Design Patterns: Proxy

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Design Patterns Proxy

• Jason Jacob

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What is a Proxy?

• A Proxy is basically a representative between the
  Client and the Component.
• It gives the Client a simple interface to the
  Component.

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How can a Proxy be used?

• It can be used to control access to restricted
  Components.
• It can be used to provide simpler access to the
  Component.
• It can be used to load data on an as needed
  basis to increase efficiency.
• Can be used as a placeholder to enhance
  efficiency.

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Simple Example on Efficiency
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The Class Diagram
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Some Code Simple Math

• public interface IMath
• double Add( double x, double y )
• double Sub( double x, double y )
• double Mul( double x, double y )
• double Div( double x, double y )
• // "RealSubject"
• class Math implements IMath
• public double Add( double x, double y ) return
  x y
• public double Sub( double x, double y ) return
  x - y
• public double Mul( double x, double y ) return
  x y
• public double Div( double x, double y ) return
  x / y

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Some More Code Math Proxy

• class MathProxy implements IMath
• Math math
• private double oldAddx 0.0
• private double oldAddy 0.0
• private double oldSum 0.0
• // more fields here for other operations...
• public MathProxy()
• public double Add( double x, double y )
• if (x oldAddx y oldAddy)
• return oldSum
• else
• oldAddx x oldAddy y oldSum
  math.Add(x,y)
• return oldSum
• // more methods here for other operations...

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Even More Code The Client

• public class ProxyApp
• public static void main(String args)
• MathProxy p new MathProxy()
• System.out.println( "4 2 ", p.Add( 4, 2 )
  )
• System.out.println( "4 2 ", p.Add( 4, 2 )
  )
• System.out.println( "5 2 ", p.Add( 5, 2 )
  )
• System.out.println( "4 - 2 ", p.Sub( 4, 2 )
  )
• System.out.println( "4 2 ", p.Mul( 4, 2 )
  )
• System.out.println( "4 / 2 ", p.Div( 4, 2 )
  )

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Sequence Diagram
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In short

• The Client needed to get something done.
• The client calls the method(s) of the Proxy.
• The Proxy may do some internal pre-processing and
  sends the real call to the Component.
• The Component sends back results.
• The Proxy may do some internal post-processing
  and sends the results back to the Client.

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Some Proxy Implementations

• Protection Proxy Controls access to Component
  based on the access rights of Clients.
• Cache Proxy Saves results temporarily so when
  Client requests same expensive operation again,
  the saved results are returned without making
  call to Component.
• Virtual Proxy Expensive objects are created on
  demand.

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Firewall Proxy
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Firewall Proxy
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Advantages

• Efficiency is generally enhanced and cost is
  lowered.
• Prevents Clients from accessing server Components
  directly.
• Keeps the housekeeping and functionality separate.

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Disadvantages

• Efficiency can suffer due to indirection.
• Implementations can be complex.

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Sources

• http//www.openloop.com/softwareEngineering/patter
  ns/designPattern/dPattern_Proxy.htm
• http//www.inf.bme.hu/ooret/1999osz/DesignPatterns
  /Proxy4/