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Architectural pattern: Interceptor. Source: POSA II pp 109 – 140 Environment: developing frameworks that can be extended transparently Recurring problem: Frameworks cannot anticipate all of the services they will need - PowerPoint PPT Presentation
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Architectural pattern: Interceptor
• Source: POSA II pp 109 – 140
• Environment: developing frameworks that can be extended transparently
• Recurring problem: – Frameworks cannot anticipate all of the services
they will need– The benefit of a framework is lost if new
services have to be implemented by applications
Requirements for extensibility
• Add a service without requiring modifications to the framework’s core architecture
• Adding application-specific services should not break existing components or applications
• Applications using a framework may need to monitor and control its behavior
Examples of framework services that might use such an extension
mechanism• Security
• Event logging
• Load balancing
• Transaction management
(Core of a) solution - interceptors• Interceptor interfaces provide hooks that
can call service code when certain events occur– Event example: arrival of a client request
• Dispatchers manage the concrete interceptors registered for framework events
• Context objects provide access to event information and the framework’s internal state and behavior
Dynamics1. Application instantiates a concrete interceptor
and registers it with the appropriate dispatcher2. Framework receives an event3. Framework passes a context object to the
dispatcher for that type of event4. Dispatcher calls registered interceptors in the
appropriate order, passing the context object5. Interceptors do their thing, using the context
object for event data and methods to control certain aspects of the framework’s behavior
Implementation1. Model framework’s internal behavior2. Identify and model interception points3. Specify context objects4. Specify the interceptors5. Specify the dispatchers
• Registration interface• Callback interface
6. Implement callback mechanisms7. Implement concrete interceptors
Known uses
• Component-based application servers– EJB, CORBA components, COM+
• CORBA ORBs
• Web browsers– Plug-ins are concrete interceptors
• Non-software: paper mail forwarding
Consequences• Benefits
– Extensibility and flexibility– Separation of concerns– Support for monitoring and control– Layer symmetry– Reuse of interceptors across applications
• Liabilities– Complex design issues– Malicious or erroneous interceptors– Risk of interception cascades