Business delegate pattern
Business delegate is a Java EE design pattern. [1] This pattern helps to reduce the coupling between presentation-tier clients and business services, and also hides the underlying implementation details of the business service, such as lookup and access details of the EJB architecture.[1][2] Business delegates acts as an adaptor to invoke business objects from the presentation tier.[3]
Structure
The client requests the business delegate to access to the underlying business service, and business delegate uses a lookup service to locate the required business service component.[1]
Components
Basic components are Business delegate, Lookup service and business service.
Business delegate
Business delegate provides control and protection for the business service. Business delegate can have two types of constructures, without ID and with ID, where ID is a string version of the reference to a remote object such as EJBHome or EJBObject.[1]
Lookup Service
Business service is located by lookup service which is used by the business delegate. The lookup service encapsulate the implementation details of business service lookup.[1]
Business Service
This a business-tier component, such as an enterprise bean or a JMS component, which provides the required service to the client.[1]
Consequences
Some consequences are as follows:
- More flexibility and maintanability as intermediate business delegate layer decouples the business layer from the presentation layer.
- Business delegate exposes a uniform API to the presentation tier to access business logic.[1][3]
Concerns
Following concers can be considered:
- Maintenance due the extra layer that increases the number of classes in the application.
- Business delegate should take care of the changes of the remote business object interfaces, and these types of changes are rare.[3]
Sample code
A sample code for a Professional Services Application(PSA), where a Web-tier cleint needs to access a session bean that implements the session façade pattern, is provided below.
Resource Delegate:
// imports
...
public class ResourceDelegate {
// Remote reference for Session Facade
private ResourceSession session;
// Class for Session Facade's Home object
private static final Class homeClazz =
corepatterns.apps.psa.ejb.ResourceSessionHome.class;
// Default Constructor. Looks up home and connects
// to session by creating a new one
public ResourceDelegate() throws ResourceException {
try {
ResourceSessionHome home = (ResourceSessionHome)
ServiceLocator.getInstance().getHome(
"Resource", homeClazz);
session = home.create();
} catch(ServiceLocatorException ex) {
// Translate Service Locator exception into
// application exception
throw new ResourceException(...);
} catch(CreateException ex) {
// Translate the Session create exception into
// application exception
throw new ResourceException(...);
} catch(RemoteException ex) {
// Translate the Remote exception into
// application exception
throw new ResourceException(...);
}
}
// Constructor that accepts an ID (Handle id) and
// reconnects to the prior session bean instead
// of creating a new one
public BusinessDelegate(String id)
throws ResourceException {
super();
reconnect(id);
}
// Returns a String ID the client can use at a
// later time to reconnect to the session bean
public String getID() {
try {
return ServiceLocator.getId(session);
} catch (Exception e) {
// Throw an application exception
}
}
// method to reconnect using String ID
public void reconnect(String id)
throws ResourceException {
try {
session = (ResourceSession)
ServiceLocator.getService(id);
} catch (RemoteException ex) {
// Translate the Remote exception into
// application exception
throw new ResourceException(...);
}
}
// The following are the business methods
// proxied to the Session Facade. If any service
// exception is encountered, these methods convert
// them into application exceptions such as
// ResourceException, SkillSetException, and so
// forth.
public ResourceTO setCurrentResource(
String resourceId)
throws ResourceException {
try {
return session.setCurrentResource(resourceId);
} catch (RemoteException ex) {
// Translate the service exception into
// application exception
throw new ResourceException(...);
}
}
public ResourceTO getResourceDetails()
throws ResourceException {
try {
return session.getResourceDetails();
} catch(RemoteException ex) {
// Translate the service exception into
// application exception
throw new ResourceException(...);
}
}
public void setResourceDetails(ResourceTO vo)
throws ResourceException {
try {
session.setResourceDetails(vo);
} catch(RemoteException ex) {
throw new ResourceException(...);
}
}
public void addNewResource(ResourceTO vo)
throws ResourceException {
try {
session.addResource(vo);
} catch(RemoteException ex) {
throw new ResourceException(...);
}
}
// all other proxy method to session bean
...
}
Remote interface for ResouceSession:
// imports
...
public class ResourceDelegate {
// Remote reference for Session Facade
private ResourceSession session;
// Class for Session Facade's Home object
private static final Class homeClazz =
corepatterns.apps.psa.ejb.ResourceSessionHome.class;
// Default Constructor. Looks up home and connects
// to session by creating a new one
public ResourceDelegate() throws ResourceException {
try {
ResourceSessionHome home = (ResourceSessionHome)
ServiceLocator.getInstance().getHome(
"Resource", homeClazz);
session = home.create();
} catch(ServiceLocatorException ex) {
// Translate Service Locator exception into
// application exception
throw new ResourceException(...);
} catch(CreateException ex) {
// Translate the Session create exception into
// application exception
throw new ResourceException(...);
} catch(RemoteException ex) {
// Translate the Remote exception into
// application exception
throw new ResourceException(...);
}
}
// Constructor that accepts an ID (Handle id) and
// reconnects to the prior session bean instead
// of creating a new one
public BusinessDelegate(String id)
throws ResourceException {
super();
reconnect(id);
}
// Returns a String ID the client can use at a
// later time to reconnect to the session bean
public String getID() {
try {
return ServiceLocator.getId(session);
} catch (Exception e) {
// Throw an application exception
}
}
// method to reconnect using String ID
public void reconnect(String id)
throws ResourceException {
try {
session = (ResourceSession)
ServiceLocator.getService(id);
} catch (RemoteException ex) {
// Translate the Remote exception into
// application exception
throw new ResourceException(...);
}
}
// The following are the business methods
// proxied to the Session Facade. If any service
// exception is encountered, these methods convert
// them into application exceptions such as
// ResourceException, SkillSetException, and so
// forth.
public ResourceTO setCurrentResource(
String resourceId)
throws ResourceException {
try {
return session.setCurrentResource(resourceId);
} catch (RemoteException ex) {
// Translate the service exception into
// application exception
throw new ResourceException(...);
}
}
public ResourceTO getResourceDetails()
throws ResourceException {
try {
return session.getResourceDetails();
} catch(RemoteException ex) {
// Translate the service exception into
// application exception
throw new ResourceException(...);
}
}
public void setResourceDetails(ResourceTO vo)
throws ResourceException {
try {
session.setResourceDetails(vo);
} catch(RemoteException ex) {
throw new ResourceException(...);
}
}
public void addNewResource(ResourceTO vo)
throws ResourceException {
try {
session.addResource(vo);
} catch(RemoteException ex) {
throw new ResourceException(...);
}
}
// all other proxy method to session bean
...
}
See also
References
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