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The Proxy Design Pattern (1) A structural design pattern Intent

The Proxy Design Pattern (1) A structural design pattern Intent Provide a surrogate or placeholder for another object to control access to it. The Proxy Design Pattern (2) Problem Situation I: Wish to control access to an object because: Housekeeping activities are required

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The Proxy Design Pattern (1) A structural design pattern Intent

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  1. The Proxy Design Pattern (1) • A structural design pattern • Intent • Provide a surrogate or placeholder for another object to control access to it

  2. The Proxy Design Pattern (2) • Problem • Situation I: Wish to control access to an object because: • Housekeeping activities are required • Protection levels exist • Objects sit in different address spaces • Actual object too expensive to build immediately • Situation II: Wish to provide transparent management of services • The client “thinks” it is using the actual service when rather it is using a proxy for the actual service

  3. The Proxy Design Pattern (3) • Non-software example (1) • Toll-free numbers • Client dials the “800” area code just like they would for a real area code • Proxy records billing information and connects to actual area code

  4. The Proxy Design Pattern (4) • Non-software example (2) • Checks • Check is a proxy for the actual funds in the bank

  5. The Proxy Design Pattern (5) • Abstract Description • Object Proxy Pattern Notice the “is-a” and “has-a” relations again.

  6. The Proxy Design Pattern (6) • Abstract Description • Sequence Diagram (typical proxy application)

  7. The Proxy Design Pattern (7) • Consequences • Isolates RealSubject from client • Forces controllable indirection • Costs indirection • True for all delegation models • May require duplicate information in Proxy and RealSubject • Must support same “properties” • These must be supported even before instantiation of RealSubject

  8. The Proxy Design Pattern (8) • Consequences • Creates second class dependent on Subject interface • Unavoidable in this pattern • Client may need to be aware of Proxy • For example, extra failure modes • Example: java.rmi.MarshalException in RMI

  9. The Proxy Design Pattern (9) • Implementation Issues • Three basic types of proxies • 1. Remote Proxy • Proxy used to “hide” the location of RealSubject • Example: RMI implementation • 2. Virtual Proxy • Performs optimizations • Example: Image rendering in HTML

  10. The Proxy Design Pattern (10) • Implementation Issues • Three basic types of proxies • 3. Housekeeping Proxy • Performs additional maintenance duties • Example: Cache Proxy • Example: Protection Proxy

  11. The Proxy Design Pattern (11) • Implementation Issues • Proxy can be written independent of RealSubject • Use Subject interface in definition of Proxy • Difficult (or impossible) if Proxy stores redundant information

  12. The Proxy Design Pattern (12) • Common Variations • Firewall Proxy (a protection proxy) • Protects targets from bad clients (or vice versa) • Synchronization Proxy (a housekeeping proxy) • Provides multiple accesses to a target object • Smart Reference Proxy (a housekeeping proxy) • Provides additional actions whenever a target object is referenced • Copy-on-Write Proxy (a virtual proxy) • Defers cloning an object until required

  13. The Proxy Design Pattern (13) • Java API Usage • java.rmi library • “Remote Method Invocation” • Allows objects in separate virtual machines to be used as if local (language specific) • Uses “stub and skeleton” to handle communication

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