Proxy Pattern Presentation based on GoF Book

1. Pattern
Structural Pattern

2. Warm-up
 You want to impress your girlfriend by
carrying a 250 lbs box (what an impression).
 It is not easy as you see!
 What to do? He he
 Create another box of exactly
the same look, but lighter.
 Now you can go!
 The ‘not that heavy’ box
is the proxy of the normal box.
 But still makes the same effect 
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3. Another Example – This time Serious
 Drawback money from the
bank!
 We will not go to a bank
branch.
 Just go to an ATM.
 Get the cash!
 So ATM is some kind of a
bank.
 With reduced functionalities.
 ATM is also a proxy to the
bank!
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4. Proxy Pattern
 Proxy pattern comes into play when we have
HeavyWeight objects and
 We want to implement a simpler version of a
HeavyWeight object.
 We don’t need the whole functionality of the HeavyWeight
object.
 We want to limit the access to the HeavyWeight object.
 Because there may be a time-delay or complex
mechanism in creating instances of HeavyWeight
objects.
 Proxy pattern is also known as «surrogate».
 Means to put something into place of another
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5. Proxy Pattern Structure
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6. Concrete Structure
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7. Participants
 Subject (Bank)
 The common interface for the proxy and the real subject
 Real Subject (Bank Branch)
 The concrete subject that implements the interface.
 Proxy (ATM)
 Provides the same interface as Real Subject (or a subset)
 Maintains a reference to the Real Subject
 Since it doesnot have all the data as Real Subject (a
lightweight), it can do many things faster than Real
Subject.
 Then invokes Real Subject (if needed).
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8. Object & Sequence Diagrams
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9. Consequences
 A level of indirection while accessing real subject.
 We can think this as another tier between real subject and
us.
 We want to support heavyweight objects, but we
want to create only when they are requested.
 Memory will not get full.
 Programs will run faster.
 It is like giving a small example of the same subject.
 Most of the times, proxy will have same interfaces as
subject.
 Programmers can declare variables without caring
whether a proxy or the heavyweight will be put.
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10. Disadvantages
 Do you think of any disadvantages?
 Identity Comparison
 We can’t do identity comparison since we don’t know
exact real subject, just a surrogate.
 Inambiguity
 Client may not be aware that the real subject it is
accessing now is not same as the previous one.
 Because client doesn’t know what else proxy is doing,
other than calling real subject.
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11. Variants
 Remote Proxy:
 Provides a local represent of an object in different
address space (or maybe in same address)
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12. Variants – cont’d
 Virtual Proxy:
 When we have so many heavyweight objects, we can
create virtual proxies for objects to load program faster.
 Program can start with proxies and real subjects may be
downloaded on background (or when requested).
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13. Variants – cont’d
 Access Control Proxy:
 Act as an controller between the real subject and clients.
 Example: Normal proxy server in modern web
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14. Variants – cont’d
 Smart Reference Proxy:
 Provides additional actions whenever a real subject is
referenced.
 Counting the number of accesses
 Locking it so that only one object can change at a time.
 Deleting an object after a certain number of accesses.
 Cache Proxy:
 Provides temporary storage of results of the expensive
target operations.
 Make some certain operations faster!
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15. Relation with Other Patterns
 Adapter Pattern
 Implements a difference interface to the object it adapts
 Proxy Pattern
 Implements same interface as its subject
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16. Relation with Other Patterns – cont’d
 Decorator Pattern vs Proxy Pattern
 Both patterns describe a level of indirection to an object
 Implementations of both keeps a reference to another
object
 But they are intended for different purposes
 Decorator is adding more responsibilities to the object while
decorating
 Proxy is controlling access to it
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17. Demo Time
 An image displayer
 First shows the proxy with a default image.
 When load pressed, it will load the actual image.
(Bigger and HD)
 UML diagram is on the right.
 See the proxy pattern?
 Demo from
Design Patterns in Java 2nd Edition
Steven John Metsker, William C. Wake
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18. Thanks
Questions?
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19. References
 GoF Design Patterns Book
 The ‘not that strong’ man: http://www.revolutionhealth.com/articles/proper-lifting-
technique/hw206944
 The impressed girl: http://www.shutterstock.com/pic-10016596/stock-photo-surprised-
girl-pointing-at-copy-space-portrait-of-a-beautiful-young-business-woman-looking-
up.html
 The bank: http://chinarsuvidhaacenter.org/
 The ATM Machine: http://www.clipartheaven.com/show/clipart/money/atm-gif.html
 Variant images: O’Reilly Head First Design Patterns book
 Access Control Proxy image: http://www.democracybroadcastingnews.com/?p=12329
 http://www.site.uottawa.ca:4321/oose/index.html#proxy
 Anti-pattern from: The patterns handbook: techniques, strategies, and
applications By Linda Rising (google books has this book)
 http://blog.decayingcode.com/post/anti-pattern-god-object.aspx
 http://www.oodesign.com/proxy-pattern.html
 http://www.javacamp.org/designPattern/proxy.html
 Demo from book: http://www.informit.com/store/product.aspx?isbn=0321333020
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20. Bonus: Introducing Anti-Patterns
 Anti-patterns are a way of making the things wrong.
 Remember Edison?
 Tried hundreds of possible materials for flament till
building the first electric light.
 At each failure, he knows what he shouldn’t do next time!
 Designs may end up with an anti-pattern by:
 Developer or designer’s not knowing better
 Not having sufficient experience for specific problems
 Or applying a perfectly good pattern in the wrong context.
 One example is «God Object»
 An object that knows too much or does too much
 Result of a mis-use of facade pattern
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