Core J2EE Patterns

Presentation Tier Patterns

• Intercepting Filter intercepts incoming requests and outgoing responses and applies a filter. These filters may be added and removed in a declarative manner, allowing them to be applied unobtrusively in a variety of combinations. After this preprocessing and/or post-processing is complete, the final filter in the group vectors control to the original target object. For an incoming request, this is often a Front Controller, but may be a View.

Intercepting Filter intercepts incoming requests and outgoing responses and applies a filter. These filters may be added and removed in a declarative manner, allowing them to be applied unobtrusively in a variety of combinations. After this preprocessing and/or post-processing is complete, the final filter in the group vectors control to the original target object. For an incoming request, this is often a Front Controller, but may be a View. To summarize, the Intercepting Filter pattern provides the ability to manipulate a request prior to processing or to manipulate the response before sending the results of the request. 

UML Representation

Benefits

The following lists the benefits of using the Intercepting Filter pattern:

• Centralizes pre-processing of requests
• Centralizes post-processing of responses

When to Use

You should use the Intercepting Filter pattern when:

• You need to pre-process a request or response.
• You need to post-process a request or response.

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• Front Controller is a container to hold the common processing logic that occurs within the presentation tier and that may otherwise be erroneously placed in a View. A controller handles requests and manages content retrieval, security, view management, and navigation, delegating to a Dispatcher component to dispatch to a View. 

Front Controller is a container to hold the common processing logic that occurs within the presentation tier and that may otherwise be erroneously placed in a View. A controller handles requests and manages content retrieval, security, view management, and navigation, delegating to a Dispatcher component to dispatch to a View. The Front Controller pattern creates central control logic for presentation request handling. The Front Controller is different from the Intercepting Filter in that the Front Controller is determining processing based on the request and an Intercepting Filter is modifying the request.

UML representation

Benefits

The following lists the benefits of using the Front Controller pattern:

• Centralizes control logic
• Improves reusability
• Improves separation of concerns

When to Use

You should use the Front Controller pattern to:

• Apply common logic to multiple requests
• Separate processing logic from view

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• Application Controller centralizes control, retrieval, and invocation of view and command processing. While a Front Controller acts as a centralized access point and controller for incoming requests, the Application Controller is responsible for identifying and invoking commands, and for identifying and dispatching to views.

• Context Object encapsulates state in a protocol-independent way to be shared throughout your application. Using Context Object makes testing easier, facilitating a more generic test environment with reduced dependence upon a specific container.

• View Helper encourages the separation of formatting-related code from other business logic. It suggests using Helper components to encapsulate logic relating to initiating content retrieval, validation, and adapting and formatting the model. The View component is then left to encapsulate the presentation formatting. Helper components typically delegate to the business services via a Business Delegate or an Application Service, while a View may be composed of multiple subcomponents to create its template.

View Helper encourages the separation of formatting-related code from other business logic. It suggests using Helper components to encapsulate logic relating to initiating content retrieval, validation, and adapting and formatting the model. The View component is then left to encapsulate the presentation formatting. Helper components typically delegate to the business services via a Business Delegate or an Application Service, while a View may be composed of multiple subcomponents to create its template. The View Helper pattern separates the processing logic from the view.

UML representation

Benefits

The following is a benefit of using the View Helper pattern:

• Separates logic from the view

When to Use

You should use the View Helper pattern to:

• Encapsulate view-processing logic

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• Composite View suggests composing a View from numerous atomic pieces. Multiple smaller views, both static and dynamic, are pieced together to create a single template. The Service to Worker and Dispatcher View patterns represent a common combination of other patterns from the catalog. The two patterns share a common structure, consisting of a controller working with a Dispatcher, Views, and Helpers. Service to Worker and Dispatcher View have similar participant roles, but differ in the division of labor among those roles. Unlike Service to Worker, Dispatcher View defers business processing until view processing has been performed.

• Service to worker performs core request handling and invoke business logic before control is passed to the view. It centralizes control and request handling to retrieve a presentation model before turning control over to the view. The view generates a dynamic response based on the presentation model.

• Dispatcher View combines a controller and dispatcher with views and helpers to handle client requests and prepare a dynamic presentation as the response. Controllers do not delegate content retrieval to helpers, because these activities are deferred to the time of view processing. A dispatcher is responsible for view management and navigation and can be encapsulated either within a controller, a view, or a separate component.

