() so.close()
return new String(fo.toByteArray())
catch (RemoteException ex) throw
new ServiceLocatorException(ex) catch
(IOException ex) throw new
ServiceLocatorException(ex)
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/ Returns the home object for the
specified JNDI name _at_param name
JNDI name of the Home interface _at_param
clazz Specific Home class _at_throws
ServiceLocatorException _at_return Home stub
/ public EJBHome getHome(String name,
Class clazz)throws ServiceLocatorException
try if(homeMap.containsKey(name))
System.out.println("Returning
from cache") return (EJBHome)
homeMap.get(name)
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else Object objrefnull
System.out.println("Looking up home using
initial context")
synchronized(context) objref
context.lookup(name)
EJBHome home (EJBHome)PortableRemoteObject.nar
row(objref, clazz)
homeMap.put(name, home) return
home catch
(NamingException ex) throw new
ServiceLocatorException(ex)
private static ServiceLocator myServiceLocator
// singleton private InitialContext context
null // connection to naming service private
Map homeMapnew HashMap() // stores already
retrieved homes
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Client Usage
try System.out.println("Using
service locator") ServiceLocator
slServiceLocator.getInstance()
TellerHome home (TellerHome)
sl.getHome("TellerBean",corej2ee.project.ejb.Telle
rHome.class) Teller teller
home.create() Customer cnew
Customer("Dan")
teller.addCustomer(c)
teller.remove()
System.out.println("Test completed") catch(Remo
teException re)
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Service Locator Summary

• Provides a simpler mechanism for clients to
  locate remote resources
• Improved performance obtained with context and
  home object caching
• Frequently implemented for other distributed
  objects
• JMS Connection Factories
• JMS Destinations Topics and Queues
• Other implementation strategies possible. Concept
  is what is important

20
Business Delegate

• Factors
• Presentation Tier makes distributed calls to
  Business Tier components. Exposed to complexity
• Performance implications if client makes too many
  remote calls
• Tight coupling. Client needs to know lookup and
  access details
• Solution
• Develop BusinessDelegate to abstract
  implementation details of the business service
• Client-Side business abstraction
• May implement caching mechanism
• Can retry when errors occur
• Wrap Remote Exceptions as a business exception

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Proxy Implementation Details

• Uses a ServiceLocator to locate the target
  business service
• Delegates business method calls to remote
  business service
• May cache data to reduce the number of remote
  calls made
• Remote exceptions translated to application
  exceptions for client
• May provide simpler interface to client
• e.g. can hold state to simplify calling of remote
  stateless session beans

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Implementation
public class TellerBusinessDelegate
publicTellerBusinessDelegate() throws
BusinessException try
TellerHome home (TellerHome)ServiceLocator.ge
tInstance().getHome(
"TellerBean", homeClazz) session
home.create() catch
(ServiceLocatorException ex)
throw new corej2ee.project.ejb.BusinessException(e
x) catch
(CreateException ex) throw new
corej2ee.project.ejb.BusinessException(ex)
catch (RemoteException ex)
throw new corej2ee.project.ejb.Busi
nessException(ex)
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public TellerBusinessDelegate(String id) throws
BusinessException reconnect(id)
public String getID() throws
BusinessException try
return corej2ee.project.ejb.ServiceLocator.getI
d(session) catch
(Exception e) throw new
corej2ee.project.ejb.BusinessException(e)
public void
reconnect(String ID) throws BusinessException
try session
(Teller)corej2ee.project.ejb.ServiceLocator.getSer
vice(ID) catch
(ServiceLocatorException ex)
throw new corej2ee.project.ejb.BusinessException(e
x)
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public void addCustomer(Customer c) throws
BusinessException try
session.addCustomer(c)
catch (java.rmi.RemoteException ex)
throw new corej2ee.project.ejb.BusinessExce
ption(ex) public
void transfer(int to,int from,double amount)
throws BusinessException try
session.transfer(to, from, amount)
catch (java.rmi.RemoteExcept
ion ex) throw new
corej2ee.project.ejb.BusinessException(ex)

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public void removeCustomer(Customer c) throws
BusinessException try
session.removeCustomer(c)
catch (java.rmi.RemoteException ex)
throw new corej2ee.project.ejb.BusinessE
xception(ex) public
void close() if(session ! null)
try session.remove()
sessionnull catch(Exception
e)
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protected void finalize() close()
/ _at_link _at_shapeType
PatternLink _at_pattern Business Delegate
Proxy Strategy _at_supplierRole Uses
/ / private BusinessException
_businessException / private Teller
session private Class homeClazz
TellerHome.class
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Client Program
package corej2ee.project.ejb public class
TellerClient public static void
main(String argv) try
TellerBusinessDelegate tellernew
TellerBusinessDelegate() Customer
cnew Customer("Dan")
teller.addCustomer(c) cnew
Customer("Cathy")
teller.addCustomer(c)
teller.close() catch
(BusinessException ex)
ex.printStackTrace()
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Business Delegate Summary

• Further simplifies client access to remote
  business services
• Hides complexity
• JNDI lookup
• Remote Exceptions
• Performance enhancements
• Home, Initial Context, Remote object reference
  caching
• Business Data caching
• Not in this case
• Consider a business delegate for the Shopping
  Cart Bean
• Could hold all items in the cart locally and skip
  remote call when the client asks for all items

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Other Sources

• http//developer.java.sun.com/developer/technicalA
  rticles/J2EE/patterns