SCEA1.5 - Few Design patterns

Service Activator: ----
Problem:a business service such as a session or entity bean provides only synchronous processing and thus presents a challenge to implementing asynchronous processingSolution: Use a Service Activator to receive asynchronous client requests and messages. On receiving a message, the Service Activator locates and invokes the necessary business methods on the business service components to fulfill the request asynchronously.
The ServiceActivator is a JMS Listener and delegation service that requires implementing the JMS message listener-making it a JMS listener object that can listen to JMS messages. The ServiceActivator can be implemented as a standalone service. Clients act as the message generator, generating events based on their activity.
Any client that needs to asynchronously invoke a business service, such as an enterprise bean, may create and send a message to the Service Activator. The Service Activator receives the message and parses it to interpret the client request. Once the client's request is parsed or unmarshalled, the Service Activator identifies and locates the necessary business service component and invokes business methods to complete processing of the client's request asynchronously.
The Service Activator may optionally send an acknowledgement to the client after successfully completing the request processing. The Service Activator may also notify the client or other services on failure events if it fails to complete the asynchronous request processing.
The Service Activator may use the services of a Service Locator to locate a business component.

Virtual Proxy: ---Application is often a collection of components and in most of the situations it may be the case that a component should be loaded until it is first accessed by the Client. Reasons may be that the component in consideration may be using most of the system resources. For example, consider the Microsoft Word Application which is providing Printer Service and the Help System. It should not be the case that both the Printer Service and the Help System should be loaded during the Application startup. Imagine what would be the case if the Client after starting the Word Application is accessing other part of the System and not the Printer and the Help Service. Definitely it will lead to a slow-response time because the Components are un-necessarily loaded.
Virtual Proxy Pattern comes into picture here as it defers the Object Creation process of memory-intensive components thereby speeding up the Application. Now, let us see how to design the Virtual Proxy.

Composite View: --- ProblemInstead of providing a mechanism to combine modular, atomic portions of a view into a composite whole, pages are built by embedding formatting code directly within each view.modification to the layout of multiple views is difficult and error prone, due to the duplication of code.
SolutionUse composite views that are composed of multiple atomic subviews. Each component of the template may be included dynamically into the whole and the layout of the page may be managed independently of the content.
This solution provides for the creation of a composite view based on the inclusion and substitution of modular dynamic and static template fragments. It promotes the reuse of atomic portions of the view by encouraging modular design. It is appropriate to use a composite view to generate pages containing display components that may be combined in a variety of ways. This scenario occurs, for example, with portal sites that include numerous independent subviews, such as news feeds, weather information, and stock quotes on a single page. The layout of the page is managed and modified independent of the subview content.

View Helper : --- ProblemPresentation tier changes occur often and are difficult to develop and maintain when business data access logic and presentation formatting logic are interwoven. This makes the system less flexible, less reusable, and generally less resilient to change.SolutionA view contains formatting code, delegating its processing responsibilities to its helper classes, implemented as JavaBeans or custom tags. Helpers also store the view's intermediate data model and serve as business data adapters.

Service to Worker: ---The system controls flow of execution and access to business data, from which it creates presentation contentThe Service to Worker pattern, like the Dispatcher View pattern, describes a common combination of other patterns from the catalog. Both of these macro patterns describe the combination of a controller and dispatcher with views and helpers. While describing this common structure, they emphasize related but different usage patterns.
Combine a controller and dispatcher with views and helpers to handle client requests and prepare a dynamic presentation as the response. Controllers delegate content retrieval to helpers, which manage the population of the intermediate model for the view. A dispatcher is responsible for view management and navigation and can be encapsulated either within a controller or a separate component.