Business Tier Patterns

• Business Delegate reduces coupling between remote tiers and provides an entry point for accessing remote services in the business tier. A Business Delegate might also cache data as necessary to improve performance. A Business Delegate encapsulates a Session Façade and maintains a one-to-one relationship with that Session Façade. An Application Service uses a Business Delegate to invoke a Session Façade.

• Service Locator encapsulates the implementation mechanisms for looking up business service components. A Business Delegate uses a Service Locator to connect to a Session Façade. Other clients that need to locate and connect to Session Façade, other business-tier services, and web services can use a Service Locator. 

• Session Façade provides coarse-grained services to the clients by hiding the complexities of the business service interactions. A Session Façade might invoke several Application Service implementations or Business Objects. A Session Façade can also encapsulate a Value List Handler.

Session Façade provides coarse-grained services to the clients by hiding the complexities of the business service interactions. A Session Façade might invoke several Application Service implementations or Business Objects. A Session Façade can also encapsulate a Value List Handler. The Session Façade pattern provides a coarse-grained service of business components to remote clients. This is the same as a Façade pattern as described in GOF Design Patterns, but just provides an interface to a service instead of code. 

UML representation

Benefits

The following lists the benefits of using the Session Facade pattern:

• Reduces the number of calls to the business component from the client
• Reduces coupling between the tiers
• Improves performance by reducing fine-grained calls from client
• Provides a cleaner API to the client

When to Use

• You should use the Session Facade pattern when:
• You have a series of calls to make to business components from the client.

• Application Service centralizes and aggregates behavior to provide a uniform service layer to the business tier services. An Application Service might interact with other services or Business Objects. An Application Service can invoke other Application Services and thus create a layer of services in your application.

• Business Object implements your conceptual domain model using an object model. Business Objects separate business data and logic into a separate layer in your application. Business Objects typically represent persistent objects and can be transparently persisted using Domain Store.

• Composite Entity implements a Business Object using local entity beans and POJOs. When implemented with bean-managed persistence, a Composite Entity uses Data Access Objects to facilitate persistence.

• The Transfer Object pattern provides the best techniques and strategies to exchange data across tiers (that is, across system boundaries) to reduce the network overhead by minimizing the number of calls to get data from another tier.

• The Transfer Object Assembler constructs a composite Transfer Object from various sources. These sources could be EJB components, Data Access Objects, or other arbitrary Java objects. This pattern is most useful when the client needs to obtain data for the application model or part of the model.

• The Value List Handler uses the GoF iterator pattern to provide query execution and processing services. The Value List Handler caches the results of the query execution and return subsets of the result to the clients as requested. By using this pattern, it is possible to avoid overheads associated with finding large numbers of entity beans. The Value List Handler uses a Data Access Object to execute a query and fetch the results from a persistent store.

The Value List Handler uses the GoF iterator pattern to provide query execution and processing services. The Value List Handler caches the results of the query execution and return subsets of the result to the clients as requested. By using this pattern, it is possible to avoid overheads associated with finding large numbers of entity beans. The Value List Handler uses a Data Access Object to execute a query and fetch the results from a persistent store. The Value List Handler pattern caches results and allows the client to traverse and select from the results. 

UML representation

Benefits

The following lists the benefits of using the Value List Handler pattern:

• Caches search results
• Improves network performance
• Improves separation of concerns

When to Use

You should use the Value List Handler pattern when you:

• Want to iterate through a set of objects.
• Implement read-only lists without transactions.

Integration Tier Patterns

• Data Access Object enables loose coupling between the business and resource tiers. Data Access Object encapsulates all the data access logic to create, retrieve, delete, and update data from a persistent store. Data Access Object uses Transfer Object to send and receive data.

• Service Activator enables asynchronous processing in your enterprise applications using JMS. A Service Activator can invoke Application Service, Session Façade or Business Objects. You can also use several Service Activators to provide parallel asynchronous processing for long running tasks.

• Domain Store provides a powerful mechanism to implement transparent persistence for your object model. It combines and links several other patterns including Data Access Objects.

• Web Service Broker exposes and brokers one or more services in your application to external clients as a web service using XML and standard web protocols. A Web Service Broker can interact with Application Service and Session Façade. A Web Service Broker uses one or more Service Activators to perform asynchronous processing of a request.